[PATCH v3] line-range: Fix infinite loop bug with degenerate '$' regex

[Lars Kellogg-Stedman <lars@oddbit.com>]

When the -L argument to "git log" is passed the degenerate regular
expression "$" (as in "-L :$:line-range.c"), this results in an
infinite loop in find_funcname_matching_regexp().

Modify find_funcname_matching_regexp to correctly match the entire line
instead of the zero-width match at eol and update the loop condition to
prevent an infinite loop in the event of other undiscovered corner cases.

Originally reported in <https://stackoverflow.com/q/74690545/147356>.

Signed-off-by: Lars Kellogg-Stedman <lars@oddbit.com>
---
This modifies my earlier patch so that instead of failing, the
regular expression '$' correctly matches any function name (and so
always returns the first function in the named file). This makes '$'
behave the same as '.' and '^'.

The primary change is that we pre-decrement the beginning-of-line
marker ('bol') before comparing it to '\n'. In the case of '$', where
we start with bol == eol, this ensures that bol will find the
beginning of the line on which the match occurred.

 line-range.c        |  6 +++---
 t/t4211-line-log.sh | 22 ++++++++++++++++++++++
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/line-range.c b/line-range.c
index 955a8a9535..b0e26e7f9d 100644
--- a/line-range.c
+++ b/line-range.c
@@ -135,7 +135,7 @@ static const char *find_funcname_matching_regexp(xdemitconf_t *xecfg, const char
 {
 	int reg_error;
 	regmatch_t match[1];
-	while (1) {
+	while (*start) {
 		const char *bol, *eol;
 		reg_error = regexec(regexp, start, 1, match, 0);
 		if (reg_error == REG_NOMATCH)
@@ -148,8 +148,7 @@ static const char *find_funcname_matching_regexp(xdemitconf_t *xecfg, const char
 		/* determine extent of line matched */
 		bol = start+match[0].rm_so;
 		eol = start+match[0].rm_eo;
-		while (bol > start && *bol != '\n')
-			bol--;
+		while (bol > start && *--bol != '\n');
 		if (*bol == '\n')
 			bol++;
 		while (*eol && *eol != '\n')
@@ -161,6 +160,7 @@ static const char *find_funcname_matching_regexp(xdemitconf_t *xecfg, const char
 			return bol;
 		start = eol;
 	}
+	return NULL;
 }
 
 static const char *parse_range_funcname(
diff --git a/t/t4211-line-log.sh b/t/t4211-line-log.sh
index ac9e4d0928..19db07a8df 100755
--- a/t/t4211-line-log.sh
+++ b/t/t4211-line-log.sh
@@ -315,4 +315,26 @@ test_expect_success 'line-log with --before' '
 	test_cmp expect actual
 '
 
+test_expect_success 'setup tests for zero-width regular expressions' '
+	cat > expect <<-EOF
+	Modify func1() in file.c
+	Add func1() and func2() in file.c
+	EOF
+'
+
+test_expect_success 'zero-width regex $ matches any function name' '
+	git log --format="%s" --no-patch "-L:$:file.c" >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'zero-width regex ^ matches any function name' '
+	git log --format="%s" --no-patch "-L:^:file.c" >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'zero-width regex . matches any function name' '
+	git log --format="%s" --no-patch "-L:.:file.c" >actual &&
+	test_cmp expect actual
+'
+
 test_done
-- 
2.38.1