[PATCH] grep: report missing left operand of --and

Git grep allows combining two patterns with --and.  It checks and
reports if the second pattern is missing when compiling the expression.
A missing first pattern, however, is only reported later at match time.
Thus no error is returned if no matching is done, e.g. because no file
matches the also given pathspec.

When that happens we get an expression tree with an GREP_NODE_AND node
and a NULL pointer to the missing left child.  free_pattern_expr()
tries to dereference it during the cleanup at the end, which result in
a segmentation fault.

Fix this by verifying the presence of the left operand at expression
compilation time.

Reported-by: Matthew Hughes <redacted>
Signed-off-by: René Scharfe <redacted>
---
Whether the check in match_expr_eval() can now be turned into a BUG is
left as an exercise for the reader. ;-)

 grep.c          |  2 ++
 t/t7810-grep.sh | 10 ++++++++++
 2 files changed, 12 insertions(+)

diff --git a/grep.c b/grep.c
index 8f91af1cb0..7d0ea4e956 100644
--- a/grep.c
+++ b/grep.c

@@ -655,6 +655,8 @@ static struct grep_expr *compile_pattern_and(struct grep_pat **list)
 	struct grep_expr *x, *y, *z;

 	x = compile_pattern_not(list);
+	if (!x)
+		die("Not a valid grep expression");
 	p = *list;
 	if (p && p->token == GREP_AND) {
 		if (!p->next)

diff --git a/t/t7810-grep.sh b/t/t7810-grep.sh
index 5830733f3d..c581239674 100755
--- a/t/t7810-grep.sh
+++ b/t/t7810-grep.sh

@@ -11,6 +11,13 @@ export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME

 . ./test-lib.sh

+test_invalid_grep_expression() {
+	params="$@" &&
+	test_expect_success "invalid expression: grep $params" '
+		test_must_fail git grep $params -- nonexisting
+	'
+}
+
 cat >hello.c <<EOF
 #include <assert.h>
 #include <stdio.h>

@@ -89,6 +96,9 @@ test_expect_success 'grep should not segfault with a bad input' '
 	test_must_fail git grep "("
 '

+test_invalid_grep_expression -e A --and
+test_invalid_grep_expression --and -e A
+
 for H in HEAD ''
 do
 	case "$H" in
--

2.32.0