Add "tag" objects that can be used to sign other objects.
You use "git-mktag" to create them, and fsck-cache knows how to parse them.
diff --git a/git-mktag.c b/git-mktag.c
new file mode 100644
index 0000000..5d2830d
--- /dev/null
+++ b/git-mktag.c
@@ -0,0 +1,125 @@
+#include "cache.h"
+
+/*
+ * A signature file has a very simple fixed format: three lines
+ * of "object <sha1>" + "type <typename>" + "tag <tagname>",
+ * followed by some free-form signature that git itself doesn't
+ * care about, but that can be verified with gpg or similar.
+ *
+ * The first three lines are guaranteed to be at least 63 bytes:
+ * "object <sha1>\n" is 48 bytes, "type tag\n" at 9 bytes is the
+ * shortest possible type-line, and "tag .\n" at 6 bytes is the
+ * shortest single-character-tag line.
+ *
+ * We also artificially limit the size of the full object to 8kB.
+ * Just because I'm a lazy bastard, and if you can't fit a signature
+ * in that size, you're doing something wrong.
+ */
+
+// Some random size
+#define MAXSIZE (8192)
+
+/*
+ * We refuse to tag something we can't verify. Just because.
+ */
+static int verify_object(unsigned char *sha1, const char *expected_type)
+{
+ int ret = -1;
+ unsigned long mapsize;
+ void *map = map_sha1_file(sha1, &mapsize);
+
+ if (map) {
+ char type[100];
+ unsigned long size;
+ void *buffer = unpack_sha1_file(map, mapsize, type, &size);
+
+ if (buffer) {
+ if (!strcmp(type, expected_type))
+ ret = check_sha1_signature(sha1, buffer, size, type);
+ free(buffer);
+ }
+ munmap(map, mapsize);
+ }
+ return ret;
+}
+
+static int verify_tag(char *buffer, unsigned long size)
+{
+ int typelen;
+ char type[20];
+ unsigned char sha1[20];
+ const char *object, *type_line, *tag_line;
+
+ if (size < 64 || size > MAXSIZE-1)
+ return -1;
+ buffer[size] = 0;
+
+ /* Verify object line */
+ object = buffer;
+ if (memcmp(object, "object ", 7))
+ return -1;
+ if (get_sha1_hex(object + 7, sha1))
+ return -1;
+
+ /* Verify type line */
+ type_line = object + 48;
+ if (memcmp(type_line - 1, "\ntype ", 6))
+ return -1;
+
+ /* Verify tag-line */
+ tag_line = strchr(type_line, '\n');
+ if (!tag_line)
+ return -1;
+ tag_line++;
+ if (memcmp(tag_line, "tag ", 4) || tag_line[4] == '\n')
+ return -1;
+
+ /* Get the actual type */
+ typelen = tag_line - type_line - strlen("type \n");
+ if (typelen >= sizeof(type))
+ return -1;
+ memcpy(type, type_line+5, typelen);
+ type[typelen] = 0;
+
+ /* Verify that the object matches */
+ if (get_sha1_hex(object + 7, sha1))
+ return -1;
+ if (verify_object(sha1, type))
+ return -1;
+
+ /* Verify the tag-name: we don't allow control characters or spaces in it */
+ tag_line += 4;
+ for (;;) {
+ unsigned char c = *tag_line++;
+ if (c == '\n')
+ break;
+ if (c > ' ')
+ continue;
+ return -1;
+ }
+
+ /* The actual stuff afterwards we don't care about.. */
+ return 0;
+}
+
+int main(int argc, char **argv)
+{
+ unsigned long size;
+ char buffer[MAXSIZE];
+ unsigned char result_sha1[20];
+
+ if (argc != 1)
+ usage("cat <signaturefile> | git-mktag");
+
+ // Read the signature
+ size = read(0, buffer, MAXSIZE);
+
+ // Verify it for some basic sanity: it needs to start with "object <sha1>\ntag "
+ if (verify_tag(buffer, size) < 0)
+ die("invalid tag signature file");
+
+ if (write_sha1_file(buffer, size, "tag", result_sha1) < 0)
+ die("unable to write tag file");
+ printf("%s\n", sha1_to_hex(result_sha1));
+ return 0;
+}
