[PATCH v6 0/3] fetch: fix non-conflicting tags not being committed

This fixes the bug reported by David Bohman [1].

The 'git-fetch(1)' uses batched updates to perform reference updates when not using 'atomic' transactions. One scenario which was missed here, was fetching tags. When fetching conflicting tags, the `fetch_and_consume_refs()` function returns '1', which skipped committing the transaction and directly jumped to the cleanup section. This mean that no updates were applied. This also extends to backfilling tags.

The first commit, extracts out common code for committing a reference transaction and handling rejected updates. The second commit ensures any failures would also commit pending updates.

The third commit fixes another regression around failing to do post-fetch operations when ref updates fail with batched updates.

[1]: id:CAB9xhmPcHnB2+i6WeA3doAinv7RAeGs04+n0fHLGToJq=UKUNw@mail.gmail.com

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
---
Changes in v6:
- This version adds a new commit which handles another regression where
  if reference updates fail when using batched updates, we skip doing
  the post-fetch operations. Namely:
    - Updating 'FETCH_HEAD' via `commit_fetch_head()`
    - Adding upstream tracking information via `set_upstream()`
    - Setting remote 'HEAD' values when `do_set_head` is true
- Link to v5: https://patch.msgid.link/20251113-fix-tags-not-fetching-v5-0-371ea7ec638d@gmail.com

Changes in v5:
- In the previous version, I assumed that the `prune_refs()` function
  also triggers committing of batched updates. However this was
  incorrect as the transaction for batched updates, is only created
  after the call to `prune_refs()`. This makes sense, since we want to
  isolate deletions from the rest of the ref updates, to avoid
  conflicts. I've amended the commit message accordingly.
- I noticed I missed cleanup of the repos created in the test, which
  I've now done.
- Link to v4: https://patch.msgid.link/20251111-fix-tags-not-fetching-v4-0-185d836ec62a@gmail.com

Changes in v4:
- Cleanup the code in the first commit to make it simpler to read.
- In the second commit, we were specifically checking for `retcode > 0`
  for committing the transaction. This is a bit confusing since that
  begs the questions why not `retcode < 0`. There is no real reason
  there, so I've change the code to simple do `if (retcode && ...)`.
  I've also added more information about the flows which would commit
  the transaction in the commit message.
- Link to v3: https://patch.msgid.link/20251108-fix-tags-not-fetching-v3-0-a12ab6c4daef@gmail.com

Changes in v3:
- Split the patch into two commits. One for extracting out existing code
  into a new commit and the other to perform the fix.
- Add back error handling when commit via the normal flow.
- Instead of calling the commit function at every failure, make it part
  of the cleanup code.
- Link to v2: https://patch.msgid.link/20251106-fix-tags-not-fetching-v2-1-610cb4b0e7c8@gmail.com

Changes in v2:
- Add a comment to explain the purpose of `commit_ref_transaction()` and
  how it works.
- Also extend the same logic towards backfilling tags. While I was able
  to add a test for the happy path, I couldn't figure out how to test
  when `backfill_tags()` tags would fail.
  Tangentially, this flow seems to only be triggered when using the now
  deprecated 'branches/' remote format.
- Remove unneeded subshells from the tests.
- Link to v1: https://patch.msgid.link/20251103-fix-tags-not-fetching-v1-1-e63caeb6c113@gmail.com

---
 builtin/fetch.c  |  71 ++++++++++++++++----------
 t/t5510-fetch.sh | 149 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 194 insertions(+), 26 deletions(-)

Karthik Nayak (3):
      fetch: extract out reference committing logic
      fetch: fix non-conflicting tags not being committed
      fetch: fix failed batched updates skipping operations

Range-diff versus v5:

1: ab03acf218 = 1: 21f2518724 fetch: extract out reference committing logic 2: 8a9982ee75 = 2: 9ca27b08fa fetch: fix non-conflicting tags not being committed -: ---------- > 3: 33a7654bfa fetch: fix failed batched updates skipping operations

base-commit: a99f379adf116d53eb11957af5bab5214915f91d change-id: 20251103-fix-tags-not-fetching-0f1621a474d4

Thanks
- Karthik