[PATCH v5 0/2] merge-tree: fix segmentation fault in read-only repositories

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 ##