Strip existing delay prefix before adding a new one on retry

[afonsojanu]

Closes #2556.

calculate_routing_key always prepends a fresh 28-bit binary prefix to whatever routing key it's given, with no check for whether the key already carries one from an earlier call.

When a task is retried with a countdown, Celery pulls the routing key back out of the delivered message's delivery_info and republishes with it as-is. If that task fails and retries again with a delay, the routing key it hands back already has a prefix on it, so kombu stacks a second one on top. Each additional delayed retry adds another 28 bits plus separators, and once the accumulated key passes AMQP's 255-byte short string limit, publishing fails with error("'B' format requires 0 <= number <= 255").

This strips a leading 28-segment binary prefix, if the routing key already has one, before prepending the new one. A routing key that only looks like it might have a prefix (a single leading 0. or 1. segment, say) is left alone, since that's not the full 28-segment pattern this module ever produces.

Added two tests: one confirming a routing key that's already been through calculate_routing_key once doesn't grow on a second call, and one confirming a routing key that merely starts with a digit-dot isn't mistaken for a real prefix and stripped.

[Copilot]

Pull request overview

Prevents RabbitMQ native delayed-delivery routing keys from growing unbounded across repeated delayed retries by stripping an existing 28-bit delay prefix before adding a new one, avoiding AMQP short-string (255 byte) publish failures.

Changes:

• Strip a leading 28-segment [01]. delay prefix from routing_key before prepending the new countdown prefix.
• Add unit tests to ensure prefixes do not accumulate and that partial look-alike prefixes are not stripped.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
kombu/transport/native_delayed_delivery.py	Adds prefix-stripping logic before generating a new delayed-delivery routing key.
t/unit/transport/test_native_delayed_delivery.py	Adds tests covering non-accumulation on retry and avoiding stripping of partial look-alikes.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.15%. Comparing base (74f2d25) to head (6c0fce0).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2591   +/-   ##
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  Coverage   83.15%   83.15%           
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  Files          79       79           
  Lines       10626    10629    +3     
  Branches     1241     1241           
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+ Hits         8836     8839    +3     
  Misses       1584     1584           
  Partials      206      206           

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