[llvm] [llvm-exegesis] Kill process that recieve a signal (PR #86069)

[Clement Courbet via llvm-commits]

================
@@ -342,7 +342,7 @@ class SubProcessFunctionExecutorImpl
       return make_error<Failure>("Failed to attach to the child process: " +
                                  Twine(strerror(errno)));
 
-    if (wait(NULL) == -1) {
+    if (waitpid(ParentOrChildPID, NULL, 0) == -1) {
----------------
legrosbuffle wrote:

At that point `ParentOrChildPID` is always the parent, but it's not immediately obvious (I had to scroll up). What about restructuring this function to have:

```

[[noreturn]] void runChildSubprocess(int ReadFD, int FriteFD) {
   // We are in the child process, close the write end of the pipe.
      close(PipeFiles[1]);
      // Unregister handlers, signal handling is now handled through ptrace in
      // the host process.
      sys::unregisterHandlers();
      prepareAndRunBenchmark(PipeFiles[0], Key);
      llvm_unreachable("Child process didn't exit when expected.");
}

Error runParentSubprocess(pid_t PID, int ReadFD, int FriteFD) {
    const ExegesisTarget &ET = State.getExegesisTarget();
    ...
}


createSubProcessAndRunBenchmark() {
  ...
  if (ParentOrChildPID == -1) {
    ...
  }
  
  if (ParentOrChildPID == 0) {
     runChildSubprocess(PipeFiles[0], Pipefiles[1]);
      llvm_unreachable("Child process didn't exit when expected.");
  }

  return runParentSubprocess(ParentOrChildPID, PipeFiles[0], Pipefiles[1]);
}
```


https://github.com/llvm/llvm-project/pull/86069