Fix race condition between signal handler and main thread.

runtime/bin/process_macos.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "bin/process.h"
 
 #include <errno.h>
 #include <fcntl.h>
 #include <signal.h>
 #include <stdio.h>
@@ skipping 81 matching lines @@
   while ((pid = TEMP_FAILURE_RETRY(waitpid(-1, &status, WNOHANG))) > 0) {
     int exit_code = 0;
     int negative = 0;
     if (WIFEXITED(status)) {
       exit_code = WEXITSTATUS(status);
     }
     if (WIFSIGNALED(status)) {
       exit_code = WTERMSIG(status);
       negative = 1;
     }
+    // Lookup the process and extract all needed information from
+    // it. The WriteToBlocking call below can cause the deletion of
+    // the process object (because this signal handler can be running
+    // on an arbitrary thread, not just the main thread) so we cannot
+    // touch it after that call.
     ProcessInfo* process = LookupProcess(pid);
+    intptr_t exit_code_fd = process->fd();
     if (process != NULL) {
       int message[3] = { pid, exit_code, negative };
       intptr_t result =
-          FDUtils::WriteToBlocking(process->fd(), &message, sizeof(message));
+          FDUtils::WriteToBlocking(exit_code_fd, &message, sizeof(message));
       if (result != sizeof(message) && errno != EPIPE) {
         perror("ExitHandler notification failed");
       }
-      TEMP_FAILURE_RETRY(close(process->fd()));
+      TEMP_FAILURE_RETRY(close(exit_code_fd));
     }
   }
   errno = entry_errno;
 }
 
 
 static void ReportChildError(int exec_control_fd) {
   // In the case of failure in the child process write the errno and
   // the OS error message to the exec control pipe and exit.
   int child_errno = errno;
@@ skipping 236 matching lines @@
   if (result == -1) {
     return false;
   }
   return true;
 }
 
 
 void Process::Exit(intptr_t id) {
   RemoveProcess(id);
 }