[all-commits] [llvm/llvm-project] 861292: [lldb] Fix race condition in Process::WaitForProce...

[athierry-oct via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 8612926c306c5191a5fb385dd11467728c59e982
      https://github.com/llvm/llvm-project/commit/8612926c306c5191a5fb385dd11467728c59e982
  Author: athierry-oct <adrien.thierry at octasic.com>
  Date:   2025-07-15 (Tue, 15 Jul 2025)

  Changed paths:
    M lldb/include/lldb/Utility/Listener.h
    M lldb/source/Utility/Broadcaster.cpp
    M lldb/source/Utility/Listener.cpp
    M lldb/unittests/Utility/ListenerTest.cpp

  Log Message:
  -----------
  [lldb] Fix race condition in Process::WaitForProcessToStop() (#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)



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