v3diff: generate prettier filenames when using GIT_EXTERNAL_DIFF

cache.h, diff.c, path.c, t/t4020-diff-external.sh

diff --git a/cache.h b/cache.h
index b8503ad..871c984 100644
--- a/cache.h
+++ b/cache.h
@@ -614,6 +614,8 @@ extern int is_empty_blob_sha1(const unsigned char *sha1);
 
 int git_mkstemp(char *path, size_t n, const char *template);
 
+int git_mkstemps(char *path, size_t n, const char *template, int suffix_len);
+
 /*
  * NOTE NOTE NOTE!!
  *
diff --git a/diff.c b/diff.c
index f06876b..226771d 100644
--- a/diff.c
+++ b/diff.c
@@ -1960,12 +1960,47 @@ static void prep_temp_blob(const char *path, struct diff_tempfile *temp,
 			   void *blob,
 			   unsigned long size,
 			   const unsigned char *sha1,
-			   int mode)
+			   int mode,
+			   int pretty_filename)
 {
 	int fd;
 	struct strbuf buf = STRBUF_INIT;
 
-	fd = git_mkstemp(temp->tmp_path, PATH_MAX, ".diff_XXXXXX");
+	if (pretty_filename) {
+		struct strbuf pretty_name = STRBUF_INIT;
+		char *pathdup = xstrdup(path);
+		char *base = basename(pathdup);
+		char *dot = strchr(base, '.');
+		int suffix_len = 0;
+
+		if (dot) {
+			/* path has an extension, e.g. "foo.txt";
+			 * generate "foo.XXXX.txt".
+			 */
+			*dot = '\0';
+			strbuf_addstr(&pretty_name, base);
+			*dot = '.';
+			strbuf_addstr(&pretty_name, ".XXXXXX");
+			suffix_len = strlen(dot);
+			strbuf_addstr(&pretty_name, dot);
+		}
+		else {
+			/* path has no extension, e.g. "Makefile";
+			 * generate "Makefile_XXXX".
+			 */
+			strbuf_addstr(&pretty_name, base);
+			strbuf_addstr(&pretty_name, "_XXXXXX");
+		}
+
+		fd = git_mkstemps(temp->tmp_path, PATH_MAX,
+			pretty_name.buf, suffix_len);
+
+		free(pathdup);
+		strbuf_release(&pretty_name);
+	}
+	else {
+		fd = git_mkstemp(temp->tmp_path, PATH_MAX, ".diff_XXXXXX");
+	}
 	if (fd < 0)
 		die("unable to create temp-file: %s", strerror(errno));
 	if (convert_to_working_tree(path,
@@ -1984,7 +2019,8 @@ static void prep_temp_blob(const char *path, struct diff_tempfile *temp,
 }
 
 static struct diff_tempfile *prepare_temp_file(const char *name,
-		struct diff_filespec *one)
+		struct diff_filespec *one,
+		int pretty_filename)
 {
 	struct diff_tempfile *temp = claim_diff_tempfile();
 
@@ -2025,7 +2061,8 @@ static struct diff_tempfile *prepare_temp_file(const char *name,
 				       (one->sha1_valid ?
 					one->sha1 : null_sha1),
 				       (one->sha1_valid ?
-					one->mode : S_IFLNK));
+					one->mode : S_IFLNK),
+				       pretty_filename);
 		}
 		else {
 			/* we can borrow from the file in the work tree */
@@ -2048,7 +2085,7 @@ static struct diff_tempfile *prepare_temp_file(const char *name,
 		if (diff_populate_filespec(one, 0))
 			die("cannot read data blob for %s", one->path);
 		prep_temp_blob(name, temp, one->data, one->size,
-			       one->sha1, one->mode);
+			       one->sha1, one->mode, pretty_filename);
 	}
 	return temp;
 }
@@ -2074,8 +2111,9 @@ static void run_external_diff(const char *pgm,
 	if (one && two) {
 		struct diff_tempfile *temp_one, *temp_two;
 		const char *othername = (other ? other : name);
-		temp_one = prepare_temp_file(name, one);
-		temp_two = prepare_temp_file(othername, two);
+		int pretty_filename = 1;
+		temp_one = prepare_temp_file(name, one, pretty_filename);
+		temp_two = prepare_temp_file(othername, two, pretty_filename);
 		*arg++ = pgm;
 		*arg++ = name;
 		*arg++ = temp_one->name;
@@ -3577,8 +3615,9 @@ static char *run_textconv(const char *pgm, struct diff_filespec *spec,
 	const char **arg = argv;
 	struct child_process child;
 	struct strbuf buf = STRBUF_INIT;
+	int pretty_filename = 0;
 
-	temp = prepare_temp_file(spec->path, spec);
+	temp = prepare_temp_file(spec->path, spec, pretty_filename);
 	*arg++ = pgm;
 	*arg++ = temp->name;
 	*arg = NULL;
diff --git a/path.c b/path.c
index 8a0a674..6292410 100644
--- a/path.c
+++ b/path.c
@@ -12,6 +12,8 @@
  */
 #include "cache.h"
 
+#define GIT_MKSTEMPS_MAX 16384
+
 static char bad_path[] = "/bad-path/";
 
 static char *get_pathname(void)
@@ -140,6 +142,74 @@ int git_mkstemp(char *path, size_t len, const char *template)
 }
 
 
+
+/* git_mkstemps() - create tmp file with suffix honoring TMPDIR variable.
+ * This is adapted from libguile's GPLv2 mkstemp.c.
+ */
+int git_mkstemps(char *path, size_t len, const char *template, int suffix_len)
+{
+	static const char letters[] =
+		"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
+		"0123456789_";
+	const char *tmp;
+	char *pattern;
+	size_t n;
+	size_t path_noext_len, letters_len;
+	struct timeval tv;
+	uint64_t value, v;
+	int fd, count;
+
+	tmp = getenv("TMPDIR");
+	if (!tmp)
+		tmp = "/tmp";
+	n = snprintf(path, len, "%s/%s", tmp, template);
+	if (len <= n) {
+		errno = ENAMETOOLONG;
+		return -1;
+	}
+
+	letters_len = strlen(letters);
+	path_noext_len = n - suffix_len;
+
+	if (path_noext_len < 0 || strlen(template) - suffix_len < 6) {
+		errno = EINVAL;
+		return -1;
+	}
+
+	if (strncmp(path + path_noext_len - 6, "XXXXXX", 6)) {
+		errno = EINVAL;
+		return -1;
+	}
+
+	/* Replace template's XXXXXX characters with randomness.
+	 * Try GIT_MKSTEMPS_MAX different filenames.
+	 */
+	gettimeofday(&tv, NULL);
+	value = ((size_t)(tv.tv_usec << 16)) ^ tv.tv_sec ^ getpid();
+	pattern = path + path_noext_len - 6;
+	for (count = 0; count < GIT_MKSTEMPS_MAX; ++count) {
+		v = value;
+		pattern[0] = letters[v % letters_len]; v/= letters_len;
+		pattern[1] = letters[v % letters_len]; v/= letters_len;
+		pattern[2] = letters[v % letters_len]; v/= letters_len;
+		pattern[3] = letters[v % letters_len]; v/= letters_len;
+		pattern[4] = letters[v % letters_len]; v/= letters_len;
+		pattern[5] = letters[v % letters_len]; v/= letters_len;
+
+		fd = open(path, O_CREAT | O_EXCL | O_RDWR, 0600);
+		if (fd > 0)
+			return fd;
+		/* This is a random value.  It is only necessary that
+		 * the next value generated by adding 7777 is different
+		 * modulus 2^32.
+		 */
+		value += 7777;
+	}
+	errno = EINVAL;
+	return -1;
+}
+
+
 int validate_headref(const char *path)
 {
 	struct stat st;
diff --git a/t/t4020-diff-external.sh b/t/t4020-diff-external.sh
index 0720001..a9fdcb0 100755
--- a/t/t4020-diff-external.sh
+++ b/t/t4020-diff-external.sh
@@ -136,6 +136,24 @@ test_expect_success 'GIT_EXTERNAL_DIFF with more than one changed files' '
 	GIT_EXTERNAL_DIFF=echo git diff
 '
 
+test_expect_success 'GIT_EXTERNAL_DIFF generates pretty paths with no ext' '
+	touch filenoext &&
+	git add filenoext &&
+	echo no extension > filenoext &&
+	GIT_EXTERNAL_DIFF=echo git diff filenoext | grep filenoext_ &&
+	git update-index --force-remove filenoext &&
+	rm filenoext
+'
+
+test_expect_success 'GIT_EXTERNAL_DIFF generates pretty paths with ext' '
+	touch file.ext &&
+	git add file.ext &&
+	echo with extension > file.ext &&
+	GIT_EXTERNAL_DIFF=echo git diff file.ext | grep file\.......\.ext &&
+	git update-index --force-remove file.ext &&
+	rm file.ext
+'
+
 echo "#!$SHELL_PATH" >fake-diff.sh
 cat >> fake-diff.sh <<\EOF
 cat $2 >> crlfed.txt