I recently looked into issues where git merge-tree calls returned bogus data
(in one instance returning an empty tree for non-empty merge parents). By
the time I had a look at the corresponding repository, the issue was no
longer reproducible, but a closer look at the code combined with some manual
experimenting turned up the fact that missing tree objects aren't handled as
errors by git merge-tree.

While at it, I added a commit on top that tries to catch all remaining
unchecked parse_tree() calls.

This patch series is based on 5f43cf5b2e4 (merge-tree: accept 3 trees as
arguments, 2024-01-28) (the original tip commit of js/merge-tree-3-trees)
because I introduced three unchecked parse_tree() calls in that topic.

Changes since v2:

 * Fixed the new "missing tree object" test case in t4301 that succeeded for
   the wrong reason.
 * Adjusted the new "missing blob object" test case to avoid succeeding for
   the wrong reason.
 * Simplified the "missing blob object" test case.

Changes since v1:

 * Simplified the test case, avoiding a subshell and a pipe in the process.
 * Added a patch to remove a superfluous subtree->object.parsed guard around
   a parse_tree(subtree) call.

Johannes Schindelin (5):
  merge-tree: fail with a non-zero exit code on missing tree objects
  merge-ort: do check `parse_tree()`'s return value
  t4301: verify that merge-tree fails on missing blob objects
  Always check `parse_tree*()`'s return value
  cache-tree: avoid an unnecessary check

 builtin/checkout.c               | 19 ++++++++++++++++---
 builtin/clone.c                  |  3 ++-
 builtin/commit.c                 |  3 ++-
 builtin/merge-tree.c             |  6 ++++++
 builtin/read-tree.c              |  3 ++-
 builtin/reset.c                  |  4 ++++
 cache-tree.c                     |  4 ++--
 merge-ort.c                      | 16 +++++++++++-----
 merge-recursive.c                |  3 ++-
 merge.c                          |  5 ++++-
 reset.c                          |  5 +++++
 sequencer.c                      |  4 ++++
 t/t4301-merge-tree-write-tree.sh | 27 +++++++++++++++++++++++++++
 13 files changed, 87 insertions(+), 15 deletions(-)

base-commit: 5f43cf5b2e4b68386d3774bce880b0f74d801635

Submitted-As: https://lore.kernel.org/git/pull.1651.v3.git.1708612605.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.1651.git.1707212981.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.1651.v2.git.1707324461.gitgitgadget@gmail.com