[PATCH v2 3/3] format-patch: check if header output looks valid

[Kristoffer Haugsbakk <code@khaugsbakk.name>]

Implement a function based on `mailinfo.c:is_mail`.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
---

Notes (series):
    Isolating this for review as its own commit so that I can point out the
    provenance. May well be squashed into the main patch eventually.

 builtin/log.c           | 33 +++++++++++++++++++++++++++++++++
 t/t4014-format-patch.sh | 13 +++++++++++++
 2 files changed, 46 insertions(+)

diff --git a/builtin/log.c b/builtin/log.c
index bc656b5e0f8..2902c2bf6fe 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -1876,6 +1876,35 @@ static void infer_range_diff_ranges(struct strbuf *r1,
 	}
 }
 
+static int is_mail(struct strbuf *sb)
+{
+	const char *header_regex = "^[!-9;-~]+:";
+	regex_t regex;
+	int ret = 1, i;
+	struct string_list list = STRING_LIST_INIT_DUP;
+
+	if (regcomp(&regex, header_regex, REG_NOSUB | REG_EXTENDED))
+		die("invalid pattern: %s", header_regex);
+	string_list_split(&list, sb->buf, '\n', -1);
+	for (i = 0; i < list.nr; i++) {
+		/* End of header */
+		if (!*list.items[i].string && i == (list.nr - 1))
+			break;
+		/* Ignore indented folded lines */
+		if (*list.items[i].string == '\t' ||
+		    *list.items[i].string == ' ')
+			continue;
+		/* It's a header if it matches header_regex */
+		if (regexec(&regex, list.items[i].string, 0, NULL, 0)) {
+			ret = 0;
+			break;
+		}
+	}
+	string_list_clear(&list, 1);
+	regfree(&regex);
+	return ret;
+}
+
 /* Returns an owned pointer */
 static char *header_cmd_output(struct rev_info *rev, const struct commit *cmit)
 {
@@ -1902,6 +1931,10 @@ static char *header_cmd_output(struct rev_info *rev, const struct commit *cmit)
 			die(_("header-cmd %s: failed with exit code %d"),
 			    header_cmd, res);
 	}
+	if (!is_mail(&output))
+		die(_("header-cmd %s: returned output which was "
+		      "not recognized as valid RFC 2822 headers"),
+		    header_cmd);
 	return strbuf_detach(&output, NULL);
 }
 
diff --git a/t/t4014-format-patch.sh b/t/t4014-format-patch.sh
index dc85c4c28fe..533a5b246e5 100755
--- a/t/t4014-format-patch.sh
+++ b/t/t4014-format-patch.sh
@@ -255,6 +255,19 @@ test_expect_success '--header-cmd with no output works' '
 	git format-patch --header-cmd=true --stdout main..side
 '
 
+test_expect_success '--header-cmd without headers-like output fails' '
+	write_script cmd <<-\EOF &&
+	printf "X-S: $GIT_FP_HEADER_CMD_HASH\n"
+	printf "\n"
+	printf "X-C: $GIT_FP_HEADER_CMD_COUNT\n"
+	EOF
+	cat > expect <<-\EOF &&
+	fatal: header-cmd ./cmd: returned output which was not recognized as valid RFC 2822 headers
+	EOF
+	test_must_fail git format-patch --header-cmd=./cmd --stdout main..side >actual 2>&1 &&
+	test_cmp expect actual
+'
+
 test_expect_success '--header-cmd reports failed command' '
 	cat > expect <<-\EOF &&
 	fatal: header-cmd false: failed with exit code 1
-- 
2.44.0.144.g29ae9861142