Preserve a ref's retention policy when its snapshot moves

[1fanwang]

Rationale for this change

Refs can be created with retention settings, but two move paths rebuild the ref without carrying those settings forward: branch writes and current-snapshot moves. The next metadata update then clears max ref age, max snapshot age, and min snapshots to keep.

Fast-forward already preserves the policy. This makes branch writes, set-current-snapshot, and rollback do the same. Refs created without a policy still stay unset.

Are these changes tested?

Targeted unit regression tests fail on current main and pass with this branch. Branch write also passes against the REST catalog.

Raw logs

$ F=pyiceberg/table/update/snapshot.py
$ T=tests/table/test_manage_snapshots.py
$ git checkout origin/main -- "$F"
$ uv run pytest "$T" -k retention -q
..FFF...FFF...                                                           [100%]
E       assert None == 86400000
E       assert None == 99999
6 failed, 8 passed, 20 deselected in 4.99s

$ git checkout HEAD -- "$F"
$ uv run pytest "$T" -q
..................................                                       [100%]
34 passed in 1.20s

$ make test-integration-setup
$ I=tests/integration/test_writes/test_writes.py
$ uv run pytest "$I" -m integration -k retention -q
.                                                                        [100%]
1 passed, 123 deselected in 6.74s
$ make test-integration-cleanup

Are there any user-facing changes?

A ref keeps the retention policy it was created with when its snapshot moves. There is no API change.

AI Disclosure

AI disclosure: GitHub Copilot CLI (GPT-6 Astra) helped rebase the stale PR, resolve the current main conflict, refresh the PR description, and rerun the tests.

[Copilot]

Pull request overview

Fixes a retention-policy regression in the snapshot commit/write path: when writing a new snapshot to an existing branch, the branch’s retention configuration (max_ref_age_ms, max_snapshot_age_ms, min_snapshots_to_keep) is now preserved instead of being dropped by the emitted set-snapshot-ref update.

Changes:

• Preserve existing branch retention fields when _SnapshotProducer._commit() updates a branch ref during writes.
• Add unit tests validating retention is preserved across multiple writes and remains unset when not configured.
• Add an integration test covering the REST catalog scenario where branch retention previously regressed.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
pyiceberg/table/update/snapshot.py	Carries existing ref retention fields into the SetSnapshotRefUpdate emitted during snapshot commits to a branch.
tests/table/test_manage_snapshots.py	Adds unit coverage to ensure branch retention survives repeated writes and remains unset if not configured.
tests/integration/test_writes/test_writes.py	Adds an integration regression test for preserving branch retention on repeated writes via REST catalog.

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[github-actions]

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[github-actions]

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