Fix a race condition when querying op metadata

[rostan-t]

Category:

Bug fix (non-breaking change which fixes an issue)

Description:

When getting the ndim, dtype or layout of an operator in dynamic mode, the invocation first checks that it has no result and if that's the case uses the operator to infer metadata statically. This can cause a race condition if another thread is executing the operator because the _operator attribute of the invocation is set to None when running is complete.

Additional information:

Affected modules and functionalities:

Dynamic mode.

Key points relevant for the review:

Tests:

• Existing tests apply
• New tests added
  • Python tests
  • GTests
  • Benchmark
  • Other
• N/A

Checklist

Documentation

• Existing documentation applies
• Documentation updated
  • Docstring
  • Doxygen
  • RST
  • Jupyter
  • Other
• N/A

DALI team only

Requirements

• Implements new requirements
• Affects existing requirements
• N/A

REQ IDs: N/A

JIRA TASK: DALI-4668

[greptile-apps]

Greptile Summary

This PR fixes a race condition in Invocation._try_get_metadata (new helper): the old ndim/dtype/layout methods read self._operator after the self._results is None guard, giving another thread a window to null out _operator (in _run_impl) between those two steps. The fix captures self._operator into a local variable first, then re-checks _results — ensuring a valid reference is always held if metadata inference proceeds. A new multithreaded stress test covers the scenario directly.

Confidence Score: 5/5

Safe to merge — the fix is minimal, correct, and directly tested.

The race condition is precisely diagnosed and the fix is tight: capture-then-recheck is the standard pattern for this class of race. CPython's GIL guarantees that any thread observing self._operator = None has already observed self._results = tuple(r) (assignment ordering within the same thread is preserved), so the inner _results is None re-check is a valid sentinel. The added truthiness guard on output_desc is a minor defensive improvement. No P0/P1 issues remain.

No files require special attention.

Important Files Changed

Filename	Overview
dali/python/nvidia/dali/experimental/dynamic/_invocation.py	Extracts metadata-lookup logic into _try_get_metadata that atomically captures self._operator before re-checking self._results, correctly eliminating the race window where another thread could null out _operator between the outer _results guard and the attribute access.
dali/test/python/experimental_mode/test_multithreading.py	Adds test_parallel_op_metadata_query which hammers .layout, .dtype, and .ndim across all available threads in a tight loop, providing direct regression coverage for the fixed race condition.

Sequence Diagram

sequenceDiagram
    participant A as Thread A
    participant B as Thread B
    participant I as Invocation

    Note over A,I: Race avoided - B completes first
    A->>I: operator = self._operator
    B->>I: self._results = tuple(r)
    B->>I: self._operator = None
    A->>I: if self._results is None?
    I-->>A: False - return None safely

    Note over A,I: Race avoided - A queries first
    A->>I: operator = self._operator
    A->>I: if self._results is None?
    I-->>A: True
    A->>I: assert operator is not None
    A->>I: init_spec and return metadata

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_{Reviews (2): Last reviewed commit: "Apply review suggestions" | Re-trigger Greptile}

[rostan-t]

!build

[dali-automaton]

CI MESSAGE: [49276376]: BUILD STARTED

[jantonguirao]

[Nit] OpSpec::OutputDesc always returns a fixed 5-tuple (truthy), so the walrus check is equivalent to a non-None check here. Still, if output_desc := ... is not None: (or just unconditional indexing) would express the intent more clearly and stay correct if the API ever returns an empty container.

[rostan-t]

If OutputDesc ever returns an empty tuple, we want this to be falsy because it would otherwise result on an IndexError on the next line

[dali-automaton]

CI MESSAGE: [49276376]: BUILD FAILED

[rostan-t]

!build

[dali-automaton]

CI MESSAGE: [49295377]: BUILD STARTED

[dali-automaton]

CI MESSAGE: [49295377]: BUILD PASSED