[PATCH] mktag: don't check SHA-1 object length under SHA-256

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Change the hardcoded minimum tag length size to vary based on whether
we're operating in the SHA-1 or SHA-256 mode, and update the "mktag"
documentation, tests and code comments to reflect that this command
can take either SHA-1 or SHA-256 input.

Let's make the code more self-documenting by verbosely inlining what a
minimum tag looks like. The fixed-string strlen() will be optimized
away by any modern compiler. This also future-proofs us for any future
hash function transition.

Then change the tests amended in acb49d1cc8b (t3800: make hash-size
independent, 2019-08-18) even more for SHA-256. Some of the tests were
failing for the wrong reasons. E.g. yes <some sha-1 length garbage> is
an invalid SHA-1, but we should test <some sha256 length garbage> when
under SHA-256. For testing an invalid non-hexadecimal object ID let's
use ROT13.

For the "object line label check" test the "139e9b339..." SHA-1 didn't
exist, but what's actually being tested there is getting "xxxxxx"
instead of "object", so let's provide a valid SHA there
instead. There's no need to make or hardcode a nonexisting object ID.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Documentation/git-mktag.txt |  2 +-
 builtin/mktag.c             | 26 +++++++++++++-------
 t/t3800-mktag.sh            | 47 ++++++++++++++++++++++++++-----------
 3 files changed, 52 insertions(+), 23 deletions(-)

diff --git a/Documentation/git-mktag.txt b/Documentation/git-mktag.txt
index fa6a7561236..bbcc0a086bf 100644
--- a/Documentation/git-mktag.txt
+++ b/Documentation/git-mktag.txt
@@ -23,7 +23,7 @@ Tag Format
 A tag signature file, to be fed to this command's standard input,
 has a very simple fixed format: four lines of
 
-  object <sha1>
+  object <sha>
   type <typename>
   tag <tagname>
   tagger <tagger>
diff --git a/builtin/mktag.c b/builtin/mktag.c
index 4982d3a93ef..3fa17243e34 100644
--- a/builtin/mktag.c
+++ b/builtin/mktag.c
@@ -5,13 +5,15 @@
 
 /*
  * A signature file has a very simple fixed format: four lines
- * of "object <sha1>" + "type <typename>" + "tag <tagname>" +
+ * of "object <sha>" + "type <typename>" + "tag <tagname>" +
  * "tagger <committer>", followed by a blank line, a free-form tag
  * message and a signature block that git itself doesn't care about,
  * but that can be verified with gpg or similar.
  *
- * The first four lines are guaranteed to be at least 83 bytes:
- * "object <sha1>\n" is 48 bytes, "type tag\n" at 9 bytes is the
+ * The first four lines are guaranteed to be either 83 or 107 bytes;
+ * depending on whether we're referencing a SHA-1 or SHA-256 tag.
+ *
+ * "object <sha1>\n" is 48 or a 72 bytes, "type tag\n" at 9 bytes is the
  * shortest possible type-line, "tag .\n" at 6 bytes is the shortest
  * single-character-tag line, and "tagger . <> 0 +0000\n" at 20 bytes is
  * the shortest possible tagger-line.
@@ -46,9 +48,17 @@ static int verify_tag(char *buffer, unsigned long size)
 	struct object_id oid;
 	const char *object, *type_line, *tag_line, *tagger_line, *lb, *rb, *p;
 	size_t len;
-
-	if (size < 84)
-		return error("wanna fool me ? you obviously got the size wrong !");
+	int minimum_size =
+		/* Minimum possible input, see t/t3800-mktag.sh */
+		strlen("object ") + the_hash_algo->hexsz + strlen("\n") +
+		strlen("type tag\n") +
+		strlen("tag x\n") +
+		strlen("tagger . <> 0 +0000\n") +
+		strlen("\n");
+
+	if (size < minimum_size)
+		return error("got %"PRIuMAX" bytes of input, need at least %d bytes",
+			     size, minimum_size);
 
 	buffer[size] = 0;
 
@@ -58,7 +68,7 @@ static int verify_tag(char *buffer, unsigned long size)
 		return error("char%d: does not start with \"object \"", 0);
 
 	if (parse_oid_hex(object + 7, &oid, &p))
-		return error("char%d: could not get SHA1 hash", 7);
+		return error("char%d: expected object ID, got garbage", 7);
 
 	/* Verify type line */
 	type_line = p + 1;
@@ -166,7 +176,7 @@ int cmd_mktag(int argc, const char **argv, const char *prefix)
 	}
 
 	/* Verify it for some basic sanity: it needs to start with
-	   "object <sha1>\ntype\ntagger " */
+	   "object <sha>\ntype\ntagger " */
 	if (verify_tag(buf.buf, buf.len) < 0)
 		die("invalid tag signature file");
 
diff --git a/t/t3800-mktag.sh b/t/t3800-mktag.sh
index d696aa4e52e..93a19bb8df9 100755
--- a/t/t3800-mktag.sh
+++ b/t/t3800-mktag.sh
@@ -26,24 +26,42 @@ test_expect_success 'setup' '
 	echo Hello >A &&
 	git update-index --add A &&
 	git commit -m "Initial commit" &&
-	head=$(git rev-parse --verify HEAD)
+	head=$(git rev-parse --verify HEAD) &&
+
+	git tag -m"some tag" annotated &&
+	annotated=$(git rev-parse --verify annotated)
 '
 
 ############################################################
 #  1. length check
-
-cat >tag.sig <<EOF
-too short for a tag
-EOF
-
-check_verify_failure 'Tag object length check' \
-	'^error: .*size wrong.*$'
+for objType in t ta tag
+do
+	cat >tag.sig <<-EOF
+	object $annotated
+	type $objType
+	tag x
+	tagger . <> 0 +0000
+
+	EOF
+	len=$(wc -c tag.sig)
+
+	if test $objType = "tag"
+	then
+		test_expect_success "Tag object length check $len passed" '
+			git mktag <tag.sig >.git/refs/tags/x 2>message &&
+			git rev-parse refs/tags/x
+		'
+	else
+		check_verify_failure "Tag object length check on length $len" \
+			'^error: got .* bytes of input, need at least'
+	fi
+done
 
 ############################################################
 #  2. object line label check
 
 cat >tag.sig <<EOF
-xxxxxx 139e9b33986b1c2670fff52c5067603117b3e895
+xxxxxx $head
 type tag
 tag mytag
 tagger . <> 0 +0000
@@ -53,17 +71,18 @@ EOF
 check_verify_failure '"object" line label check' '^error: char0: .*"object "$'
 
 ############################################################
-#  3. object line SHA1 check
+#  3. object line SHA check
 
+invalid_sha=$(echo $head | tr A-Za-z N-ZA-Mn-za-m)
 cat >tag.sig <<EOF
-object zz9e9b33986b1c2670fff52c5067603117b3e895
+object $invalid_sha
 type tag
 tag mytag
 tagger . <> 0 +0000
 
 EOF
 
-check_verify_failure '"object" line SHA1 check' '^error: char7: .*SHA1 hash$'
+check_verify_failure '"object" line object check' '^error: char7: .*expected object ID, got garbage'
 
 ############################################################
 #  4. type line label check
@@ -125,7 +144,7 @@ check_verify_failure '"type" line type-name length check' \
 	'^error: char.*: type too long$'
 
 ############################################################
-#  9. verify object (SHA1/type) check
+#  9. verify object (SHA/type) check
 
 cat >tag.sig <<EOF
 object $(test_oid deadbeef)
@@ -135,7 +154,7 @@ tagger . <> 0 +0000
 
 EOF
 
-check_verify_failure 'verify object (SHA1/type) check' \
+check_verify_failure 'verify object (SHA/type) check' \
 	'^error: char7: could not verify object.*$'
 
 ############################################################
-- 
2.29.2.222.g5d2a92d10f8