Fix debugger hang when expanding objects with blocking property getters (#2053) - #2055
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Rich Chiodo (rchiodo) wants to merge 1 commit into
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Fix debugger hang when expanding objects with blocking property getters (#2053)#2055Rich Chiodo (rchiodo) wants to merge 1 commit into
Rich Chiodo (rchiodo) wants to merge 1 commit into
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…rs (#2053) Inspecting an object whose property getters block on a background thread (e.g. lancedb's LanceDBConnection, which dispatches to a daemon event-loop thread) hung the debugger. At a breakpoint all threads are suspended, so expanding the variable evaluated a property that waited forever on the suspended background thread -> deadlock. repr() was unaffected because it never touches the properties. Fixes: - Add PYDEVD_UNBLOCK_THREADS_ON_VARIABLES_TIMEOUT (default 3.0s). When resolving a variable's children takes too long, resume the other threads until it finishes, then re-suspend -- so the debugger recovers instead of hanging. Reuses the existing evaluate-path unblock machinery, extracted into a shared unblock_threads_on_timeout context manager. - DefaultResolver._get_py_dictionary now calls getattr a single time (try/except AttributeError) instead of hasattr + getattr, which evaluated every property/descriptor twice. Adds a regression test reproducing the deadlock via the variables request. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
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Summary
Fixes #2053.
Inspecting a
LanceDBConnectionin the debugger hung on Python 3.13. The root cause is not a slowrepr— it's a thread deadlock during variable expansion.At a breakpoint, debugpy suspends all threads (in
multi_threads_single_notificationmode), including background daemon threads.LanceDBConnectionexposes@propertygetters such asread_consistency_intervalthat dispatch work to a background asyncio event loop running in a daemon thread and block on the result (asyncio.run_coroutine_threadsafe(...).result()). When the variables view expands the object, debugpy evaluates those properties on the suspended thread, which waits forever on the also-suspended background loop thread → deadlock.This is why:
repr(db)is fast — it never touches the properties.vars(db)works — it only reads the instance__dict__, no property evaluation.db = Nonebefore the breakpoint avoids the hang.Changes
1. Recover instead of hang (the real cure)
PYDEVD_UNBLOCK_THREADS_ON_VARIABLES_TIMEOUT(default3.0s). When resolving a variable's children takes longer than the timeout, the other threads are resumed until the resolution finishes, then re-suspended — so a property that depends on another thread can complete instead of deadlocking.unblock_threads_on_timeoutcontext manager inpydevd_vars.py._run_with_unblock_threadsnow uses it (evaluate-path behavior unchanged).internal_get_variable_json(the DAP variables request).2. Dedupe double
getattrDefaultResolver._get_py_dictionarypreviously didhasattr()+getattr(), evaluating every property/descriptor twice. It now callsgetattr()a single time insidetry/except AttributeError. This halves the cost (and side effects) of expanding objects with expensive properties.Tests
_debugger_case_deadlock_thread_variables.pyandtest_debugger_case_deadlock_thread_variablesreproduce the exact scenario (a property getter blocking on a suspended background thread). The test passes with the fix and deadlocks/fails when the timeout is disabled, proving it catches the regression.test_debugger_case_deadlock_thread_eval,test_debugger_case_breakpoint_on_unblock_thread_eval,test_debugger_case_unblock_manually,test_debugger_case_deadlock_notify_evaluate_timeout,test_debugger_case_deadlock_interrupt_thread).test_hasattr_failure,test_getattr_warning, and the broader variable/resolver test set pass, confirming thegetattrdedupe is behavior-preserving.All changes are in pure-Python pydevd modules (no Cython regeneration needed).