https://github.com/athierry-oct created https://github.com/llvm/llvm-project/pull/144919
This PR addresses a race condition encountered when using LLDB through the Python scripting interface. I'm relatively new to LLDB, so feedback is very welcome, especially if there's a more appropriate way to address this issue. ### Bug Description When running a script that repeatedly calls `debugger.GetListener().WaitForEvent()` in a loop, and at some point invokes `process.Kill()` from within that loop to terminate the session, a race condition can occur if `process.Kill()` is called around the same time a breakpoint is hit. ### Race Condition Details The issue arises when the following sequence of events happens: 1. The process's **private state** transitions to `stopped` when the breakpoint is hit. 2. `process.Kill()` calls `Process::Destroy()`, which invokes `Process::WaitForProcessToStop()`. At this point: - `private_state = stopped` - `public_state = running` (the public state has not yet been updated) 3. The **public stop event** is broadcast **before** the hijack listener is installed. 4. As a result, the stop event is delivered to the **non-hijack listener**. 5. The interrupt request sent by `Process::StopForDestroyOrDetach()` is ignored because the process is already stopped (`private_state = stopped`). 6. No public stop event reaches the hijack listener. 7. `Process::WaitForProcessToStop()` hangs waiting for a public stop event, but the event is never received. 8. `process.Kill()` times out after 20 seconds ### Fix Summary This patch modifies `Process::WaitForProcessToStop()` to ensure that any pending events in the non-hijack listener queue are processed before checking the hijack listener. This guarantees that any missed public state change events are handled, preventing the hang. ### Additional Context A discussion of this issue, including a script to reproduce the bug, can be found here: [LLDB hangs when killing process at the same time a breakpoint is hit](https://discourse.llvm.org/t/lldb-hangs-when-killing-process-at-the-same-time-a-breakpoint-is-hit) >From dfe59d058645bcf83e7956cec14ad43a7ada5349 Mon Sep 17 00:00:00 2001 From: Adrien Thierry <adrien.thie...@octasic.com> Date: Thu, 19 Jun 2025 10:00:46 -0400 Subject: [PATCH] [lldb] Fix race condition in Process::WaitForProcessToStop() This patch addresses a race condition that can occur when a script using the Python API calls SBProcess::Kill() around the same time a breakpoint is hit. If a stop event is broadcast before the hijack listener is installed, it may be delivered to the default listener and missed entirely by the hijack listener. This causes Process::WaitForProcessToStop() to hang until it times out, waiting for a public stop event that is never received. To fix this, Process::WaitForProcessToStop() now checks for pending events in the non-hijack listener before falling back to the hijack listener, ensuring that any missed public state change events are properly handled. --- lldb/source/Target/Process.cpp | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) diff --git a/lldb/source/Target/Process.cpp b/lldb/source/Target/Process.cpp index 61a3d05bc3746..c75acdb1ecd80 100644 --- a/lldb/source/Target/Process.cpp +++ b/lldb/source/Target/Process.cpp @@ -683,7 +683,18 @@ StateType Process::WaitForProcessToStop( while (state != eStateInvalid) { EventSP event_sp; - state = GetStateChangedEvents(event_sp, timeout, hijack_listener_sp); + + // A stop event may have been queued in the non-hijack listener before + // the hijack listener was installed (e.g., if a breakpoint is hit + // around the same time SBProcess::Kill() is called). Check and process + // those events first to avoid missing them. + if (PeekAtStateChangedEvents()) { + // Get event from non-hijack listener + state = GetStateChangedEvents(event_sp, timeout, nullptr); + } else { + state = GetStateChangedEvents(event_sp, timeout, hijack_listener_sp); + } + if (event_sp_ptr && event_sp) *event_sp_ptr = event_sp; _______________________________________________ lldb-commits mailing list lldb-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-commits
