[backport] Backport sweep for 9.1

[valkeyrie-ops]

Backport sweep for 9.1

Automated cherry-picks from PRs marked "To be backported".

Applied

Source PR	Title	Detail
#4104	Avoid offloading writes to IO threads for the slot migration export job while snapshotting.	conflicts resolved by Claude Code

AI resolution details are posted as comments on this PR when available.

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Generated by valkey-ci-agent using Claude Code.

[valkeyrie-ops]

AI conflict resolution: source PR #4104

Avoid offloading writes to IO threads for the slot migration export job while snapshotting.

1 conflicted file

Claude Summary

Both conflicts are resolved and the result matches the upstream PR exactly in size and shape (28 insertions, 9 deletions across the same 3 files). I did not run git add, per the constraints, so the test file still shows as unmerged in git status — but its contents are fully resolved with no conflict markers.

What I did:

• tests/unit/cluster/cluster-migrateslots.tcl — Both conflicts were on start_cluster header lines for the two slot-migration-max-failover-repl-bytes test blocks. The PR's only change to those lines was adding the network tag; the repl-timeout 3600 override on the incoming side came from an unrelated upstream commit that isn't on the 9.1 branch. I merged by adding just the network tag while keeping 9.1's existing overrides (no repl-timeout 3600), consistent with the other seven start_cluster headers that auto-merged cleanly with only the tag added.

• src/cluster_migrateslots.c and src/io_threads.c — Auto-merged identically to the upstream diff (skip SIGUSR1 when the child exit pipe is closed, defer snapshotting while client IO state isn't idle, and skip IO-thread write offload for slot-migration export clients). No adaptation needed.

Verification: I confirmed all symbols the new code relies on exist on 9.1 (clusterSlotMigrationShouldInstallWriteHandler, slot_migration_child_exit_pipe, io_read_state/io_write_state, CLIENT_IDLE, and the network tag which is recognized in test_helper.tcl and already used by other test files). Both C files pass gcc -fsyntax-only, and no conflict markers remain in any of the three files.

AI-resolved conflicted files

• tests/unit/cluster/cluster-migrateslots.tcl — view diff

Full backport commit diff: commit 270c167226a6.

Please review these AI resolutions for correctness before merging.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.67%. Comparing base (d27f9ba) to head (66b7144).

Additional details and impacted files

@@            Coverage Diff             @@
##              9.1    #4225      +/-   ##
==========================================
- Coverage   76.67%   76.67%   -0.01%     
==========================================
  Files         163      163              
  Lines       81286    81292       +6     
==========================================
+ Hits        62330    62334       +4     
- Misses      18956    18958       +2     

Files with missing lines	Coverage Δ
src/cluster_migrateslots.c	92.17% <100.00%> (+0.03%)	⬆️
src/io_threads.c	76.09% <100.00%> (+0.05%)	⬆️

... and 19 files with indirect coverage changes

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