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From: "Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>
To: git@vger.kernel.org
Cc: Elijah Newren <newren@gmail.com>, Taylor Blau <me@ttaylorr.com>,
	Johannes Schindelin <johannes.schindelin@gmx.de>
Subject: [PATCH v5 0/2] merge-tree: fix segmentation fault in read-only repositories
Date: Wed, 28 Sep 2022 07:29:20 +0000	[thread overview]
Message-ID: <pull.1362.v5.git.1664350162.gitgitgadget@gmail.com> (raw)
In-Reply-To: <pull.1362.v4.git.1664229348.gitgitgadget@gmail.com>

Turns out that the segmentation fault reported by Taylor
[https://lore.kernel.org/git/YyopQD+LvPucnz3w@nand.local/] happened while
testing merge-ort in a read-only repository, and that the upstream version
of git merge-tree is as affected as GitHub's internal version.

Changes since v4:

 * Corrected the commit message that claimed that the second patch was not
   fixing a real bug but just avoided unnecessary work. As pointed out by
   Elijah, there are scenarios where this second patch fixes a very real
   bug.
 * Added Elijah's "Reviewed-by" footer.

Changes since v3:

 * I now consistently use the pattern int ret = 0; ... if (...) ret = -1.
 * I added a commit to properly propagate write failures through the
   handle_content_merge() call path, even if it is not really critical (it
   just fails sooner, but the merge would have failed just the same without
   this patch).

Changes since v2:

 * Completely changed the approach by no longer touching
   builtin/merge-tree.c but instead fixing the underlying merge-ort layer:
   we no longer ignore the return value of write_tree() and
   write_object_file().

Changes since v1:

 * Using the SANITY prereq now
 * If the merge was aborted due to a write error, exit with a non-zero code
   even if the (potentially partial) merge is clean
 * The test case now also verifies that the git merge-tree command fails in
   a read-only repository even if the merge would have succeeded

Johannes Schindelin (2):
  merge-ort: fix segmentation fault in read-only repositories
  merge-ort: return early when failing to write a blob

 merge-ort.c                      | 94 ++++++++++++++++++++------------
 t/t4301-merge-tree-write-tree.sh |  9 +++
 2 files changed, 69 insertions(+), 34 deletions(-)


base-commit: dda7228a83e2e9ff584bf6adbf55910565b41e14
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1362%2Fdscho%2Fmerge-tree-in-read-only-repos-v5
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1362/dscho/merge-tree-in-read-only-repos-v5
Pull-Request: https://github.com/gitgitgadget/git/pull/1362

Range-diff vs v4:

 1:  ab6df092eba ! 1:  63be9f9f717 merge-ort: fix segmentation fault in read-only repositories
     @@ Commit message
          Let's stop ignoring the return value of `write_object_file()` and
          `write_tree()` and set `clean = -1` in the error case.
      
     +    Reviewed-by: Elijah Newren <newren@gmail.com>
          Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
      
       ## merge-ort.c ##
 2:  087207ae0b0 ! 2:  bfc71a2d8ad merge-ort: return early when failing to write a blob
     @@ Commit message
          that already exists in the database). And this can fail, too, but we
          ignore that write failure so far.
      
     -    Since we will always write out a new tree object in addition to the blob
     -    (and if the blob did not exist in the database yet, we can be certain
     -    that the tree object did not exist yet), the merge will _still_ fail at
     -    that point, but it does unnecessary work by continuing after the blob
     -    could not be written.
     -
          Let's pay close attention and error out early if the blob could not be
          written. This reduces the error output of t4301.25 ("merge-ort fails
          gracefully in a read-only repository") from:
     @@ Commit message
                  error: error: Unable to add numbers to database
                  fatal: failure to merge
      
     +    This is _not_ just a cosmetic change: Even though one might assume that
     +    the operation would have failed anyway at the point when the new tree
     +    object is written (and the corresponding tree object _will_ be new if it
     +    contains a blob that is new), but that is not so: As pointed out by
     +    Elijah Newren, when Git has previously been allowed to add loose objects
     +    via `sudo` calls, it is very possible that the blob object cannot be
     +    written (because the corresponding `.git/objects/??/` directory may be
     +    owned by `root`) but the tree object can be written (because the
     +    corresponding objects directory is owned by the current user). This
     +    would result in a corrupt repository because it is missing the blob
     +    object, and with this here patch we prevent that.
     +
          Note: This patch adjusts two variable declarations from `unsigned` to
          `int` because their purpose is to hold the return value of
          `handle_content_merge()`, which is of type `int`. The existing users of
          those variables are only interested whether that variable is zero or
          non-zero, therefore this type change does not affect the existing code.
      
     +    Reviewed-by: Elijah Newren <newren@gmail.com>
          Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
      
       ## merge-ort.c ##

-- 
gitgitgadget

  parent reply	other threads:[~2022-09-28  7:40 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-21 15:30 [PATCH] merge-tree: fix segmentation fault in read-only repositories Johannes Schindelin via GitGitGadget
2022-09-21 15:42 ` Taylor Blau
2022-09-21 20:12   ` Johannes Schindelin
2022-09-21 18:12 ` Junio C Hamano
2022-09-21 20:13   ` Johannes Schindelin
2022-09-21 22:08 ` [PATCH v2] " Johannes Schindelin via GitGitGadget
2022-09-21 22:24   ` Junio C Hamano
2022-09-22 19:52     ` Johannes Schindelin
2022-09-21 22:56   ` Elijah Newren
2022-09-21 23:11     ` Junio C Hamano
2022-09-22 17:24     ` Johannes Schindelin
2022-09-22 19:50     ` Johannes Schindelin
2022-09-23  1:47       ` Elijah Newren
2022-09-22 19:46   ` [PATCH v3] merge-ort: " Johannes Schindelin via GitGitGadget
2022-09-23  1:38     ` Elijah Newren
2022-09-23  9:55       ` Johannes Schindelin
2022-09-26 21:55     ` [PATCH v4 0/2] merge-tree: " Johannes Schindelin via GitGitGadget
2022-09-26 21:55       ` [PATCH v4 1/2] merge-ort: " Johannes Schindelin via GitGitGadget
2022-09-26 21:55       ` [PATCH v4 2/2] merge-ort: return early when failing to write a blob Johannes Schindelin via GitGitGadget
2022-09-26 22:07         ` Junio C Hamano
2022-09-27  8:05         ` Elijah Newren
2022-09-27 16:45           ` Junio C Hamano
2022-09-27  8:11       ` [PATCH v4 0/2] merge-tree: fix segmentation fault in read-only repositories Elijah Newren
2022-09-28  7:29       ` Johannes Schindelin via GitGitGadget [this message]
2022-09-28  7:29         ` [PATCH v5 1/2] merge-ort: " Johannes Schindelin via GitGitGadget
2022-09-28  7:29         ` [PATCH v5 2/2] merge-ort: return early when failing to write a blob Johannes Schindelin via GitGitGadget
2022-09-28 15:53           ` Junio C Hamano
2022-09-29  1:36             ` Elijah Newren
2022-09-29  1:49               ` Junio C Hamano

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