Fix delayed-delivery routing key prefix accumulation on retries (#2556)

[apoorvdarshan]

Summary

When RabbitMQ Native Delayed Delivery is used (Quorum Queues), a task that repeatedly calls self.retry(countdown=...) accumulates a routing-key prefix on every delayed retry. Once the accumulated routing key exceeds AMQP's 255-byte short-string limit, the retry fails with:

celery.exceptions.Reject: (error("'B' format requires 0 <= number <= 255"), False)

Root cause: kombu/transport/native_delayed_delivery.py::calculate_routing_key() unconditionally prepended a 28-bit binary prefix. On each delayed retry, Celery re-reads the already-prefixed routing key from request.delivery_info and passes it back into calculate_routing_key(), so a fresh 56-character prefix (28 bits + 28 dots) was prepended each time. After ~5 retries the key crossed 255 bytes.

Fixes #2556.

Fix

Before prepending a new prefix, strip any existing delayed-delivery prefix from the routing key. The prefix is a fixed 28-token dotted-binary pattern (MAX_NUMBER_OF_BITS_TO_USE digits, each 0/1, each terminated by a dot), matched by an anchored regex ^(?:[01]\.){28}. This keeps the routing key at a constant length across any number of retries while leaving ordinary (unprefixed) routing keys — including ones that contain dots — untouched.

Tests

Added two regression tests in t/unit/transport/test_native_delayed_delivery.py (both fail on main, pass with the fix):

• test_calculate_routing_key_with_existing_prefix — a routing key that already carries a prefix gets the old prefix stripped, not doubled.
• test_calculate_routing_key_does_not_accumulate_on_retries — repeated calls keep the key identical and <= 255 bytes.

All 22 tests in the file pass, and the surrounding transport unit suite is green (unrelated pre-existing failures only where optional broker deps are absent). flake8 and isort are clean on the changed files.

Disclosure: prepared with AI assistance; reviewed and verified locally.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.81%. Comparing base (4281680) to head (8c5a75f).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2566   +/-   ##
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  Coverage   82.81%   82.81%           
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  Files          79       79           
  Lines       10333    10336    +3     
  Branches     1187     1187           
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+ Hits         8557     8560    +3     
  Misses       1575     1575           
  Partials      201      201           

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