merge-tree: fix segmentation fault in read-only repositories

Turns out that the segmentation fault reported by Taylor
[https://lore.kernel.org/git/[email protected]/] 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 <[email protected]>

## 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 <[email protected]>

## merge-ort.c ##

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