git diff -I<regex> does not work with empty +/- lines

From: Johannes Altmanninger <aclopte@gmail.com>
Date: Nov 28, 2021, 09:15 UTC
Message-ID: <20211128091521.7ysocurtt4qlgcf6@gmail.com>

diff -I<regex> suppresses hunks where all +/- lines match <regex>. it is useful to filter away boilerplate changes.

Unfortunately, it doesn't help if a hunk has a blank line, because the one obvious way to filter out blank lines (^$) match *every* line, surprisingly. This is because for a line "01\n" we have a zero-width match here ^$ (offset 3).

IOW, while we succesfully ignore deleted blank lines

	printf '\n' | git diff --no-index - /dev/null -I'^$'
	diff --git a/- b/-
	deleted file mode 100644

we also ignore non-blank lines (very surprising)

	printf 'non-blank-line\n' | git diff --no-index - /dev/null -I'^$'
	diff --git a/- b/-
	deleted file mode 100644

unless they don't end in a newline (special case)

	printf 'line without ending newline' | git diff --no-index - /dev/null -I'^$'
	diff --git a/- b/-
	deleted file mode 100644
	--- a/-
	+++ /dev/null
	@@ -1 +0,0 @@
	-line without ending newline
	\ No newline at end of file

This patch fixes the second example. Is this the right direction? Do we want to honor core.eol, so we preserve the \r when we have Unix endings?

In any case -I<regex> won't be able to discern between "line\n" and "line" but that's not important to me.

diff --git a/xdiff/xdiffi.c b/xdiff/xdiffi.c
index a4542c05b6..23325022b9 100644
--- a/xdiff/xdiffi.c
+++ b/xdiff/xdiffi.c
@@ -1016,10 +1016,17 @@ static void xdl_mark_ignorable_lines(xdchange_t *xscr, xdfenv_t *xe, long flags)
 static int record_matches_regex(xrecord_t *rec, xpparam_t const *xpp) {
 	regmatch_t regmatch;
 	int i;
+	const char *end = rec->ptr + rec->size;
+
+	if (rec->size >= 2 && end[-2] == '\r' && end[-1] == '\n') {
+		end -= 2;
+	} else if (rec->size && end[-1] == '\n') {
+		end -= 1;
+	}
 
 	for (i = 0; i < xpp->ignore_regex_nr; i++)
-		if (!regexec_buf(xpp->ignore_regex[i], rec->ptr, rec->size, 1,
-				 &regmatch, 0))
+		if (!regexec_buf(xpp->ignore_regex[i], rec->ptr,
+				 end - rec->ptr, 1, &regmatch, 0))
 			return 1;
 
 	return 0;