llvm-project

8612926c - [lldb] Fix race condition in Process::WaitForProcessToStop() (#144919)

[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)