receive-pack.c: consolidate find header logic

There are two functions that have very similar logic of finding a header
value. find_commit_header, and find_header. We can conslidate the logic
by introducing a new function find_header_mem, which is equivalent to
find_commit_header except it takes a len parameter that determines how
many bytes will be read. find_commit_header and find_header can then both
call find_header_mem.

This reduces duplicate logic, as the logic for finding header values
can now all live in one place.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -1631,12 +1631,20 @@ struct commit_list **commit_list_append(struct commit *commit,
 	return &new_commit->next;
 }
 
-const char *find_commit_header(const char *msg, const char *key, size_t *out_len)
+const char *find_header_mem(const char *msg, size_t len,
+			const char *key, size_t *out_len)
 {
 	int key_len = strlen(key);
 	const char *line = msg;
 
-	while (line) {
+	/*
+	 * NEEDSWORK: It's possible for strchrnul() to scan beyond the range
+	 * given by len. However, current callers are safe because they compute
+	 * len by scanning a NUL-terminated block of memory starting at msg.
+	 * Nonetheless, it would be better to ensure the function does not look
+	 * at msg beyond the len provided by the caller.
+	 */
+	while (line && line < msg + len) {
 		const char *eol = strchrnul(line, '\n');
 
 		if (line == eol)
@@ -1653,6 +1661,10 @@ const char *find_commit_header(const char *msg, const char *key, size_t *out_len
 	return NULL;
 }
 
+const char *find_commit_header(const char *msg, const char *key, size_t *out_len)
+{
+	return find_header_mem(msg, strlen(msg), key, out_len);
+}
 /*
  * Inspect the given string and determine the true "end" of the log message, in
  * order to find where to put a new Signed-off-by trailer.  Ignored are