refs: loosen the OID parsing

We used to require loose references to contain only an OID (possibly
after trimming the string). This is however not enough for letting us
lookup FETCH_HEAD, which can have a lot of content after the initial
OID.

Change the parsing rules so that a loose refernce must e at least 40
bytes long and the 41st (if it's there) must be accepted by
isspace(3). This makes the trim unnecessary, so only do it for
symrefs. This fixes #977.

src/refs.c

@@ -155,11 +155,26 @@ static int loose_parse_symbolic(git_reference *ref, git_buf *file_content)
 
 static int loose_parse_oid(git_oid *oid, git_buf *file_content)
 {
-	/* File format: 40 chars (OID) */
-	if (git_buf_len(file_content) == GIT_OID_HEXSZ &&
-		git_oid_fromstr(oid, git_buf_cstr(file_content)) == 0)
+	size_t len;
+	const char *str;
+
+	len = git_buf_len(file_content);
+	if (len < GIT_OID_HEXSZ)
+		goto corrupted;
+
+	/* str is guranteed to be zero-terminated */
+	str = git_buf_cstr(file_content);
+
+	/* If the file is longer than 40 chars, the 41st must be a space */
+	if (git_oid_fromstr(oid, git_buf_cstr(file_content)) < 0)
+		goto corrupted;
+
+	/* If the file is longer than 40 chars, the 41st must be a space */
+	str += GIT_OID_HEXSZ;
+	if (*str == '\0' || git__isspace(*str))
 		return 0;
 
+corrupted:
 	giterr_set(GITERR_REFERENCE, "Corrupted loose reference file");
 	return -1;
 }

tests-clar/refs/read.c

@@ -212,6 +212,7 @@ void test_refs_read__trailing(void)
 	cl_git_pass(git_reference_lookup(&test, g_repo, "refs/heads/test"));
 	cl_git_pass(git_reference_lookup(&trailing, g_repo, "refs/heads/trailing"));
 	cl_git_pass(git_oid_cmp(git_reference_oid(test), git_reference_oid(trailing)));
+	cl_git_pass(git_reference_lookup(&trailing, g_repo, "FETCH_HEAD"));
 
 	git_reference_free(test);
 	git_reference_free(trailing);

tests-clar/resources/testrepo.git/FETCH_HEAD

+a65fedf39aefe402d3bb6e24df4d4f5fe4547750		branch 'master' of git://example.com/git/testrepo.git
+258f0e2a959a364e40ed6603d5d44fbb24765b10	not-for-merge  branch 'haacked' of git://example.com/git/testrepo.git