fix(frontend): evict async tokenizer executors when the tokenizer is retired

[panpan0000]

Summary

Fixes #13336.

The frontend cached one make_async wrapper per tokenizer in a process-global dict keyed by id(tokenizer), with no eviction or shutdown path. Since vLLM's make_async closes over both the tokenizer and its ThreadPoolExecutor, every retired tokenizer (model/frontend churn) was pinned in memory together with its executor and worker thread for the process lifetime. Keying by id() also risked aliasing a stale wrapper when a new tokenizer reused a freed address.

The pool is now an identity-keyed weak-reference registry storing (weakref, executor) pairs. Each lookup verifies the referent identity, and a matching weak-reference callback removes the entry and calls executor.shutdown(wait=False) when the tokenizer is collected. The cheap make_async closure is rebuilt per call. Tokenizers that do not support weak references fall back to the previous process-lifetime caching behavior.

Validation

• New unit tests in tests/frontend/test_prepost_tokenizer_pool.py:
  • same tokenizer reuses a single executor and the wrapper stays functional;
  • collecting the tokenizer evicts the pool entry and calls executor.shutdown exactly once;
  • a new tokenizer after eviction gets a fresh executor (no stale id() aliasing);
  • a non-weakrefable (__slots__) tokenizer falls back to the strong pool and keeps working;
  • an in-flight tokenizer call is evicted from the worker thread with shutdown(wait=False).
• Test style follows the existing tests/frontend/test_prepost*.py conventions (HAS_VLLM guard, unit/gpu_0/pre_merge markers); assertions rely only on CPython refcounting plus gc.collect(), not on GC timing.
• pre-commit run --files ... passes (black, flake8, ruff, codespell, marker validation).

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Summary by CodeRabbit

• Bug Fixes

  • Improved asynchronous tokenizer execution management for more reliable resource cleanup.
  • Reuses execution resources for the same tokenizer and releases them when no longer needed.
  • Added fallback handling for tokenizers that do not support automatic cleanup.

• Tests

  • Added coverage for executor reuse, cleanup, recreation, shutdown, and fallback behavior.

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[github-actions]

👋 Hi panpan0000! Thank you for contributing to ai-dynamo/dynamo.

Just a reminder: The NVIDIA Test Github Validation CI runs an essential subset of the testing framework to quickly catch errors.Your PR reviewers may elect to test the changes comprehensively before approving your changes.

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[coderabbitai]

Walkthrough

The frontend now maintains one executor per tokenizer. Weak-referenceable tokenizers trigger executor shutdown after collection. Non-weak-referenceable tokenizers use a strong-reference fallback. Tests cover reuse, cleanup, recreation, and fallback behavior.

Changes

Tokenizer executor lifecycle

Layer / File(s)	Summary
Executor storage and tokenizer integration components/src/dynamo/frontend/prepost.py	The frontend stores executors by tokenizer, shuts them down through weak-reference finalizers, and uses a strong pool when weak references are unavailable. _get_async_tokenizer passes the executor to make_async.
Executor lifecycle tests tests/frontend/test_prepost_tokenizer_pool.py	The tests verify executor reuse, tokenizer results, weak-pool eviction and shutdown, fresh executor creation, and strong-pool fallback behavior.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: 🟡 Moderate · up to 5082d

The change fixes executor retention for weak-referenceable tokenizers, but the non-weak-referenceable fallback can still reuse an executor for a different tokenizer after object-ID reuse, creating a concrete correctness and resource-lifetime risk. The test cleanup should also explicitly shut down executors before clearing them, so this PR is not merge-ready until these issues are addressed.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes address issue #13336 by adding executor reuse, tokenizer-based eviction, shutdown, stale-ID prevention, and regression tests.
Out of Scope Changes check	✅ Passed	The source changes and tests directly support the linked issue and PR objectives without unrelated modifications.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Title check	✅ Passed	The title clearly identifies the main change: evicting async tokenizer executors when tokenizers are retired.
Description check	✅ Passed	The description clearly explains the issue, implementation, fallback behavior, validation, and linked issue. It does not use the template headings exactly and does not explicitly identify where the re…

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[coderabbitai]

Actionable comments posted: 2

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Inline comments:
In `@components/src/dynamo/frontend/prepost.py`:
- Around line 68-70: Update the strong fallback tokenizer pool and its access
logic to retain each tokenizer object together with its ThreadPoolExecutor,
preventing reuse of an executor after an object ID is recycled. Preserve
executor reuse for the same tokenizer, and update the fallback-pool test to
assert the stored tokenizer has the original identity.

In `@tests/frontend/test_prepost_tokenizer_pool.py`:
- Around line 67-68: Update the teardown for
test_non_weakrefable_tokenizer_falls_back_to_strong_pool to shut down every
executor from snapshots of both _ASYNC_TOKENIZER_EXECUTORS and
_STRONG_ASYNC_TOKENIZER_EXECUTORS before clearing either pool, ensuring worker
threads are reliably stopped.

🪄 Autofix

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between a7e0eac and 5082d1b.

📒 Files selected for processing (2)

• components/src/dynamo/frontend/prepost.py
• tests/frontend/test_prepost_tokenizer_pool.py

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[devin-ai-integration]

Devin Review found 2 potential issues.

[biswapanda]

P1: WeakKeyDictionary is not identity-keyed: weak references compare and hash through their referents. Two distinct tokenizer instances that implement equal value semantics will therefore share the entry, but each gets its own finalizer. When the first is collected, its finalizer shuts down the shared executor while the second tokenizer (and its async wrapper) is still live, causing later tokenization to fail. Please use an identity-based weak registry (or a weak-reference wrapper whose equality/hash are identity-only), and add a regression with two distinct but == tokenizers where one is collected before using the other.

[rmccorm4]

Hi @panpan0000, just following up on this - can you address the comments from @biswapanda above and fix the merge conflicts?

[panpan0000]

@biswapanda @rmccorm4 Thank you and fixed identity-keying issue.

1. The weak registry is now indexed by object ID but stores a weak reference and verifies ref() is tokenizer on every lookup, so distinct tokenizers with equal value semantics never share an executor and recycled IDs cannot reuse stale entries. The weak-reference callback removes only its own entry and shuts down its executor with wait=False.

2. I also added the requested regression: two distinct but equal/hash-equal tokenizers get separate executors; after the first is collected, only its executor is shut down and the second tokenizer can still tokenize successfully. Current main was merged while preserving its frontend changes, and the generated API page deleted on main was not resurrected. The focused tokenizer-pool suite passes (5 tests), and all applicable pre-commit hooks pass.

assisted by codex

[krishung5]

/ok to test 8a1cbf8

[krishung5]

nit: The PR description still describes a WeakKeyDictionary with weakref.finalize, but the latest implementation uses an identity-keyed registry with weakref callbacks. Could we update the Summary and Validation sections to match the final design?

[panpan0000]

nit: The PR description still describes a WeakKeyDictionary with weakref.finalize, but the latest implementation uses an identity-keyed registry with weakref callbacks. Could we update the Summary and Validation sections to match the final design?

fixed ,thank you @krishung5 .

[krishung5]

/ok to test 1bfab05