tags: Fixed the tag parser to correctly treat the message field as optional.

This fix makes libgit2 capable of parsing annotated tag objects that lack
the optional message/description field.
Previously, libgit2 treated this field as mandatory and raised a tag_error on
such tags. However, the message field is optional.

An example of such a tag is refs/tags/v2.6.16.31-rc1 in Linux:

$ git cat-file tag refs/tags/v2.6.16.31-rc1
object afaa018cefb6af63befef1df7d8febaae904434f
type commit
tag v2.6.16.31-rc1
tagger Adrian Bunk <bunk@stusta.de> 1162716505 +0100
$

src/tag.c

@@ -139,16 +139,19 @@ int git_tag__parse_buffer(git_tag *tag, const char *buffer, size_t length)
 			return -1;
 	}
 
-	if( *buffer != '\n' )
-		return tag_error("No new line before message");
+	tag->message = NULL;
+	if (buffer < buffer_end) {
+		if( *buffer != '\n' )
+			return tag_error("No new line before message");
 
-	text_len = buffer_end - ++buffer;
+		text_len = buffer_end - ++buffer;
 
-	tag->message = git__malloc(text_len + 1);
-	GITERR_CHECK_ALLOC(tag->message);
+		tag->message = git__malloc(text_len + 1);
+		GITERR_CHECK_ALLOC(tag->message);
 
-	memcpy(tag->message, buffer, text_len);
-	tag->message[text_len] = '\0';
+		memcpy(tag->message, buffer, text_len);
+		tag->message[text_len] = '\0';
+	}
 
 	return 0;
 }

tests-clar/object/tag/read.c

@@ -7,6 +7,8 @@ static const char *tag2_id = "7b4384978d2493e851f9cca7858815fac9b10980";
 static const char *tagged_commit = "e90810b8df3e80c413d903f631643c716887138d";
 static const char *bad_tag_id = "eda9f45a2a98d4c17a09d681d88569fa4ea91755";
 static const char *badly_tagged_commit = "e90810b8df3e80c413d903f631643c716887138d";
+static const char *short_tag_id = "5da7760512a953e3c7c4e47e4392c7a4338fb729";
+static const char *short_tagged_commit = "4a5ed60bafcf4638b7c8356bd4ce1916bfede93c";
 
 static git_repository *g_repo;
 
@@ -83,3 +85,34 @@ void test_object_tag_read__parse_without_tagger(void)
    git_commit_free(commit);
    git_repository_free(bad_tag_repo);
 }
+
+void test_object_tag_read__parse_without_message(void)
+{
+   // read and parse a tag without a message field
+   git_repository *short_tag_repo;
+   git_tag *short_tag;
+   git_commit *commit;
+   git_oid id, id_commit;
+
+   // TODO: This is a little messy
+   cl_git_pass(git_repository_open(&short_tag_repo, cl_fixture("short_tag.git")));
+
+   git_oid_fromstr(&id, short_tag_id);
+   git_oid_fromstr(&id_commit, short_tagged_commit);
+
+   cl_git_pass(git_tag_lookup(&short_tag, short_tag_repo, &id));
+   cl_assert(short_tag != NULL);
+
+   cl_assert_equal_s(git_tag_name(short_tag), "no_description");
+   cl_assert(git_oid_cmp(&id, git_tag_id(short_tag)) == 0);
+   cl_assert(short_tag->message == NULL);
+
+   cl_git_pass(git_tag_target((git_object **)&commit, short_tag));
+   cl_assert(commit != NULL);
+
+   cl_assert(git_oid_cmp(&id_commit, git_commit_id(commit)) == 0);
+
+   git_tag_free(short_tag);
+   git_commit_free(commit);
+   git_repository_free(short_tag_repo);
+}

tests-clar/resources/short_tag.git/HEAD

@@ -0,0 +1 @@
+ref: refs/heads/master

tests-clar/resources/short_tag.git/config

@@ -0,0 +1,5 @@
+[core]
+	repositoryformatversion = 0
+	filemode = true
+	bare = true
+	logallrefupdates = true

tests-clar/resources/short_tag.git/objects/5d/a7760512a953e3c7c4e47e4392c7a4338fb729

@@ -0,0 +1 @@
+x
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E+‚pÐV¹Ð¡SƆ¨‘d/m(R¯°áïJ€%çÄ
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tests-clar/resources/short_tag.git/packed-refs

@@ -0,0 +1 @@
+5da7760512a953e3c7c4e47e4392c7a4338fb729 refs/tags/no_description

tests-clar/resources/short_tag.git/refs/heads/master

@@ -0,0 +1 @@
+4a5ed60bafcf4638b7c8356bd4ce1916bfede93c