Re: [PATCH 6/6] hash-object: use fsck for object checks

From: Taylor Blau <hidden>
Date: 2023-01-18 21:34:25

On Wed, Jan 18, 2023 at 03:44:12PM -0500, Jeff King wrote:

This is obviously going to be a user-visible behavior change, and the
test changes earlier in this series show the scope of the impact. But
I'd argue that this is OK:

  - the documentation for hash-object is already vague about which
    checks we might do, saying that --literally will allow "any
    garbage[...] which might not otherwise pass standard object parsing
    or git-fsck checks". So we are already covered under the documented
    behavior.

  - users don't generally run hash-object anyway. There are a lot of
    spots in the tests that needed to be updated because creating
    garbage objects is something that Git's tests disproportionately do.

  - it's hard to imagine anyone thinking the new behavior is worse. Any
    object we reject would be a potential problem down the road for the
    user. And if they really want to create garbage, --literally is
    already the escape hatch they need.

This is the discussion I was pointing out earlier in the series as
evidence for making this behavior the new default without "--literally".

That being said, let me play devil's advocate for a second. Do the new
fsck checks slow anything in hash-object down significantly? If so, then
it's plausible to imagine a hash-object caller who (a) doesn't use
`--literally`, but (b) does care about throughput if they're writing a
large number of objects at once.

I don't know if such a situation exists, or if these new fsck checks
even slow hash-object down enough to care. But I didn't catch a
discussion of this case in your series, so I figured I'd bring it up
here just in case.

  - the resulting messages are much better. For example:

      [before]
      $ echo 'tree 123' | git hash-object -t commit --stdin
      error: bogus commit object 0000000000000000000000000000000000000000
      fatal: corrupt commit

      [after]
      $ echo 'tree 123' | git.compile hash-object -t commit --stdin
      error: object fails fsck: badTreeSha1: invalid 'tree' line format - bad sha1
      fatal: refusing to create malformed object

Much nicer, well done.

quoted hunk ↗ jump to hunk

Signed-off-by: Jeff King <redacted>
---
 object-file.c          | 55 ++++++++++++++++++------------------------
 t/t1007-hash-object.sh | 11 +++++++++
 2 files changed, 34 insertions(+), 32 deletions(-)

diff --git a/object-file.c b/object-file.c
index 80a0cd3b35..5c96384803 100644
--- a/object-file.c
+++ b/object-file.c

@@ -33,6 +33,7 @@
 #include "object-store.h"
 #include "promisor-remote.h"
 #include "submodule.h"
+#include "fsck.h"

 /* The maximum size for an object header. */
 #define MAX_HEADER_LEN 32

@@ -2298,32 +2299,21 @@ int repo_has_object_file(struct repository *r,
 	return repo_has_object_file_with_flags(r, oid, 0);
 }

-static void check_tree(const void *buf, size_t size)
-{
-	struct tree_desc desc;
-	struct name_entry entry;
-
-	init_tree_desc(&desc, buf, size);
-	while (tree_entry(&desc, &entry))
-		/* do nothing
-		 * tree_entry() will die() on malformed entries */
-		;
-}
-
-static void check_commit(const void *buf, size_t size)
-{
-	struct commit c;
-	memset(&c, 0, sizeof(c));
-	if (parse_commit_buffer(the_repository, &c, buf, size, 0))
-		die(_("corrupt commit"));
-}
-
-static void check_tag(const void *buf, size_t size)
-{
-	struct tag t;
-	memset(&t, 0, sizeof(t));
-	if (parse_tag_buffer(the_repository, &t, buf, size))
-		die(_("corrupt tag"));

OK, here we're getting rid of all of the lightweight checks that
hash-object used to implement on its own.

quoted hunk ↗ jump to hunk

+/*
+ * We can't use the normal fsck_error_function() for index_mem(),
+ * because we don't yet have a valid oid for it to report. Instead,
+ * report the minimal fsck error here, and rely on the caller to
+ * give more context.
+ */
+static int hash_format_check_report(struct fsck_options *opts,
+				     const struct object_id *oid,
+				     enum object_type object_type,
+				     enum fsck_msg_type msg_type,
+				     enum fsck_msg_id msg_id,
+				     const char *message)
+{
+	error(_("object fails fsck: %s"), message);
+	return 1;
 }

 static int index_mem(struct index_state *istate,

@@ -2350,12 +2340,13 @@ static int index_mem(struct index_state *istate,
 		}
 	}
 	if (flags & HASH_FORMAT_CHECK) {
-		if (type == OBJ_TREE)
-			check_tree(buf, size);
-		if (type == OBJ_COMMIT)
-			check_commit(buf, size);
-		if (type == OBJ_TAG)
-			check_tag(buf, size);
+		struct fsck_options opts = FSCK_OPTIONS_DEFAULT;
+
+		opts.strict = 1;
+		opts.error_func = hash_format_check_report;
+		if (fsck_buffer(null_oid(), type, buf, size, &opts))
+			die(_("refusing to create malformed object"));
+		fsck_finish(&opts);
 	}

And here's the main part of the change, which is delightfully simple and
appears correct to me.

quoted hunk ↗ jump to hunk

diff --git a/t/t1007-hash-object.sh b/t/t1007-hash-object.sh
index 2d2148d8fa..ac3d173767 100755
--- a/t/t1007-hash-object.sh
+++ b/t/t1007-hash-object.sh

@@ -222,6 +222,17 @@ test_expect_success 'empty filename in tree' '
 	grep "empty filename in tree entry" err
 '

+test_expect_success 'duplicate filename in tree' '
+	hex_oid=$(echo foo | git hash-object --stdin -w) &&
+	bin_oid=$(echo $hex_oid | hex2oct) &&
+	{
+		printf "100644 file\0$bin_oid" &&
+		printf "100644 file\0$bin_oid"
+	} >tree-with-duplicate-filename &&
+	test_must_fail git hash-object -t tree tree-with-duplicate-filename 2>err &&
+	grep "duplicateEntries" err
+'
+

For what it's worth, I think that this is sufficient coverage for the
new fsck checks.

Thanks,
Taylor

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