SentinelChannel: add backward-compatible fanout for mixed-version clusters

[TheCodingLand]

Commit 06a3b23 fixed SentinelChannel to format keyprefix_fanout with the actual database number (e.g. /{db}. → /0.), aligning sentinel with the base Channel behavior.

However, this changed the PUB/SUB topic that sentinel workers use for fanout (broadcast) messages, which means workers running kombu < 5.4.0 and >= 5.4.0 publish and subscribe to different Redis channels and can no longer exchange control commands (ping, shutdown, rate_limit, etc.) during rolling upgrades.

This commit adds a sentinel_fanout_compat transport option (default False) that, when explicitly enabled:

• Publishes fanout messages to both the new formatted topic (/0.exchange) and the legacy literal topic (/{db}.exchange)
• Subscribes to both topic patterns
• Deduplicates messages received on both topics using a bounded hash set

This allows mixed-version clusters to communicate during rolling upgrades. Users performing a rolling upgrade should set sentinel_fanout_compat=True in their transport_options until all workers run kombu >= 5.4.0, then remove it.

app = Celery('myapp', broker='sentinel://sentinel:26379/0',
             broker_transport_options={
                 'master_name': 'mymaster',
                 'sentinel_fanout_compat': True,  # enable during rolling upgrade
             })

Closes #2152

[codecov]

Codecov Report

❌ Patch coverage is 82.41758% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.12%. Comparing base (92006e7) to head (2230ac9).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
kombu/transport/redis.py	82.41%	5 Missing and 11 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2457      +/-   ##
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+ Coverage   81.11%   81.12%   +0.01%     
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  Files          77       77              
  Lines        9738     9828      +90     
  Branches     1098     1119      +21     
==========================================
+ Hits         7899     7973      +74     
- Misses       1631     1636       +5     
- Partials      208      219      +11     

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[Copilot]

Pull request overview

Adds an opt-in Redis Sentinel fanout compatibility mode so mixed kombu versions (pre/post 5.4.0 topic formatting change) can still exchange broadcast/control commands during rolling upgrades.

Changes:

• Introduces sentinel_fanout_compat transport option on SentinelChannel to dual-publish and dual-subscribe to both formatted (/0.exchange) and legacy (/{db}.exchange) topics.
• Implements deduplication logic to avoid double-delivery when listening on both topics.
• Adds unit tests covering default behavior, dual publish/subscribe, unsubscribe, and dedup behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
kombu/transport/redis.py	Adds sentinel_fanout_compat option and SentinelChannel logic for dual topic fanout + dedup.
t/unit/transport/test_redis.py	Adds tests validating compat option behavior, topic selection, and dedup.

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[TheCodingLand]

I updated the PR based on review feedback

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

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[Copilot]

Deduplication keys are currently just delivery_tag (or hash(raw) fallback). If two different fanout exchanges ever reuse the same delivery_tag (or if the hash fallback collides / same payload bytes are published on two exchanges), messages could be incorrectly dropped. Consider namespacing the dedup key by exchange/channel (e.g., (exchange, delivery_tag)), and for the fallback prefer a stable digest of the raw bytes plus channel instead of Python’s salted hash().

[auvipy]

@TheCodingLand can you please cross check the suggestions? and share your findings

[Copilot]

When compat is enabled, _put_fanout publishes via super()._put_fanout(...) and then opens a second conn_or_acquire() context to publish the legacy topic. This results in two separate client acquisitions per fanout publish. Consider acquiring a client once and publishing to both topics on the same connection to reduce overhead.

[auvipy]

also, please fix the merge conflict carefully

[auvipy]

please, fix the merge conflicts