patchgit-cat-file: fix output when format string contains no variables

From: Sven Brauch <svenbrauch@googlemail.com>
Date: Tue, 5 Nov 2013 20:06:21 +0100
Subject: [PATCH] git-cat-file: fix output when format string contains no
 variables

When the format string for git-cat-object --batch-check contained no variables, the function would not actually look for the object on disk, but just verify that the hash is correct. Thus it would report no error if asking for objects which did not actually exist on disk if the SHA hash looked ok.

Example of buggy behaviour: echo "XYZ" | git hash-object --stdin | git cat-file --batch-check="found" would report "found" although the manpage claims it would report an error.

Signed-off-by: Sven Brauch <svenbrauch@googlemail.com>
---

Notes:
    This fixes a bug where git-cat-file --batch-check would erroneously tell
    that objects exist while they did in fact not in case the argument to
    --batch-check was just a constant strig (i.e. no %(...) variables).
    The reason was that calling sha1_object_info_extended while requesting no
    properties of the object would not even verify this object existed, or more
    exactly, sha1_loose_object_info would not do that.
    
    I'm entirely unfamiliar with the git codebase; the suggested fix ensures
    that always at least one property is requested. If there's a better way
    to fix this issue, please let me know.

 builtin/cat-file.c | 9 +++++++++
 1 file changed, 9 insertions(+)

Sven Brauch <svenbrauch@googlemail.com> writes:

Thanks; first some procedural issues:

 - Omit the first line "From 2e7b..."; that does not belong to any
   patch submission (it is a separator used between messages in the
   mailbox formatted file "format-patch --stdout" produces).

 - The second line "From: Sven..." records exactly the same address
   as the e-mail you are sending out, so it should be omitted as
   well.

 - The third line "Date: ..." is not the time we the general public
   sees your fix for the first time, which is what "Date: " of your
   e-mail header already records, so we do not need it either.

 - And the last one "Subject: ..." is redundant; we can see it in
   your e-mail header.

In general, the latter three lines are produced by format-patch to help you fill header fields in the MUA of your choice by cutting (not copying) and pasting. Unless there is a valid reason to have values different from what recipients would see in the e-mail header (and there often isn't, unless you are forwarding somebody else's patch, in which case you may want to use "From: ", or you are responding to an ongoing discussion with a patch, in which case you may want to use "Subject: "), please remove them after copying them out to your e-mail header.

An excellent log message. It would have been even better to add a new test to t1006 based on this reproduction recipe.

I think the real problem is that sha1_loose_object_info() is called by sha1_object_info_extended(), when it does not find a cached or a packed object, and the callee assumes that it is asked to fill in only the requested pieces of information while the caller does not even bother to check if such an object actually exists.

How about doing it like the attached instead?

-- >8 --
Subject: sha1_loose_object_info(): do not return success on missing object

Since 052fe5ea (sha1_loose_object_info: make type lookup optional, 2013-07-12), sha1_loose_object_info() returns happily without checking if the object in question exists, which is not what the the caller sha1_object_info_extended() expects; the caller does not even bother checking the existence of the object itself.

Noticed-by: Sven Brauch <svenbrauch@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---

  Oh, by the way, there is this one iffy bit in batch_one_object():

          if (get_sha1(obj_name, data->sha1)) {
                  printf("%s missing\n", obj_name);
                  fflush(stdout);
                  return 0;
          }

  At this point, the object _may_ be missing, but the obj_name may be
  malformed, so saying "missing" is not strictly correct.  If, for
  example, you misspelled the name of the master branch, you would get
  this:

          $ echo mastre | git cat-file --batch-check=foo
          mastre missing

  I however doubt that it is a good idea to reword this message by
  adding a logic to tell misspelled object name and missing object
  name apart.  The users of "cat-file --batch-check" are not
  expecting to be able to distinguish these two classes of errors
  anyway.

 sha1_file.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

On Wed, Nov 06, 2013 at 10:00:57AM -0800, Junio C Hamano wrote:

Yes, exactly. This is over-optimization on the part of my 052fe5ea. The caller asked for "nothing", so we happily optimize the request to do nothing. But I forgot there is always an implicit "does it even exist" query in the call. I do not see any point in adding an "exists" query field to "struct object_info". :)

This is definitely the right fix. Commit message and patch look good to me. A few extra bits are in the patch below.

Yes, I noticed that while doing the original series, but left it intact for the exact reason you mention. Note that it is going to stdout and is part of the actual data stream (so there is a good chance scripts are parsing it).

Here's a squashable patch which expands the comment above the code you're tweaking, and adds a test to t1006.

I notice that t1006 could use some modernization, whitespace-fixing, and typo-fixing, but I left that out of the patch for clarity.

Thanks both of you for working on this.

-Peff

---

On Wednesday 06 November 2013 10:00:57 Junio C Hamano wrote:

Thanks, I will take care of the mentioned points for future submissions.

Yes; this seems more like a proper fix. I would prefer it over my suggestion. It is illogical that sha1_loose_object_info sometimes returns an error if the object does not exist and sometimes not, depending on which properties are requested.

Sven