[Lldb-commits] [lldb] [lldb] Handle SIGINT via the MainLoop signal thread (on POSIX) (PR #195959)

[Jonas Devlieghere via lldb-commits]

JDevlieghere wrote:

> The SIGURG thing confused me a bit. The commit message makes it sound like receiving a SIGINT does not interrupt a blocking syscall. This isn't true because, while the signal _callback_ runs on a different thread, the signal _handler_ (`static SignalHandler` in MainLoopPosix.cpp) runs on the thread chosen by the kernel (same as before), and it is installed without SA_RESTART, so it should interrupt syscalls. I think what's happening is (and the code comments allude to that) is that the syscall gets interrupted, but then the code checks some flags (which aren't set because we haven't called `DispatchInputInterrupt` yet), and goes back to sleep. The subsequent SIGURG yanks it out of that.

Yes, that's exactly right. The EINTR doesn't have an effect because Python's signal-check finds nothing pending yet. As you figured out, the SIGURG arrives later to give Python a second chance after the trip flag is set.

> I'm wondering if we could avoid grabbing another signal for this (SIGURG could in theory be used by some other part of the application). I think the same effect could be achieved by sending another SIGINT to the main thread (obviously, after we've restored the handler) -- and then setting a flag to ignore the next callback invocation. WDYT?

Wouldn't that be race-y with a real external SIGINT, which would look identical to the self-sent one? I guess we could check `siginfo_t.si_pid` in the MainLoop to ignore it if it's our own PID? Let me know if you prefer this over the `SIGURG` approach.

My experience with a library installing signal handlers has been nothing but disastrous, so I'd be okay with preserving the current behavior and limitations to avoid that. 

---

The crash I'm looking at is a bit more involved than what I described in the PR description. I don't think what I'm seeing is the problem you're describing. This is what the crashlog looks like: 

```
0  lldb                     0x00000001025160c4 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 56
1  lldb                     0x0000000102514784 llvm::sys::RunSignalHandlers() + 112
2  lldb                     0x00000001025169cc SignalHandler(int, __siginfo*, void*) + 300
3  libsystem_platform.dylib 0x0000000191c0e744 _sigtramp + 56
4  libsystem_pthread.dylib  0x0000000191c04888 pthread_kill + 296
5  libsystem_c.dylib        0x0000000191ad2d30 raise + 32
6  LLDB                     0x000000011d17ba0c SignalHandler(int, __siginfo*, void*) + 324
7  libsystem_platform.dylib 0x0000000191c0e744 _sigtramp + 56
8  libsystem_trace.dylib    0x0000000191932618 _os_log_impl_flatten_and_send + 2544
9  libsystem_trace.dylib    0x0000000191947668 __os_signpost_emit_impl + 212
10 libsystem_trace.dylib    0x0000000191935188 _os_signpost_emit_with_name_impl + 40
11 LLDB                     0x000000011d127e10 llvm::SignpostEmitter::startInterval(void const*, llvm::StringRef) + 160
12 LLDB                     0x000000011cfd946c lldb_private::instrumentation::Instrumenter::Instrumenter(llvm::StringRef, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>&&) + 136
13 LLDB                     0x0000000117a321a0 lldb::SBDebugger::DispatchInputInterrupt() + 200
14 lldb                     0x00000001024ffd90 sigint_handler(int) + 56
15 libsystem_platform.dylib 0x0000000191c0e744 _sigtramp + 56
16 libSystem.B.dylib        0x00000001a14ee5f4 libSystem_atfork_child + 56
17 libSystem.B.dylib        0x00000001a14ee5f4 libSystem_atfork_child + 56
18 libsystem_c.dylib        0x0000000191ab4d50 fork + 112
19 DebugSymbols             0x00000001abbc3bac CreateDSYMPathUsingShellCommand(__CFString const*, __CFString const*) + 164
20 DebugSymbols             0x00000001abbb902c CopyDSYMURLForUUIDWithOptions(__CFUUID const*, void const*, void const*, void const*, void const*) + 564
21 DebugSymbols             0x00000001abbb9114 DBGCopyFullDSYMURLForUUIDWithOptions + 48
22 LLDB                     0x0000000117ccbb54 lldb_private::SymbolLocatorDebugSymbols::LocateExecutableObjectFile(lldb_private::ModuleSpec const&) + 568
```



https://github.com/llvm/llvm-project/pull/195959