[gomoripeti]

Proposed Changes

There was a bug in the CQ shared message store, the recovery process after an unclean shutdown counted every live message one too many, when it goes through every per-queue index to look for shared messages that are still referenced by any queue.

Bug introduced in commit 18acc01 included in 4.0.0

Details: rabbit_msg_store:count_msg_refs/3 registers references with ets:update_counter/4 and a default object carrying ref_count=1. The increment is applied to a freshly inserted default as well, so every message came out of dirty recovery with one reference too many. That reference was never released: after a crash, every message that was in the store at the time stayed on disk for good, with its index entry, once all queues had consumed it. Fan-out messages were affected the same way (N queues gave N+1 references).

Start the default at 0. Add a regression test, and make msg_store_v1_compat remove each message once, as its second remove of a zero-reference entry now crashes the store the way it would in production.

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[Copilot]

🟢 Approval recommended

The focused fix matches ets:update_counter/4 semantics and is covered by appropriate recovery tests.

Pull request overview

Fixes dirty recovery so each shared message-store reference is counted exactly once.

Changes:

• Initializes recovered reference counters at zero before incrementing
• Adds regression coverage for single and fan-out references
• Updates legacy compatibility expectations

File summaries

File	Description
deps/rabbit/src/rabbit_msg_store.erl	Corrects recovered reference counting
deps/rabbit/test/backing_queue_SUITE.erl	Adds regression coverage and updates compatibility tests

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Balanced

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[lhoguin]

Thank you!!

[lhoguin]

Did manual backport in #17354