v9, 8 partsconvert: add support for different encodings

Documentation/gitattributes.txt, convert.c, t/t0028-working-tree-encoding.sh, utf8.h

diff --git a/Documentation/gitattributes.txt b/Documentation/gitattributes.txt
index 11315054f4..aa3deae392 100644
--- a/Documentation/gitattributes.txt
+++ b/Documentation/gitattributes.txt
@@ -297,14 +297,16 @@ number of pitfalls:
   in your repository, then it is strongly recommended to ensure that all
   clients working with the repository support it.

-  If you declare `*.proj` files as UTF-16 and you add `foo.proj` with an
-  `working-tree-encoding` enabled Git client, then `foo.proj` will be
+  For example, Microsoft Visual Studio resources files (`*.rc`) or
+  PowerShell script files (`*.ps1`) are sometimes encoded in UTF-16.
+  If you declare `*.ps1` as files as UTF-16 and you add `foo.ps1` with
+  a `working-tree-encoding` enabled Git client, then `foo.ps1` will be
   stored as UTF-8 internally. A client without `working-tree-encoding`
-  support will checkout `foo.proj` as UTF-8 encoded file. This will
+  support will checkout `foo.ps1` as UTF-8 encoded file. This will
   typically cause trouble for the users of this file.

   If a Git client, that does not support the `working-tree-encoding`
-  attribute, adds a new file `bar.proj`, then `bar.proj` will be
+  attribute, adds a new file `bar.ps1`, then `bar.ps1` will be
   stored "as-is" internally (in this example probably as UTF-16).
   A client with `working-tree-encoding` support will interpret the
   internal contents as UTF-8 and try to convert it to UTF-16 on checkout.
@@ -325,22 +327,22 @@ Use the `working-tree-encoding` attribute only if you cannot store a file
 in UTF-8 encoding and if you want Git to be able to process the content
 as text.

-As an example, use the following attributes if your '*.proj' files are
+As an example, use the following attributes if your '*.ps1' files are
 UTF-16 encoded with byte order mark (BOM) and you want Git to perform
 automatic line ending conversion based on your platform.

 ------------------------
-*.proj		text working-tree-encoding=UTF-16
+*.ps1		text working-tree-encoding=UTF-16
 ------------------------

-Use the following attributes if your '*.proj' files are UTF-16 little
+Use the following attributes if your '*.ps1' files are UTF-16 little
 endian encoded without BOM and you want Git to use Windows line endings
 in the working directory. Please note, it is highly recommended to
 explicitly define the line endings with `eol` if the `working-tree-encoding`
 attribute is used to avoid ambiguity.

 ------------------------
-*.proj 		text working-tree-encoding=UTF-16LE eol=CRLF
+*.ps1		text working-tree-encoding=UTF-16LE eol=CRLF
 ------------------------

 You can get a list of all available encodings on your platform with the
@@ -354,7 +356,7 @@ If you do not know the encoding of a file, then you can use the `file`
 command to guess the encoding:

 ------------------------
-file foo.proj
+file foo.ps1
 ------------------------


diff --git a/convert.c b/convert.c
index 398cd9cf7b..6cbb2b2618 100644
--- a/convert.c
+++ b/convert.c
@@ -266,6 +266,53 @@ static int will_convert_lf_to_crlf(size_t len, struct text_stat *stats,

 }

+static int validate_encoding(const char *path, const char *enc,
+		      const char *data, size_t len, int die_on_error)
+{
+	if (!memcmp("UTF-", enc, 4)) {
+		/*
+		 * Check for detectable errors in UTF encodings
+		 */
+		if (has_prohibited_utf_bom(enc, data, len)) {
+			const char *error_msg = _(
+				"BOM is prohibited in '%s' if encoded as %s");
+			/*
+			 * This advice is shown for UTF-??BE and UTF-??LE
+			 * encodings. We truncate the encoding name to 6
+			 * chars with %.6s to cut off the last two "byte
+			 * order" characters.
+			 */
+			const char *advise_msg = _(
+				"The file '%s' contains a byte order "
+				"mark (BOM). Please use %.6s as "
+				"working-tree-encoding.");
+			advise(advise_msg, path, enc);
+			if (die_on_error)
+				die(error_msg, path, enc);
+			else {
+				return error(error_msg, path, enc);
+			}
+
+		} else if (is_missing_required_utf_bom(enc, data, len)) {
+			const char *error_msg = _(
+				"BOM is required in '%s' if encoded as %s");
+			const char *advise_msg = _(
+				"The file '%s' is missing a byte order "
+				"mark (BOM). Please use %sBE or %sLE "
+				"(depending on the byte order) as "
+				"working-tree-encoding.");
+			advise(advise_msg, path, enc, enc);
+			if (die_on_error)
+				die(error_msg, path, enc);
+			else {
+				return error(error_msg, path, enc);
+			}
+		}
+
+	}
+	return 0;
+}
+
 static void trace_encoding(const char *context, const char *path,
 			   const char *encoding, const char *buf, size_t len)
 {
@@ -297,8 +344,12 @@ static int check_roundtrip(const char* enc_name)
 	 * Search for the given encoding in that string.
 	 */
 	const char *found = strcasestr(check_roundtrip_encoding, enc_name);
-	const char *next = found + strlen(enc_name);
-	int len = strlen(check_roundtrip_encoding);
+	const char *next;
+	int len;
+	if (!found)
+		return 0;
+	next = found + strlen(enc_name);
+	len = strlen(check_roundtrip_encoding);
 	return (found && (
 			/*
 			 * check that the found encoding is at the
@@ -322,17 +373,14 @@ static int check_roundtrip(const char* enc_name)
 		));
 }

-static struct encoding {
-	const char *name;
-	struct encoding *next;
-} *encoding, **encoding_tail;
 static const char *default_encoding = "UTF-8";

 static int encode_to_git(const char *path, const char *src, size_t src_len,
-			 struct strbuf *buf, struct encoding *enc, int conv_flags)
+			 struct strbuf *buf, const char *enc, int conv_flags)
 {
 	char *dst;
 	int dst_len;
+	int die_on_error = conv_flags & CONV_WRITE_OBJECT;

 	/*
 	 * No encoding is specified or there is nothing to encode.
@@ -350,39 +398,11 @@ static int encode_to_git(const char *path, const char *src, size_t src_len,
 	if (!buf && !src)
 		return 1;

-	if (has_prohibited_utf_bom(enc->name, src, src_len)) {
-		const char *error_msg = _(
-			"BOM is prohibited in '%s' if encoded as %s");
-		/*
-		 * This advise is shown for UTF-??BE and UTF-??LE encodings.
-		 * We truncate the encoding name to 6 chars with %.6s to cut
-		 * off the last two "byte order" characters.
-		 */
-		const char *advise_msg = _(
-			"The file '%s' contains a byte order mark (BOM). "
-			"Please use %.6s as working-tree-encoding.");
-		advise(advise_msg, path, enc->name);
-		if (conv_flags & CONV_WRITE_OBJECT)
-			die(error_msg, path, enc->name);
-		else
-			error(error_msg, path, enc->name);
-
-	} else if (is_missing_required_utf_bom(enc->name, src, src_len)) {
-		const char *error_msg = _(
-			"BOM is required in '%s' if encoded as %s");
-		const char *advise_msg = _(
-			"The file '%s' is missing a byte order mark (BOM). "
-			"Please use %sBE or %sLE (depending on the byte order) "
-			"as working-tree-encoding.");
-		advise(advise_msg, path, enc->name, enc->name);
-		if (conv_flags & CONV_WRITE_OBJECT)
-			die(error_msg, path, enc->name);
-		else
-			error(error_msg, path, enc->name);
-	}
+	if (validate_encoding(path, enc, src, src_len, die_on_error))
+		return 0;

-	trace_encoding("source", path, enc->name, src, src_len);
-	dst = reencode_string_len(src, src_len, default_encoding, enc->name,
+	trace_encoding("source", path, enc, src, src_len);
+	dst = reencode_string_len(src, src_len, default_encoding, enc,
 				  &dst_len);
 	if (!dst) {
 		/*
@@ -392,10 +412,12 @@ static int encode_to_git(const char *path, const char *src, size_t src_len,
 		 * working tree. Let's try to avoid this by screaming loud.
 		 */
 		const char* msg = _("failed to encode '%s' from %s to %s");
-		if (conv_flags & CONV_WRITE_OBJECT)
-			die(msg, path, enc->name, default_encoding);
-		else
-			error(msg, path, enc->name, default_encoding);
+		if (die_on_error)
+			die(msg, path, enc, default_encoding);
+		else {
+			error(msg, path, enc, default_encoding);
+			return 0;
+		}
 	}
 	trace_encoding("destination", path, default_encoding, dst, dst_len);

@@ -418,23 +440,23 @@ static int encode_to_git(const char *path, const char *src, size_t src_len,
 	 * [1] http://unicode.org/faq/utf_bom.html#gen2
 	 * [2] https://support.microsoft.com/en-us/help/170559/prb-conversion-problem-between-shift-jis-and-unicode
 	 */
-	if ((conv_flags & CONV_WRITE_OBJECT) && check_roundtrip(enc->name)) {
+	if (die_on_error && check_roundtrip(enc)) {
 		char *re_src;
 		int re_src_len;

 		re_src = reencode_string_len(dst, dst_len,
-					     enc->name, default_encoding,
+					     enc, default_encoding,
 					     &re_src_len);

-		trace_printf("Checking roundtrip encoding for %s...\n", enc->name);
-		trace_encoding("reencoded source", path, enc->name,
+		trace_printf("Checking roundtrip encoding for %s...\n", enc);
+		trace_encoding("reencoded source", path, enc,
 			       re_src, re_src_len);

 		if (!re_src || src_len != re_src_len ||
 		    memcmp(src, re_src, src_len)) {
 			const char* msg = _("encoding '%s' from %s to %s and "
 					    "back is not the same");
-			die(msg, path, enc->name, default_encoding);
+			die(msg, path, enc, default_encoding);
 		}

 		free(re_src);
@@ -445,7 +467,7 @@ static int encode_to_git(const char *path, const char *src, size_t src_len,
 }

 static int encode_to_worktree(const char *path, const char *src, size_t src_len,
-			      struct strbuf *buf, struct encoding *enc)
+			      struct strbuf *buf, const char *enc)
 {
 	char *dst;
 	int dst_len;
@@ -457,11 +479,11 @@ static int encode_to_worktree(const char *path, const char *src, size_t src_len,
 	if (!enc || (src && !src_len))
 		return 0;

-	dst = reencode_string_len(src, src_len, enc->name, default_encoding,
+	dst = reencode_string_len(src, src_len, enc, default_encoding,
 				  &dst_len);
 	if (!dst) {
 		error("failed to encode '%s' from %s to %s",
-			path, enc->name, default_encoding);
+			path, default_encoding, enc);
 		return 0;
 	}

@@ -1182,33 +1204,23 @@ static int ident_to_worktree(const char *path, const char *src, size_t len,
 	return 1;
 }

-static struct encoding *git_path_check_encoding(struct attr_check_item *check)
+static const char *git_path_check_encoding(struct attr_check_item *check)
 {
 	const char *value = check->value;
-	struct encoding *enc;

 	if (ATTR_TRUE(value) || ATTR_FALSE(value) || ATTR_UNSET(value) ||
 	    !strlen(value))
 		return NULL;

-	for (enc = encoding; enc; enc = enc->next)
-		if (!strcasecmp(value, enc->name))
-			return enc;
-
 	/* Don't encode to the default encoding */
 	if (!strcasecmp(value, default_encoding))
 		return NULL;

-	enc = xcalloc(1, sizeof(*enc));
 	/*
 	 * Ensure encoding names are always upper case (e.g. UTF-8) to
 	 * simplify subsequent string comparisons.
 	 */
-	enc->name = xstrdup_toupper(value);
-	*encoding_tail = enc;
-	encoding_tail = &(enc->next);
-
-	return enc;
+	return xstrdup_toupper(value);
 }

 static enum crlf_action git_path_check_crlf(struct attr_check_item *check)
@@ -1266,7 +1278,7 @@ struct conv_attrs {
 	enum crlf_action attr_action; /* What attr says */
 	enum crlf_action crlf_action; /* When no attr is set, use core.autocrlf */
 	int ident;
-	struct encoding *working_tree_encoding; /* Supported encoding or default encoding if NULL */
+	const char *working_tree_encoding; /* Supported encoding or default encoding if NULL */
 };

 static void convert_attrs(struct conv_attrs *ca, const char *path)
@@ -1278,7 +1290,6 @@ static void convert_attrs(struct conv_attrs *ca, const char *path)
 					 "eol", "text", "working-tree-encoding",
 					 NULL);
 		user_convert_tail = &user_convert;
-		encoding_tail = &encoding;
 		git_config(read_convert_config, NULL);
 	}

diff --git a/t/t0028-working-tree-encoding.sh b/t/t0028-working-tree-encoding.sh
index e34c21eb29..23e89ae623 100755
--- a/t/t0028-working-tree-encoding.sh
+++ b/t/t0028-working-tree-encoding.sh
@@ -6,7 +6,7 @@ test_description='working-tree-encoding conversion via gitattributes'

 GIT_TRACE_WORKING_TREE_ENCODING=1 && export GIT_TRACE_WORKING_TREE_ENCODING

-test_expect_success 'setup test repo' '
+test_expect_success 'setup test files' '
 	git config core.eol lf &&

 	text="hallo there!\ncan you read me?" &&
@@ -14,164 +14,135 @@ test_expect_success 'setup test repo' '
 	printf "$text" >test.utf8.raw &&
 	printf "$text" | iconv -f UTF-8 -t UTF-16 >test.utf16.raw &&
 	printf "$text" | iconv -f UTF-8 -t UTF-32 >test.utf32.raw &&
-	cp test.utf16.raw test.utf16 &&
-	cp test.utf32.raw test.utf32 &&
+
+	# Line ending tests
+	printf "one\ntwo\nthree\n" >lf.utf8.raw &&
+	printf "one\r\ntwo\r\nthree\r\n" >crlf.utf8.raw &&
+
+	# BOM tests
+	printf "\0a\0b\0c"                         >nobom.utf16be.raw &&
+	printf "a\0b\0c\0"                         >nobom.utf16le.raw &&
+	printf "\376\777\0a\0b\0c"                 >bebom.utf16be.raw &&
+	printf "\777\376a\0b\0c\0"                 >lebom.utf16le.raw &&
+	printf "\0\0\0a\0\0\0b\0\0\0c"             >nobom.utf32be.raw &&
+	printf "a\0\0\0b\0\0\0c\0\0\0"             >nobom.utf32le.raw &&
+	printf "\0\0\376\777\0\0\0a\0\0\0b\0\0\0c" >bebom.utf32be.raw &&
+	printf "\777\376\0\0a\0\0\0b\0\0\0c\0\0\0" >lebom.utf32le.raw &&

 	# Add only UTF-16 file, we will add the UTF-32 file later
+	cp test.utf16.raw test.utf16 &&
+	cp test.utf32.raw test.utf32 &&
 	git add .gitattributes test.utf16 &&
 	git commit -m initial
 '

 test_expect_success 'ensure UTF-8 is stored in Git' '
-	git cat-file -p :test.utf16 >test.utf16.git &&
-	test_cmp_bin test.utf8.raw test.utf16.git &&
+	test_when_finished "rm -f test.utf16.git" &&

-	# cleanup
-	rm test.utf16.git
+	git cat-file -p :test.utf16 >test.utf16.git &&
+	test_cmp_bin test.utf8.raw test.utf16.git
 '

 test_expect_success 're-encode to UTF-16 on checkout' '
+	test_when_finished "rm -f test.utf16.raw" &&
+
 	rm test.utf16 &&
 	git checkout test.utf16 &&
-	test_cmp_bin test.utf16.raw test.utf16 &&
-
-	# cleanup
-	rm test.utf16.raw
+	test_cmp_bin test.utf16.raw test.utf16
 '

 test_expect_success 'check $GIT_DIR/info/attributes support' '
-	echo "*.utf32 text working-tree-encoding=utf-32" >.git/info/attributes &&
+	test_when_finished "rm -f test.utf8.raw test.utf32.raw test.utf32.git" &&
+	test_when_finished "git reset --hard HEAD" &&

+	echo "*.utf32 text working-tree-encoding=utf-32" >.git/info/attributes &&
 	git add test.utf32 &&

 	git cat-file -p :test.utf32 >test.utf32.git &&
-	test_cmp_bin test.utf8.raw test.utf32.git &&
-
-	# cleanup
-	git reset --hard HEAD &&
-	rm test.utf8.raw test.utf32.raw test.utf32.git
+	test_cmp_bin test.utf8.raw test.utf32.git
 '

-test_expect_success 'check prohibited UTF BOM' '
-	printf "\0a\0b\0c"                         >nobom.utf16be.raw &&
-	printf "a\0b\0c\0"                         >nobom.utf16le.raw &&
-	printf "\376\777\0a\0b\0c"                 >bebom.utf16be.raw &&
-	printf "\777\376a\0b\0c\0"                 >lebom.utf16le.raw &&
+for i in 16 32
+do
+	test_expect_success "check prohibited UTF-${i} BOM" '
+		test_when_finished "git reset --hard HEAD" &&

-	printf "\0\0\0a\0\0\0b\0\0\0c"             >nobom.utf32be.raw &&
-	printf "a\0\0\0b\0\0\0c\0\0\0"             >nobom.utf32le.raw &&
-	printf "\0\0\376\777\0\0\0a\0\0\0b\0\0\0c" >bebom.utf32be.raw &&
-	printf "\777\376\0\0a\0\0\0b\0\0\0c\0\0\0" >lebom.utf32le.raw &&
+		echo "*.utf${i}be text working-tree-encoding=utf-${i}be" >>.gitattributes &&
+		echo "*.utf${i}le text working-tree-encoding=utf-${i}le" >>.gitattributes &&

-	echo "*.utf16be text working-tree-encoding=utf-16be" >>.gitattributes &&
-	echo "*.utf16le text working-tree-encoding=utf-16le" >>.gitattributes &&
-	echo "*.utf32be text working-tree-encoding=utf-32be" >>.gitattributes &&
-	echo "*.utf32le text working-tree-encoding=utf-32le" >>.gitattributes &&
-
-	# Here we add a UTF-16 files with BOM (big-endian and little-endian)
-	# but we tell Git to treat it as UTF-16BE/UTF-16LE. In these cases
-	# the BOM is prohibited.
-	cp bebom.utf16be.raw bebom.utf16be &&
-	test_must_fail git add bebom.utf16be 2>err.out &&
-	test_i18ngrep "fatal: BOM is prohibited .* UTF-16BE" err.out &&
-
-	cp lebom.utf16le.raw lebom.utf16be &&
-	test_must_fail git add lebom.utf16be 2>err.out &&
-	test_i18ngrep "fatal: BOM is prohibited .* UTF-16BE" err.out &&
-
-	cp bebom.utf16be.raw bebom.utf16le &&
-	test_must_fail git add bebom.utf16le 2>err.out &&
-	test_i18ngrep "fatal: BOM is prohibited .* UTF-16LE" err.out &&
-
-	cp lebom.utf16le.raw lebom.utf16le &&
-	test_must_fail git add lebom.utf16le 2>err.out &&
-	test_i18ngrep "fatal: BOM is prohibited .* UTF-16LE" err.out &&
-
-	# ... and the same for UTF-32
-	cp bebom.utf32be.raw bebom.utf32be &&
-	test_must_fail git add bebom.utf32be 2>err.out &&
-	test_i18ngrep "fatal: BOM is prohibited .* UTF-32BE" err.out &&
-
-	cp lebom.utf32le.raw lebom.utf32be &&
-	test_must_fail git add lebom.utf32be 2>err.out &&
-	test_i18ngrep "fatal: BOM is prohibited .* UTF-32BE" err.out &&
-
-	cp bebom.utf32be.raw bebom.utf32le &&
-	test_must_fail git add bebom.utf32le 2>err.out &&
-	test_i18ngrep "fatal: BOM is prohibited .* UTF-32LE" err.out &&
-
-	cp lebom.utf32le.raw lebom.utf32le &&
-	test_must_fail git add lebom.utf32le 2>err.out &&
-	test_i18ngrep "fatal: BOM is prohibited .* UTF-32LE" err.out &&
-
-	# cleanup
-	git reset --hard HEAD
-'
+		# Here we add a UTF-16 (resp. UTF-32) files with BOM (big/little-endian)
+		# but we tell Git to treat it as UTF-16BE/UTF-16LE (resp. UTF-32).
+		# In these cases the BOM is prohibited.
+		cp bebom.utf${i}be.raw bebom.utf${i}be &&
+		test_must_fail git add bebom.utf${i}be 2>err.out &&
+		test_i18ngrep "fatal: BOM is prohibited .* UTF-${i}BE" err.out &&
+
+		cp lebom.utf${i}le.raw lebom.utf${i}be &&
+		test_must_fail git add lebom.utf${i}be 2>err.out &&
+		test_i18ngrep "fatal: BOM is prohibited .* UTF-${i}BE" err.out &&

-test_expect_success 'check required UTF BOM' '
-	echo "*.utf32 text working-tree-encoding=utf-32" >>.gitattributes &&
+		cp bebom.utf${i}be.raw bebom.utf${i}le &&
+		test_must_fail git add bebom.utf${i}le 2>err.out &&
+		test_i18ngrep "fatal: BOM is prohibited .* UTF-${i}LE" err.out &&

-	cp nobom.utf16be.raw nobom.utf16 &&
-	test_must_fail git add nobom.utf16 2>err.out &&
-	test_i18ngrep "fatal: BOM is required .* UTF-16" err.out &&
+		cp lebom.utf${i}le.raw lebom.utf${i}le &&
+		test_must_fail git add lebom.utf${i}le 2>err.out &&
+		test_i18ngrep "fatal: BOM is prohibited .* UTF-${i}LE" err.out
+	'

-	cp nobom.utf16le.raw nobom.utf16 &&
-	test_must_fail git add nobom.utf16 2>err.out &&
-	test_i18ngrep "fatal: BOM is required .* UTF-16" err.out &&
+	test_expect_success "check required UTF-${i} BOM" '
+		test_when_finished "git reset --hard HEAD" &&

-	cp nobom.utf32be.raw nobom.utf32 &&
-	test_must_fail git add nobom.utf32 2>err.out &&
-	test_i18ngrep "fatal: BOM is required .* UTF-32" err.out &&
+		echo "*.utf${i} text working-tree-encoding=utf-${i}" >>.gitattributes &&

-	cp nobom.utf32le.raw nobom.utf32 &&
-	test_must_fail git add nobom.utf32 2>err.out &&
-	test_i18ngrep "fatal: BOM is required .* UTF-32" err.out &&
+		cp nobom.utf${i}be.raw nobom.utf${i} &&
+		test_must_fail git add nobom.utf${i} 2>err.out &&
+		test_i18ngrep "fatal: BOM is required .* UTF-${i}" err.out &&

-	# cleanup
-	rm nobom.utf16 nobom.utf32 &&
-	git reset --hard HEAD
+		cp nobom.utf${i}le.raw nobom.utf${i} &&
+		test_must_fail git add nobom.utf${i} 2>err.out &&
+		test_i18ngrep "fatal: BOM is required .* UTF-${i}" err.out
 	'

-test_expect_success 'eol conversion for UTF-16 encoded files on checkout' '
-	printf "one\ntwo\nthree\n" >lf.utf8.raw &&
-	printf "one\r\ntwo\r\nthree\r\n" >crlf.utf8.raw &&
+	test_expect_success "eol conversion for UTF-${i} encoded files on checkout" '
+		test_when_finished "rm -f crlf.utf${i}.raw lf.utf${i}.raw" &&
+		test_when_finished "git reset --hard HEAD^" &&

-	cat lf.utf8.raw | iconv -f UTF-8 -t UTF-16 >lf.utf16.raw &&
-	cat crlf.utf8.raw | iconv -f UTF-8 -t UTF-16 >crlf.utf16.raw &&
-	cp crlf.utf16.raw eol.utf16 &&
+		cat lf.utf8.raw | iconv -f UTF-8 -t UTF-${i} >lf.utf${i}.raw &&
+		cat crlf.utf8.raw | iconv -f UTF-8 -t UTF-${i} >crlf.utf${i}.raw &&
+		cp crlf.utf${i}.raw eol.utf${i} &&

-	cat >expectIndexLF <<-\EOF &&
-		i/lf    w/-text attr/text             	eol.utf16
+		cat >expectIndexLF <<-EOF &&
+			i/lf    w/-text attr/text             	eol.utf${i}
 		EOF

-	git add eol.utf16 &&
+		git add eol.utf${i} &&
 		git commit -m eol &&

-	# UTF-16 with CRLF (Windows line endings)
-	rm eol.utf16 &&
-	git -c core.eol=crlf checkout eol.utf16 &&
-	test_cmp_bin crlf.utf16.raw eol.utf16 &&
+		# UTF-${i} with CRLF (Windows line endings)
+		rm eol.utf${i} &&
+		git -c core.eol=crlf checkout eol.utf${i} &&
+		test_cmp_bin crlf.utf${i}.raw eol.utf${i} &&

-	# Although the file has CRLF in the working tree, ensure LF in the index
-	git ls-files --eol eol.utf16 >actual &&
+		# Although the file has CRLF in the working tree,
+		# ensure LF in the index
+		git ls-files --eol eol.utf${i} >actual &&
 		test_cmp expectIndexLF actual &&

-	# UTF-16 with LF (Unix line endings)
-	rm eol.utf16 &&
-	git -c core.eol=lf checkout eol.utf16 &&
-	test_cmp_bin lf.utf16.raw eol.utf16 &&
+		# UTF-${i} with LF (Unix line endings)
+		rm eol.utf${i} &&
+		git -c core.eol=lf checkout eol.utf${i} &&
+		test_cmp_bin lf.utf${i}.raw eol.utf${i} &&

 		# The file LF in the working tree, ensure LF in the index
-	git ls-files --eol eol.utf16 >actual &&
-	test_cmp expectIndexLF actual&&
-
-	rm crlf.utf16.raw crlf.utf8.raw lf.utf16.raw lf.utf8.raw &&
-
-	# cleanup
-	git reset --hard HEAD^
+		git ls-files --eol eol.utf${i} >actual &&
+		test_cmp expectIndexLF actual
 	'
+done

 test_expect_success 'check unsupported encodings' '
+	test_when_finished "rm -f err.out" &&
+	test_when_finished "git reset --hard HEAD" &&

 	echo "*.nothing text working-tree-encoding=" >>.gitattributes &&
 	printf "nothing" >t.nothing &&
@@ -180,34 +151,33 @@ test_expect_success 'check unsupported encodings' '
 	echo "*.garbage text working-tree-encoding=garbage" >>.gitattributes &&
 	printf "garbage" >t.garbage &&
 	test_must_fail git add t.garbage 2>err.out &&
-	test_i18ngrep "fatal: failed to encode" err.out &&
-
-	# cleanup
-	rm err.out &&
-	git reset --hard HEAD
+	test_i18ngrep "fatal: failed to encode" err.out
 '

 test_expect_success 'error if encoding round trip is not the same during refresh' '
 	BEFORE_STATE=$(git rev-parse HEAD) &&
-
-	# Skip the UTF-16 filter for the added file
-	# This simulates a Git version that has no working tree encoding support
-	echo "hallo" >nonsense.utf16 &&
-	TEST_HASH=$(git hash-object --no-filters -w nonsense.utf16) &&
-	git update-index --add --cacheinfo 100644 $TEST_HASH nonsense.utf16 &&
+	test_when_finished "rm -f err.out" &&
+	test_when_finished "git reset --hard $BEFORE_STATE" &&
+
+	# Add and commit a UTF-16 file but skip the "working-tree-encoding"
+	# filter. Consequently, the in-repo representation is UTF-16 and not
+	# UTF-8. This simulates a Git version that has no working tree encoding
+	# support.
+	echo "*.utf16le text working-tree-encoding=utf-16le" >.gitattributes &&
+	echo "hallo" >nonsense.utf16le &&
+	TEST_HASH=$(git hash-object --no-filters -w nonsense.utf16le) &&
+	git update-index --add --cacheinfo 100644 $TEST_HASH nonsense.utf16le &&
 	COMMIT=$(git commit-tree -p $(git rev-parse HEAD) -m "plain commit" $(git write-tree)) &&
 	git update-ref refs/heads/master $COMMIT &&

 	test_must_fail git checkout HEAD^ 2>err.out &&
-	test_i18ngrep "error: .* overwritten by checkout:" err.out &&
-
-	# cleanup
-	rm err.out &&
-	git reset --hard $BEFORE_STATE
+	test_i18ngrep "error: .* overwritten by checkout:" err.out
 '

 test_expect_success 'error if encoding garbage is already in Git' '
 	BEFORE_STATE=$(git rev-parse HEAD) &&
+	test_when_finished "rm -f err.out" &&
+	test_when_finished "git reset --hard $BEFORE_STATE" &&

 	# Skip the UTF-16 filter for the added file
 	# This simulates a Git version that has no checkoutEncoding support
@@ -218,52 +188,46 @@ test_expect_success 'error if encoding garbage is already in Git' '
 	git update-ref refs/heads/master $COMMIT &&

 	git diff 2>err.out &&
-	test_i18ngrep "error: BOM is required" err.out &&
-
-	# cleanup
-	rm err.out &&
-	git reset --hard $BEFORE_STATE
+	test_i18ngrep "error: BOM is required" err.out
 '

 test_expect_success 'check roundtrip encoding' '
+	test_when_finished "rm -f roundtrip.shift roundtrip.utf16" &&
+	test_when_finished "git reset --hard HEAD" &&
+
 	text="hallo there!\nroundtrip test here!" &&
 	printf "$text" | iconv -f UTF-8 -t SHIFT-JIS >roundtrip.shift &&
 	printf "$text" | iconv -f UTF-8 -t UTF-16 >roundtrip.utf16 &&
 	echo "*.shift text working-tree-encoding=SHIFT-JIS" >>.gitattributes &&

 	# SHIFT-JIS encoded files are round-trip checked by default...
-	GIT_TRACE=1 git add .gitattributes roundtrip.shift 2>&1 >/dev/null |
+	GIT_TRACE=1 git add .gitattributes roundtrip.shift 2>&1 |
 		grep "Checking roundtrip encoding for SHIFT-JIS" &&
 	git reset &&

 	# ... unless we overwrite the Git config!
-	test_config core.checkRoundtripEncoding "garbage" &&
-	! GIT_TRACE=1 git add .gitattributes roundtrip.shift 2>&1 >/dev/null |
+	! GIT_TRACE=1 git -c core.checkRoundtripEncoding=garbage \
+		add .gitattributes roundtrip.shift 2>&1 |
 		grep "Checking roundtrip encoding for SHIFT-JIS" &&
-	test_unconfig core.checkRoundtripEncoding &&
 	git reset &&

 	# UTF-16 encoded files should not be round-trip checked by default...
-	! GIT_TRACE=1 git add roundtrip.utf16 2>&1 >/dev/null |
+	! GIT_TRACE=1 git add roundtrip.utf16 2>&1 |
 		grep "Checking roundtrip encoding for UTF-16" &&
 	git reset &&

 	# ... unless we tell Git to check it!
-	test_config_global core.checkRoundtripEncoding "UTF-16, UTF-32" &&
-	GIT_TRACE=1 git add roundtrip.utf16 2>&1 >/dev/null |
+	GIT_TRACE=1 git -c core.checkRoundtripEncoding="UTF-16, UTF-32" \
+		add roundtrip.utf16 2>&1 |
 		grep "Checking roundtrip encoding for UTF-16" &&
 	git reset &&

 	# ... unless we tell Git to check it!
 	# (here we also check that the casing of the encoding is irrelevant)
-	test_config_global core.checkRoundtripEncoding "UTF-32, utf-16" &&
-	GIT_TRACE=1 git add roundtrip.utf16 2>&1 >/dev/null |
+	GIT_TRACE=1 git -c core.checkRoundtripEncoding="UTF-32, utf-16" \
+		add roundtrip.utf16 2>&1 |
 		grep "Checking roundtrip encoding for UTF-16" &&
-	git reset &&
-
-	# cleanup
-	rm roundtrip.shift roundtrip.utf16 &&
-	git reset --hard HEAD
+	git reset
 '

 test_done
diff --git a/utf8.h b/utf8.h
index 62f86fba64..cce654a64a 100644
--- a/utf8.h
+++ b/utf8.h
@@ -71,9 +71,9 @@ void strbuf_utf8_align(struct strbuf *buf, align_type position, unsigned int wid
 		       const char *s);

 /*
- * Whenever a data stream is declared to be UTF-16BE, UTF-16LE, UTF-32BE
- * or UTF-32LE a BOM must not be used [1]. The function returns true if
- * this is the case.
+ * If a data stream is declared as UTF-16BE or UTF-16LE, then a UTF-16
+ * BOM must not be used [1]. The same applies for the UTF-32 equivalents.
+ * The function returns true if this rule is violated.
  *
  * [1] http://unicode.org/faq/utf_bom.html#bom10
  */
@@ -83,10 +83,13 @@ int has_prohibited_utf_bom(const char *enc, const char *data, size_t len);
  * If the endianness is not defined in the encoding name, then we
  * require a BOM. The function returns true if a required BOM is missing.
  *
- * The Unicode standard instructs to assume big-endian if there
- * in no BOM for UTF-16/32 [1][2]. However, the W3C/WHATWG
- * encoding standard used in HTML5 recommends to assume
- * little-endian to "deal with deployed content" [3].
+ * The Unicode standard instructs to assume big-endian if there in no
+ * BOM for UTF-16/32 [1][2]. However, the W3C/WHATWG encoding standard
+ * used in HTML5 recommends to assume little-endian to "deal with
+ * deployed content" [3].
+ *
+ * Therefore, strictly requiring a BOM seems to be the safest option for
+ * content in Git.
  *
  * [1] http://unicode.org/faq/utf_bom.html#gen6
  * [2] http://www.unicode.org/versions/Unicode10.0.0/ch03.pdf


### Patches

Lars Schneider (8):
  strbuf: remove unnecessary NUL assignment in xstrdup_tolower()
  strbuf: add xstrdup_toupper()
  utf8: add function to detect prohibited UTF-16/32 BOM
  utf8: add function to detect a missing UTF-16/32 BOM
  convert: add 'working-tree-encoding' attribute
  convert: check for detectable errors in UTF encodings
  convert: add tracing for 'working-tree-encoding' attribute
  convert: add round trip check based on 'core.checkRoundtripEncoding'

 Documentation/config.txt         |   6 +
 Documentation/gitattributes.txt  |  88 +++++++++++++
 config.c                         |   5 +
 convert.c                        | 267 ++++++++++++++++++++++++++++++++++++++-
 convert.h                        |   2 +
 environment.c                    |   1 +
 sha1_file.c                      |   2 +-
 strbuf.c                         |  13 +-
 strbuf.h                         |   1 +
 t/t0028-working-tree-encoding.sh | 233 ++++++++++++++++++++++++++++++++++
 utf8.c                           |  37 ++++++
 utf8.h                           |  28 ++++
 12 files changed, 680 insertions(+), 3 deletions(-)
 create mode 100755 t/t0028-working-tree-encoding.sh


base-commit: 8a2f0888555ce46ac87452b194dec5cb66fb1417
--
2.16.2

diff --git a/strbuf.c b/strbuf.c
index 1df674e919..55b7daeb35 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -781,7 +781,6 @@ char *xstrdup_tolower(const char *string)
 	result = xmallocz(len);
 	for (i = 0; i < len; i++)
 		result[i] = tolower(string[i]);
-	result[i] = '\0';
 	return result;
 }
 

utf8.c, utf8.h

diff --git a/utf8.c b/utf8.c
index 2c27ce0137..914881cd1f 100644
--- a/utf8.c
+++ b/utf8.c
@@ -538,6 +538,30 @@ char *reencode_string_len(const char *in, int insz,
 }
 #endif
 
+static int has_bom_prefix(const char *data, size_t len,
+			  const char *bom, size_t bom_len)
+{
+	return (len >= bom_len) && !memcmp(data, bom, bom_len);
+}
+
+static const char utf16_be_bom[] = {0xFE, 0xFF};
+static const char utf16_le_bom[] = {0xFF, 0xFE};
+static const char utf32_be_bom[] = {0x00, 0x00, 0xFE, 0xFF};
+static const char utf32_le_bom[] = {0xFF, 0xFE, 0x00, 0x00};
+
+int has_prohibited_utf_bom(const char *enc, const char *data, size_t len)
+{
+	return (
+	  (!strcmp(enc, "UTF-16BE") || !strcmp(enc, "UTF-16LE")) &&
+	  (has_bom_prefix(data, len, utf16_be_bom, sizeof(utf16_be_bom)) ||
+	   has_bom_prefix(data, len, utf16_le_bom, sizeof(utf16_le_bom)))
+	) || (
+	  (!strcmp(enc, "UTF-32BE") || !strcmp(enc, "UTF-32LE")) &&
+	  (has_bom_prefix(data, len, utf32_be_bom, sizeof(utf32_be_bom)) ||
+	   has_bom_prefix(data, len, utf32_le_bom, sizeof(utf32_le_bom)))
+	);
+}
+
 /*
  * Returns first character length in bytes for multi-byte `text` according to
  * `encoding`.
diff --git a/utf8.h b/utf8.h
index 6bbcf31a83..0db1db4519 100644
--- a/utf8.h
+++ b/utf8.h
@@ -70,4 +70,13 @@ typedef enum {
 void strbuf_utf8_align(struct strbuf *buf, align_type position, unsigned int width,
 		       const char *s);
 
+/*
+ * If a data stream is declared as UTF-16BE or UTF-16LE, then a UTF-16
+ * BOM must not be used [1]. The same applies for the UTF-32 equivalents.
+ * The function returns true if this rule is violated.
+ *
+ * [1] http://unicode.org/faq/utf_bom.html#bom10
+ */
+int has_prohibited_utf_bom(const char *enc, const char *data, size_t len);
+
 #endif

strbuf.c, strbuf.h

diff --git a/strbuf.c b/strbuf.c
index 55b7daeb35..b635f0bdc4 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -784,6 +784,18 @@ char *xstrdup_tolower(const char *string)
 	return result;
 }
 
+char *xstrdup_toupper(const char *string)
+{
+	char *result;
+	size_t len, i;
+
+	len = strlen(string);
+	result = xmallocz(len);
+	for (i = 0; i < len; i++)
+		result[i] = toupper(string[i]);
+	return result;
+}
+
 char *xstrvfmt(const char *fmt, va_list ap)
 {
 	struct strbuf buf = STRBUF_INIT;
diff --git a/strbuf.h b/strbuf.h
index 14c8c10d66..df7ced53ed 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -607,6 +607,7 @@ __attribute__((format (printf,2,3)))
 extern int fprintf_ln(FILE *fp, const char *fmt, ...);
 
 char *xstrdup_tolower(const char *);
+char *xstrdup_toupper(const char *);
 
 /**
  * Create a newly allocated string using printf format. You can do this easily

utf8.c, utf8.h

diff --git a/utf8.c b/utf8.c
index 914881cd1f..5113d26e56 100644
--- a/utf8.c
+++ b/utf8.c
@@ -562,6 +562,19 @@ int has_prohibited_utf_bom(const char *enc, const char *data, size_t len)
 	);
 }
 
+int is_missing_required_utf_bom(const char *enc, const char *data, size_t len)
+{
+	return (
+	   !strcmp(enc, "UTF-16") &&
+	   !(has_bom_prefix(data, len, utf16_be_bom, sizeof(utf16_be_bom)) ||
+	     has_bom_prefix(data, len, utf16_le_bom, sizeof(utf16_le_bom)))
+	) || (
+	   !strcmp(enc, "UTF-32") &&
+	   !(has_bom_prefix(data, len, utf32_be_bom, sizeof(utf32_be_bom)) ||
+	     has_bom_prefix(data, len, utf32_le_bom, sizeof(utf32_le_bom)))
+	);
+}
+
 /*
  * Returns first character length in bytes for multi-byte `text` according to
  * `encoding`.
diff --git a/utf8.h b/utf8.h
index 0db1db4519..cce654a64a 100644
--- a/utf8.h
+++ b/utf8.h
@@ -79,4 +79,23 @@ void strbuf_utf8_align(struct strbuf *buf, align_type position, unsigned int wid
  */
 int has_prohibited_utf_bom(const char *enc, const char *data, size_t len);
 
+/*
+ * If the endianness is not defined in the encoding name, then we
+ * require a BOM. The function returns true if a required BOM is missing.
+ *
+ * The Unicode standard instructs to assume big-endian if there in no
+ * BOM for UTF-16/32 [1][2]. However, the W3C/WHATWG encoding standard
+ * used in HTML5 recommends to assume little-endian to "deal with
+ * deployed content" [3].
+ *
+ * Therefore, strictly requiring a BOM seems to be the safest option for
+ * content in Git.
+ *
+ * [1] http://unicode.org/faq/utf_bom.html#gen6
+ * [2] http://www.unicode.org/versions/Unicode10.0.0/ch03.pdf
+ *     Section 3.10, D98, page 132
+ * [3] https://encoding.spec.whatwg.org/#utf-16le
+ */
+int is_missing_required_utf_bom(const char *enc, const char *data, size_t len);
+
 #endif

Documentation/gitattributes.txt, convert.c, convert.h, sha1_file.c, t/t0028-working-tree-encoding.sh

diff --git a/Documentation/gitattributes.txt b/Documentation/gitattributes.txt
index 30687de81a..31a4f92840 100644
--- a/Documentation/gitattributes.txt
+++ b/Documentation/gitattributes.txt
@@ -272,6 +272,86 @@ few exceptions.  Even though...
   catch potential problems early, safety triggers.
 
 
+`working-tree-encoding`
+^^^^^^^^^^^^^^^^^^^^^^^
+
+Git recognizes files encoded in ASCII or one of its supersets (e.g.
+UTF-8, ISO-8859-1, ...) as text files. Files encoded in certain other
+encodings (e.g. UTF-16) are interpreted as binary and consequently
+built-in Git text processing tools (e.g. 'git diff') as well as most Git
+web front ends do not visualize the contents of these files by default.
+
+In these cases you can tell Git the encoding of a file in the working
+directory with the `working-tree-encoding` attribute. If a file with this
+attribute is added to Git, then Git reencodes the content from the
+specified encoding to UTF-8. Finally, Git stores the UTF-8 encoded
+content in its internal data structure (called "the index"). On checkout
+the content is reencoded back to the specified encoding.
+
+Please note that using the `working-tree-encoding` attribute may have a
+number of pitfalls:
+
+- Alternative Git implementations (e.g. JGit or libgit2) and older Git
+  versions (as of March 2018) do not support the `working-tree-encoding`
+  attribute. If you decide to use the `working-tree-encoding` attribute
+  in your repository, then it is strongly recommended to ensure that all
+  clients working with the repository support it.
+
+  For example, Microsoft Visual Studio resources files (`*.rc`) or
+  PowerShell script files (`*.ps1`) are sometimes encoded in UTF-16.
+  If you declare `*.ps1` as files as UTF-16 and you add `foo.ps1` with
+  a `working-tree-encoding` enabled Git client, then `foo.ps1` will be
+  stored as UTF-8 internally. A client without `working-tree-encoding`
+  support will checkout `foo.ps1` as UTF-8 encoded file. This will
+  typically cause trouble for the users of this file.
+
+  If a Git client, that does not support the `working-tree-encoding`
+  attribute, adds a new file `bar.ps1`, then `bar.ps1` will be
+  stored "as-is" internally (in this example probably as UTF-16).
+  A client with `working-tree-encoding` support will interpret the
+  internal contents as UTF-8 and try to convert it to UTF-16 on checkout.
+  That operation will fail and cause an error.
+
+- Reencoding content requires resources that might slow down certain
+  Git operations (e.g 'git checkout' or 'git add').
+
+Use the `working-tree-encoding` attribute only if you cannot store a file
+in UTF-8 encoding and if you want Git to be able to process the content
+as text.
+
+As an example, use the following attributes if your '*.ps1' files are
+UTF-16 encoded with byte order mark (BOM) and you want Git to perform
+automatic line ending conversion based on your platform.
+
+------------------------
+*.ps1		text working-tree-encoding=UTF-16
+------------------------
+
+Use the following attributes if your '*.ps1' files are UTF-16 little
+endian encoded without BOM and you want Git to use Windows line endings
+in the working directory. Please note, it is highly recommended to
+explicitly define the line endings with `eol` if the `working-tree-encoding`
+attribute is used to avoid ambiguity.
+
+------------------------
+*.ps1		text working-tree-encoding=UTF-16LE eol=CRLF
+------------------------
+
+You can get a list of all available encodings on your platform with the
+following command:
+
+------------------------
+iconv --list
+------------------------
+
+If you do not know the encoding of a file, then you can use the `file`
+command to guess the encoding:
+
+------------------------
+file foo.ps1
+------------------------
+
+
 `ident`
 ^^^^^^^
 
diff --git a/convert.c b/convert.c
index b976eb968c..2f864df258 100644
--- a/convert.c
+++ b/convert.c
@@ -7,6 +7,7 @@
 #include "sigchain.h"
 #include "pkt-line.h"
 #include "sub-process.h"
+#include "utf8.h"
 
 /*
  * convert.c - convert a file when checking it out and checking it in.
@@ -265,6 +266,78 @@ static int will_convert_lf_to_crlf(size_t len, struct text_stat *stats,
 
 }
 
+static const char *default_encoding = "UTF-8";
+
+static int encode_to_git(const char *path, const char *src, size_t src_len,
+			 struct strbuf *buf, const char *enc, int conv_flags)
+{
+	char *dst;
+	int dst_len;
+	int die_on_error = conv_flags & CONV_WRITE_OBJECT;
+
+	/*
+	 * No encoding is specified or there is nothing to encode.
+	 * Tell the caller that the content was not modified.
+	 */
+	if (!enc || (src && !src_len))
+		return 0;
+
+	/*
+	 * Looks like we got called from "would_convert_to_git()".
+	 * This means Git wants to know if it would encode (= modify!)
+	 * the content. Let's answer with "yes", since an encoding was
+	 * specified.
+	 */
+	if (!buf && !src)
+		return 1;
+
+	dst = reencode_string_len(src, src_len, default_encoding, enc,
+				  &dst_len);
+	if (!dst) {
+		/*
+		 * We could add the blob "as-is" to Git. However, on checkout
+		 * we would try to reencode to the original encoding. This
+		 * would fail and we would leave the user with a messed-up
+		 * working tree. Let's try to avoid this by screaming loud.
+		 */
+		const char* msg = _("failed to encode '%s' from %s to %s");
+		if (die_on_error)
+			die(msg, path, enc, default_encoding);
+		else {
+			error(msg, path, enc, default_encoding);
+			return 0;
+		}
+	}
+
+	strbuf_attach(buf, dst, dst_len, dst_len + 1);
+	return 1;
+}
+
+static int encode_to_worktree(const char *path, const char *src, size_t src_len,
+			      struct strbuf *buf, const char *enc)
+{
+	char *dst;
+	int dst_len;
+
+	/*
+	 * No encoding is specified or there is nothing to encode.
+	 * Tell the caller that the content was not modified.
+	 */
+	if (!enc || (src && !src_len))
+		return 0;
+
+	dst = reencode_string_len(src, src_len, enc, default_encoding,
+				  &dst_len);
+	if (!dst) {
+		error("failed to encode '%s' from %s to %s",
+			path, default_encoding, enc);
+		return 0;
+	}
+
+	strbuf_attach(buf, dst, dst_len, dst_len + 1);
+	return 1;
+}
+
 static int crlf_to_git(const struct index_state *istate,
 		       const char *path, const char *src, size_t len,
 		       struct strbuf *buf,
@@ -978,6 +1051,25 @@ static int ident_to_worktree(const char *path, const char *src, size_t len,
 	return 1;
 }
 
+static const char *git_path_check_encoding(struct attr_check_item *check)
+{
+	const char *value = check->value;
+
+	if (ATTR_TRUE(value) || ATTR_FALSE(value) || ATTR_UNSET(value) ||
+	    !strlen(value))
+		return NULL;
+
+	/* Don't encode to the default encoding */
+	if (!strcasecmp(value, default_encoding))
+		return NULL;
+
+	/*
+	 * Ensure encoding names are always upper case (e.g. UTF-8) to
+	 * simplify subsequent string comparisons.
+	 */
+	return xstrdup_toupper(value);
+}
+
 static enum crlf_action git_path_check_crlf(struct attr_check_item *check)
 {
 	const char *value = check->value;
@@ -1033,6 +1125,7 @@ struct conv_attrs {
 	enum crlf_action attr_action; /* What attr says */
 	enum crlf_action crlf_action; /* When no attr is set, use core.autocrlf */
 	int ident;
+	const char *working_tree_encoding; /* Supported encoding or default encoding if NULL */
 };
 
 static void convert_attrs(struct conv_attrs *ca, const char *path)
@@ -1041,7 +1134,8 @@ static void convert_attrs(struct conv_attrs *ca, const char *path)
 
 	if (!check) {
 		check = attr_check_initl("crlf", "ident", "filter",
-					 "eol", "text", NULL);
+					 "eol", "text", "working-tree-encoding",
+					 NULL);
 		user_convert_tail = &user_convert;
 		git_config(read_convert_config, NULL);
 	}
@@ -1064,6 +1158,7 @@ static void convert_attrs(struct conv_attrs *ca, const char *path)
 			else if (eol_attr == EOL_CRLF)
 				ca->crlf_action = CRLF_TEXT_CRLF;
 		}
+		ca->working_tree_encoding = git_path_check_encoding(ccheck + 5);
 	} else {
 		ca->drv = NULL;
 		ca->crlf_action = CRLF_UNDEFINED;
@@ -1144,6 +1239,13 @@ int convert_to_git(const struct index_state *istate,
 		src = dst->buf;
 		len = dst->len;
 	}
+
+	ret |= encode_to_git(path, src, len, dst, ca.working_tree_encoding, conv_flags);
+	if (ret && dst) {
+		src = dst->buf;
+		len = dst->len;
+	}
+
 	if (!(conv_flags & CONV_EOL_KEEP_CRLF)) {
 		ret |= crlf_to_git(istate, path, src, len, dst, ca.crlf_action, conv_flags);
 		if (ret && dst) {
@@ -1167,6 +1269,7 @@ void convert_to_git_filter_fd(const struct index_state *istate,
 	if (!apply_filter(path, NULL, 0, fd, dst, ca.drv, CAP_CLEAN, NULL))
 		die("%s: clean filter '%s' failed", path, ca.drv->name);
 
+	encode_to_git(path, dst->buf, dst->len, dst, ca.working_tree_encoding, conv_flags);
 	crlf_to_git(istate, path, dst->buf, dst->len, dst, ca.crlf_action, conv_flags);
 	ident_to_git(path, dst->buf, dst->len, dst, ca.ident);
 }
@@ -1198,6 +1301,12 @@ static int convert_to_working_tree_internal(const char *path, const char *src,
 		}
 	}
 
+	ret |= encode_to_worktree(path, src, len, dst, ca.working_tree_encoding);
+	if (ret) {
+		src = dst->buf;
+		len = dst->len;
+	}
+
 	ret_filter = apply_filter(
 		path, src, len, -1, dst, ca.drv, CAP_SMUDGE, dco);
 	if (!ret_filter && ca.drv && ca.drv->required)
@@ -1664,6 +1773,9 @@ struct stream_filter *get_stream_filter(const char *path, const unsigned char *s
 	if (ca.drv && (ca.drv->process || ca.drv->smudge || ca.drv->clean))
 		return NULL;
 
+	if (ca.working_tree_encoding)
+		return NULL;
+
 	if (ca.crlf_action == CRLF_AUTO || ca.crlf_action == CRLF_AUTO_CRLF)
 		return NULL;
 
diff --git a/convert.h b/convert.h
index 65ab3e5167..1d9539ed0b 100644
--- a/convert.h
+++ b/convert.h
@@ -12,6 +12,7 @@ struct index_state;
 #define CONV_EOL_RNDTRP_WARN  (1<<1) /* Warn if CRLF to LF to CRLF is different */
 #define CONV_EOL_RENORMALIZE  (1<<2) /* Convert CRLF to LF */
 #define CONV_EOL_KEEP_CRLF    (1<<3) /* Keep CRLF line endings as is */
+#define CONV_WRITE_OBJECT     (1<<4) /* Content is written to the index */
 
 extern int global_conv_flags_eol;
 
diff --git a/sha1_file.c b/sha1_file.c
index 6bc7c6ada9..e2f319d677 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -138,7 +138,7 @@ static int get_conv_flags(unsigned flags)
 	if (flags & HASH_RENORMALIZE)
 		return CONV_EOL_RENORMALIZE;
 	else if (flags & HASH_WRITE_OBJECT)
-	  return global_conv_flags_eol;
+		return global_conv_flags_eol | CONV_WRITE_OBJECT;
 	else
 		return 0;
 }
diff --git a/t/t0028-working-tree-encoding.sh b/t/t0028-working-tree-encoding.sh
new file mode 100755
index 0000000000..71e8e3700b
--- /dev/null
+++ b/t/t0028-working-tree-encoding.sh
@@ -0,0 +1,135 @@
+#!/bin/sh
+
+test_description='working-tree-encoding conversion via gitattributes'
+
+. ./test-lib.sh
+
+test_expect_success 'setup test files' '
+	git config core.eol lf &&
+
+	text="hallo there!\ncan you read me?" &&
+	echo "*.utf16 text working-tree-encoding=utf-16" >.gitattributes &&
+	printf "$text" >test.utf8.raw &&
+	printf "$text" | iconv -f UTF-8 -t UTF-16 >test.utf16.raw &&
+	printf "$text" | iconv -f UTF-8 -t UTF-32 >test.utf32.raw &&
+
+	# Line ending tests
+	printf "one\ntwo\nthree\n" >lf.utf8.raw &&
+	printf "one\r\ntwo\r\nthree\r\n" >crlf.utf8.raw &&
+
+	# BOM tests
+	printf "\0a\0b\0c"                         >nobom.utf16be.raw &&
+	printf "a\0b\0c\0"                         >nobom.utf16le.raw &&
+	printf "\376\777\0a\0b\0c"                 >bebom.utf16be.raw &&
+	printf "\777\376a\0b\0c\0"                 >lebom.utf16le.raw &&
+	printf "\0\0\0a\0\0\0b\0\0\0c"             >nobom.utf32be.raw &&
+	printf "a\0\0\0b\0\0\0c\0\0\0"             >nobom.utf32le.raw &&
+	printf "\0\0\376\777\0\0\0a\0\0\0b\0\0\0c" >bebom.utf32be.raw &&
+	printf "\777\376\0\0a\0\0\0b\0\0\0c\0\0\0" >lebom.utf32le.raw &&
+
+	# Add only UTF-16 file, we will add the UTF-32 file later
+	cp test.utf16.raw test.utf16 &&
+	cp test.utf32.raw test.utf32 &&
+	git add .gitattributes test.utf16 &&
+	git commit -m initial
+'
+
+test_expect_success 'ensure UTF-8 is stored in Git' '
+	test_when_finished "rm -f test.utf16.git" &&
+
+	git cat-file -p :test.utf16 >test.utf16.git &&
+	test_cmp_bin test.utf8.raw test.utf16.git
+'
+
+test_expect_success 're-encode to UTF-16 on checkout' '
+	test_when_finished "rm -f test.utf16.raw" &&
+
+	rm test.utf16 &&
+	git checkout test.utf16 &&
+	test_cmp_bin test.utf16.raw test.utf16
+'
+
+test_expect_success 'check $GIT_DIR/info/attributes support' '
+	test_when_finished "rm -f test.utf8.raw test.utf32.raw test.utf32.git" &&
+	test_when_finished "git reset --hard HEAD" &&
+
+	echo "*.utf32 text working-tree-encoding=utf-32" >.git/info/attributes &&
+	git add test.utf32 &&
+
+	git cat-file -p :test.utf32 >test.utf32.git &&
+	test_cmp_bin test.utf8.raw test.utf32.git
+'
+
+for i in 16 32
+do
+	test_expect_success "eol conversion for UTF-${i} encoded files on checkout" '
+		test_when_finished "rm -f crlf.utf${i}.raw lf.utf${i}.raw" &&
+		test_when_finished "git reset --hard HEAD^" &&
+
+		cat lf.utf8.raw | iconv -f UTF-8 -t UTF-${i} >lf.utf${i}.raw &&
+		cat crlf.utf8.raw | iconv -f UTF-8 -t UTF-${i} >crlf.utf${i}.raw &&
+		cp crlf.utf${i}.raw eol.utf${i} &&
+
+		cat >expectIndexLF <<-EOF &&
+			i/lf    w/-text attr/text             	eol.utf${i}
+		EOF
+
+		git add eol.utf${i} &&
+		git commit -m eol &&
+
+		# UTF-${i} with CRLF (Windows line endings)
+		rm eol.utf${i} &&
+		git -c core.eol=crlf checkout eol.utf${i} &&
+		test_cmp_bin crlf.utf${i}.raw eol.utf${i} &&
+
+		# Although the file has CRLF in the working tree,
+		# ensure LF in the index
+		git ls-files --eol eol.utf${i} >actual &&
+		test_cmp expectIndexLF actual &&
+
+		# UTF-${i} with LF (Unix line endings)
+		rm eol.utf${i} &&
+		git -c core.eol=lf checkout eol.utf${i} &&
+		test_cmp_bin lf.utf${i}.raw eol.utf${i} &&
+
+		# The file LF in the working tree, ensure LF in the index
+		git ls-files --eol eol.utf${i} >actual &&
+		test_cmp expectIndexLF actual
+	'
+done
+
+test_expect_success 'check unsupported encodings' '
+	test_when_finished "rm -f err.out" &&
+	test_when_finished "git reset --hard HEAD" &&
+
+	echo "*.nothing text working-tree-encoding=" >>.gitattributes &&
+	printf "nothing" >t.nothing &&
+	git add t.nothing &&
+
+	echo "*.garbage text working-tree-encoding=garbage" >>.gitattributes &&
+	printf "garbage" >t.garbage &&
+	test_must_fail git add t.garbage 2>err.out &&
+	test_i18ngrep "fatal: failed to encode" err.out
+'
+
+test_expect_success 'error if encoding round trip is not the same during refresh' '
+	BEFORE_STATE=$(git rev-parse HEAD) &&
+	test_when_finished "rm -f err.out" &&
+	test_when_finished "git reset --hard $BEFORE_STATE" &&
+
+	# Add and commit a UTF-16 file but skip the "working-tree-encoding"
+	# filter. Consequently, the in-repo representation is UTF-16 and not
+	# UTF-8. This simulates a Git version that has no working tree encoding
+	# support.
+	echo "*.utf16le text working-tree-encoding=utf-16le" >.gitattributes &&
+	echo "hallo" >nonsense.utf16le &&
+	TEST_HASH=$(git hash-object --no-filters -w nonsense.utf16le) &&
+	git update-index --add --cacheinfo 100644 $TEST_HASH nonsense.utf16le &&
+	COMMIT=$(git commit-tree -p $(git rev-parse HEAD) -m "plain commit" $(git write-tree)) &&
+	git update-ref refs/heads/master $COMMIT &&
+
+	test_must_fail git checkout HEAD^ 2>err.out &&
+	test_i18ngrep "error: .* overwritten by checkout:" err.out
+'
+
+test_done

Documentation/config.txt, Documentation/gitattributes.txt, config.c, convert.c, convert.h, environment.c, t/t0028-working-tree-encoding.sh

diff --git a/Documentation/config.txt b/Documentation/config.txt
index 0e25b2c92b..d7a56054a5 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -530,6 +530,12 @@ core.autocrlf::
 	This variable can be set to 'input',
 	in which case no output conversion is performed.
 
+core.checkRoundtripEncoding::
+	A comma separated list of encodings that Git performs UTF-8 round
+	trip checks on if they are used in an `working-tree-encoding`
+	attribute (see linkgit:gitattributes[5]). The default value is
+	`SHIFT-JIS`.
+
 core.symlinks::
 	If false, symbolic links are checked out as small plain files that
 	contain the link text. linkgit:git-update-index[1] and
diff --git a/Documentation/gitattributes.txt b/Documentation/gitattributes.txt
index 31a4f92840..aa3deae392 100644
--- a/Documentation/gitattributes.txt
+++ b/Documentation/gitattributes.txt
@@ -312,6 +312,14 @@ number of pitfalls:
   internal contents as UTF-8 and try to convert it to UTF-16 on checkout.
   That operation will fail and cause an error.
 
+- Reencoding content to non-UTF encodings can cause errors as the
+  conversion might not be UTF-8 round trip safe. If you suspect your
+  encoding to not be round trip safe, then add it to
+  `core.checkRoundtripEncoding` to make Git check the round trip
+  encoding (see linkgit:git-config[1]). SHIFT-JIS (Japanese character
+  set) is known to have round trip issues with UTF-8 and is checked by
+  default.
+
 - Reencoding content requires resources that might slow down certain
   Git operations (e.g 'git checkout' or 'git add').
 
diff --git a/config.c b/config.c
index 1f003fbb90..d0ada9fcd4 100644
--- a/config.c
+++ b/config.c
@@ -1172,6 +1172,11 @@ static int git_default_core_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.checkroundtripencoding")) {
+		check_roundtrip_encoding = xstrdup(value);
+		return 0;
+	}
+
 	if (!strcmp(var, "core.notesref")) {
 		notes_ref_name = xstrdup(value);
 		return 0;
diff --git a/convert.c b/convert.c
index eec34a94b9..6cbb2b2618 100644
--- a/convert.c
+++ b/convert.c
@@ -336,6 +336,43 @@ static void trace_encoding(const char *context, const char *path,
 	strbuf_release(&trace);
 }
 
+static int check_roundtrip(const char* enc_name)
+{
+	/*
+	 * check_roundtrip_encoding contains a string of space and/or
+	 * comma separated encodings (eg. "UTF-16, ASCII, CP1125").
+	 * Search for the given encoding in that string.
+	 */
+	const char *found = strcasestr(check_roundtrip_encoding, enc_name);
+	const char *next;
+	int len;
+	if (!found)
+		return 0;
+	next = found + strlen(enc_name);
+	len = strlen(check_roundtrip_encoding);
+	return (found && (
+			/*
+			 * check that the found encoding is at the
+			 * beginning of check_roundtrip_encoding or
+			 * that it is prefixed with a space or comma
+			 */
+			found == check_roundtrip_encoding || (
+				found > check_roundtrip_encoding &&
+				(*(found-1) == ' ' || *(found-1) == ',')
+			)
+		) && (
+			/*
+			 * check that the found encoding is at the
+			 * end of check_roundtrip_encoding or
+			 * that it is suffixed with a space or comma
+			 */
+			next == check_roundtrip_encoding + len || (
+				next < check_roundtrip_encoding + len &&
+				(*next == ' ' || *next == ',')
+			)
+		));
+}
+
 static const char *default_encoding = "UTF-8";
 
 static int encode_to_git(const char *path, const char *src, size_t src_len,
@@ -384,6 +421,47 @@ static int encode_to_git(const char *path, const char *src, size_t src_len,
 	}
 	trace_encoding("destination", path, default_encoding, dst, dst_len);
 
+	/*
+	 * UTF supports lossless conversion round tripping [1] and conversions
+	 * between UTF and other encodings are mostly round trip safe as
+	 * Unicode aims to be a superset of all other character encodings.
+	 * However, certain encodings (e.g. SHIFT-JIS) are known to have round
+	 * trip issues [2]. Check the round trip conversion for all encodings
+	 * listed in core.checkRoundtripEncoding.
+	 *
+	 * The round trip check is only performed if content is written to Git.
+	 * This ensures that no information is lost during conversion to/from
+	 * the internal UTF-8 representation.
+	 *
+	 * Please note, the code below is not tested because I was not able to
+	 * generate a faulty round trip without an iconv error. Iconv errors
+	 * are already caught above.
+	 *
+	 * [1] http://unicode.org/faq/utf_bom.html#gen2
+	 * [2] https://support.microsoft.com/en-us/help/170559/prb-conversion-problem-between-shift-jis-and-unicode
+	 */
+	if (die_on_error && check_roundtrip(enc)) {
+		char *re_src;
+		int re_src_len;
+
+		re_src = reencode_string_len(dst, dst_len,
+					     enc, default_encoding,
+					     &re_src_len);
+
+		trace_printf("Checking roundtrip encoding for %s...\n", enc);
+		trace_encoding("reencoded source", path, enc,
+			       re_src, re_src_len);
+
+		if (!re_src || src_len != re_src_len ||
+		    memcmp(src, re_src, src_len)) {
+			const char* msg = _("encoding '%s' from %s to %s and "
+					    "back is not the same");
+			die(msg, path, enc, default_encoding);
+		}
+
+		free(re_src);
+	}
+
 	strbuf_attach(buf, dst, dst_len, dst_len + 1);
 	return 1;
 }
diff --git a/convert.h b/convert.h
index 1d9539ed0b..765abfbd60 100644
--- a/convert.h
+++ b/convert.h
@@ -56,6 +56,7 @@ struct delayed_checkout {
 };
 
 extern enum eol core_eol;
+extern char *check_roundtrip_encoding;
 extern const char *get_cached_convert_stats_ascii(const struct index_state *istate,
 						  const char *path);
 extern const char *get_wt_convert_stats_ascii(const char *path);
diff --git a/environment.c b/environment.c
index 10a32c20ac..5bae9131ad 100644
--- a/environment.c
+++ b/environment.c
@@ -50,6 +50,7 @@ int check_replace_refs = 1;
 char *git_replace_ref_base;
 enum eol core_eol = EOL_UNSET;
 int global_conv_flags_eol = CONV_EOL_RNDTRP_WARN;
+char *check_roundtrip_encoding = "SHIFT-JIS";
 unsigned whitespace_rule_cfg = WS_DEFAULT_RULE;
 enum branch_track git_branch_track = BRANCH_TRACK_REMOTE;
 enum rebase_setup_type autorebase = AUTOREBASE_NEVER;
diff --git a/t/t0028-working-tree-encoding.sh b/t/t0028-working-tree-encoding.sh
index bdc487b44f..23e89ae623 100755
--- a/t/t0028-working-tree-encoding.sh
+++ b/t/t0028-working-tree-encoding.sh
@@ -191,4 +191,43 @@ test_expect_success 'error if encoding garbage is already in Git' '
 	test_i18ngrep "error: BOM is required" err.out
 '
 
+test_expect_success 'check roundtrip encoding' '
+	test_when_finished "rm -f roundtrip.shift roundtrip.utf16" &&
+	test_when_finished "git reset --hard HEAD" &&
+
+	text="hallo there!\nroundtrip test here!" &&
+	printf "$text" | iconv -f UTF-8 -t SHIFT-JIS >roundtrip.shift &&
+	printf "$text" | iconv -f UTF-8 -t UTF-16 >roundtrip.utf16 &&
+	echo "*.shift text working-tree-encoding=SHIFT-JIS" >>.gitattributes &&
+
+	# SHIFT-JIS encoded files are round-trip checked by default...
+	GIT_TRACE=1 git add .gitattributes roundtrip.shift 2>&1 |
+		grep "Checking roundtrip encoding for SHIFT-JIS" &&
+	git reset &&
+
+	# ... unless we overwrite the Git config!
+	! GIT_TRACE=1 git -c core.checkRoundtripEncoding=garbage \
+		add .gitattributes roundtrip.shift 2>&1 |
+		grep "Checking roundtrip encoding for SHIFT-JIS" &&
+	git reset &&
+
+	# UTF-16 encoded files should not be round-trip checked by default...
+	! GIT_TRACE=1 git add roundtrip.utf16 2>&1 |
+		grep "Checking roundtrip encoding for UTF-16" &&
+	git reset &&
+
+	# ... unless we tell Git to check it!
+	GIT_TRACE=1 git -c core.checkRoundtripEncoding="UTF-16, UTF-32" \
+		add roundtrip.utf16 2>&1 |
+		grep "Checking roundtrip encoding for UTF-16" &&
+	git reset &&
+
+	# ... unless we tell Git to check it!
+	# (here we also check that the casing of the encoding is irrelevant)
+	GIT_TRACE=1 git -c core.checkRoundtripEncoding="UTF-32, utf-16" \
+		add roundtrip.utf16 2>&1 |
+		grep "Checking roundtrip encoding for UTF-16" &&
+	git reset
+'
+
 test_done

convert.c, t/t0028-working-tree-encoding.sh

diff --git a/convert.c b/convert.c
index 2f864df258..9647b06679 100644
--- a/convert.c
+++ b/convert.c
@@ -266,6 +266,53 @@ static int will_convert_lf_to_crlf(size_t len, struct text_stat *stats,
 
 }
 
+static int validate_encoding(const char *path, const char *enc,
+		      const char *data, size_t len, int die_on_error)
+{
+	if (!memcmp("UTF-", enc, 4)) {
+		/*
+		 * Check for detectable errors in UTF encodings
+		 */
+		if (has_prohibited_utf_bom(enc, data, len)) {
+			const char *error_msg = _(
+				"BOM is prohibited in '%s' if encoded as %s");
+			/*
+			 * This advice is shown for UTF-??BE and UTF-??LE
+			 * encodings. We truncate the encoding name to 6
+			 * chars with %.6s to cut off the last two "byte
+			 * order" characters.
+			 */
+			const char *advise_msg = _(
+				"The file '%s' contains a byte order "
+				"mark (BOM). Please use %.6s as "
+				"working-tree-encoding.");
+			advise(advise_msg, path, enc);
+			if (die_on_error)
+				die(error_msg, path, enc);
+			else {
+				return error(error_msg, path, enc);
+			}
+
+		} else if (is_missing_required_utf_bom(enc, data, len)) {
+			const char *error_msg = _(
+				"BOM is required in '%s' if encoded as %s");
+			const char *advise_msg = _(
+				"The file '%s' is missing a byte order "
+				"mark (BOM). Please use %sBE or %sLE "
+				"(depending on the byte order) as "
+				"working-tree-encoding.");
+			advise(advise_msg, path, enc, enc);
+			if (die_on_error)
+				die(error_msg, path, enc);
+			else {
+				return error(error_msg, path, enc);
+			}
+		}
+
+	}
+	return 0;
+}
+
 static const char *default_encoding = "UTF-8";
 
 static int encode_to_git(const char *path, const char *src, size_t src_len,
@@ -291,6 +338,9 @@ static int encode_to_git(const char *path, const char *src, size_t src_len,
 	if (!buf && !src)
 		return 1;
 
+	if (validate_encoding(path, enc, src, src_len, die_on_error))
+		return 0;
+
 	dst = reencode_string_len(src, src_len, default_encoding, enc,
 				  &dst_len);
 	if (!dst) {
diff --git a/t/t0028-working-tree-encoding.sh b/t/t0028-working-tree-encoding.sh
index 71e8e3700b..5c7e36a164 100755
--- a/t/t0028-working-tree-encoding.sh
+++ b/t/t0028-working-tree-encoding.sh
@@ -62,6 +62,46 @@ test_expect_success 'check $GIT_DIR/info/attributes support' '
 
 for i in 16 32
 do
+	test_expect_success "check prohibited UTF-${i} BOM" '
+		test_when_finished "git reset --hard HEAD" &&
+
+		echo "*.utf${i}be text working-tree-encoding=utf-${i}be" >>.gitattributes &&
+		echo "*.utf${i}le text working-tree-encoding=utf-${i}le" >>.gitattributes &&
+
+		# Here we add a UTF-16 (resp. UTF-32) files with BOM (big/little-endian)
+		# but we tell Git to treat it as UTF-16BE/UTF-16LE (resp. UTF-32).
+		# In these cases the BOM is prohibited.
+		cp bebom.utf${i}be.raw bebom.utf${i}be &&
+		test_must_fail git add bebom.utf${i}be 2>err.out &&
+		test_i18ngrep "fatal: BOM is prohibited .* UTF-${i}BE" err.out &&
+
+		cp lebom.utf${i}le.raw lebom.utf${i}be &&
+		test_must_fail git add lebom.utf${i}be 2>err.out &&
+		test_i18ngrep "fatal: BOM is prohibited .* UTF-${i}BE" err.out &&
+
+		cp bebom.utf${i}be.raw bebom.utf${i}le &&
+		test_must_fail git add bebom.utf${i}le 2>err.out &&
+		test_i18ngrep "fatal: BOM is prohibited .* UTF-${i}LE" err.out &&
+
+		cp lebom.utf${i}le.raw lebom.utf${i}le &&
+		test_must_fail git add lebom.utf${i}le 2>err.out &&
+		test_i18ngrep "fatal: BOM is prohibited .* UTF-${i}LE" err.out
+	'
+
+	test_expect_success "check required UTF-${i} BOM" '
+		test_when_finished "git reset --hard HEAD" &&
+
+		echo "*.utf${i} text working-tree-encoding=utf-${i}" >>.gitattributes &&
+
+		cp nobom.utf${i}be.raw nobom.utf${i} &&
+		test_must_fail git add nobom.utf${i} 2>err.out &&
+		test_i18ngrep "fatal: BOM is required .* UTF-${i}" err.out &&
+
+		cp nobom.utf${i}le.raw nobom.utf${i} &&
+		test_must_fail git add nobom.utf${i} 2>err.out &&
+		test_i18ngrep "fatal: BOM is required .* UTF-${i}" err.out
+	'
+
 	test_expect_success "eol conversion for UTF-${i} encoded files on checkout" '
 		test_when_finished "rm -f crlf.utf${i}.raw lf.utf${i}.raw" &&
 		test_when_finished "git reset --hard HEAD^" &&
@@ -132,4 +172,21 @@ test_expect_success 'error if encoding round trip is not the same during refresh
 	test_i18ngrep "error: .* overwritten by checkout:" err.out
 '
 
+test_expect_success 'error if encoding garbage is already in Git' '
+	BEFORE_STATE=$(git rev-parse HEAD) &&
+	test_when_finished "rm -f err.out" &&
+	test_when_finished "git reset --hard $BEFORE_STATE" &&
+
+	# Skip the UTF-16 filter for the added file
+	# This simulates a Git version that has no checkoutEncoding support
+	cp nobom.utf16be.raw nonsense.utf16 &&
+	TEST_HASH=$(git hash-object --no-filters -w nonsense.utf16) &&
+	git update-index --add --cacheinfo 100644 $TEST_HASH nonsense.utf16 &&
+	COMMIT=$(git commit-tree -p $(git rev-parse HEAD) -m "plain commit" $(git write-tree)) &&
+	git update-ref refs/heads/master $COMMIT &&
+
+	git diff 2>err.out &&
+	test_i18ngrep "error: BOM is required" err.out
+'
+
 test_done

convert.c, t/t0028-working-tree-encoding.sh

diff --git a/convert.c b/convert.c
index 9647b06679..eec34a94b9 100644
--- a/convert.c
+++ b/convert.c
@@ -313,6 +313,29 @@ static int validate_encoding(const char *path, const char *enc,
 	return 0;
 }
 
+static void trace_encoding(const char *context, const char *path,
+			   const char *encoding, const char *buf, size_t len)
+{
+	static struct trace_key coe = TRACE_KEY_INIT(WORKING_TREE_ENCODING);
+	struct strbuf trace = STRBUF_INIT;
+	int i;
+
+	strbuf_addf(&trace, "%s (%s, considered %s):\n", context, path, encoding);
+	for (i = 0; i < len && buf; ++i) {
+		strbuf_addf(
+			&trace,"| \e[2m%2i:\e[0m %2x \e[2m%c\e[0m%c",
+			i,
+			(unsigned char) buf[i],
+			(buf[i] > 32 && buf[i] < 127 ? buf[i] : ' '),
+			((i+1) % 8 && (i+1) < len ? ' ' : '\n')
+		);
+	}
+	strbuf_addchars(&trace, '\n', 1);
+
+	trace_strbuf(&coe, &trace);
+	strbuf_release(&trace);
+}
+
 static const char *default_encoding = "UTF-8";
 
 static int encode_to_git(const char *path, const char *src, size_t src_len,
@@ -341,6 +364,7 @@ static int encode_to_git(const char *path, const char *src, size_t src_len,
 	if (validate_encoding(path, enc, src, src_len, die_on_error))
 		return 0;
 
+	trace_encoding("source", path, enc, src, src_len);
 	dst = reencode_string_len(src, src_len, default_encoding, enc,
 				  &dst_len);
 	if (!dst) {
@@ -358,6 +382,7 @@ static int encode_to_git(const char *path, const char *src, size_t src_len,
 			return 0;
 		}
 	}
+	trace_encoding("destination", path, default_encoding, dst, dst_len);
 
 	strbuf_attach(buf, dst, dst_len, dst_len + 1);
 	return 1;
diff --git a/t/t0028-working-tree-encoding.sh b/t/t0028-working-tree-encoding.sh
index 5c7e36a164..bdc487b44f 100755
--- a/t/t0028-working-tree-encoding.sh
+++ b/t/t0028-working-tree-encoding.sh
@@ -4,6 +4,8 @@ test_description='working-tree-encoding conversion via gitattributes'
 
 . ./test-lib.sh
 
+GIT_TRACE_WORKING_TREE_ENCODING=1 && export GIT_TRACE_WORKING_TREE_ENCODING
+
 test_expect_success 'setup test files' '
 	git config core.eol lf &&