Fix for bug dart 3639: Process.run gets stuck on Mac

runtime/bin/platform_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/platform.h"
 
 #include <crt_externs.h>
 #include <signal.h>
 #include <string.h>
 #include <unistd.h>
 
 
 bool Platform::Initialize() {
   // Turn off the signal handler for SIGPIPE as it causes the process
   // to terminate on writing to a closed pipe. Without the signal
   // handler error EPIPE is set instead.
   struct sigaction act;
   bzero(&act, sizeof(act));
   act.sa_handler = SIG_IGN;
   if (sigaction(SIGPIPE, &act, 0) != 0) {
     perror("Setting signal handler failed");
     return false;
   }
+  // Unblock SIGCHLD as waiting on spawned child process depends
+  // on successful interception of this signal.
+  sigset_t newset;
+  sigemptyset(&newset);
+  sigaddset(&newset, SIGCHLD);
+  if (sigprocmask(SIG_UNBLOCK, &newset, NULL) != 0) {
+    perror("Unblocking SIGCHLD signal failed");
+  }
+
   return true;
 }
 
 
 int Platform::NumberOfProcessors() {
   return sysconf(_SC_NPROCESSORS_ONLN);
 }
 
 
 const char* Platform::OperatingSystem() {
@@ skipping 24 matching lines @@
 }
 
 
 char* Platform::StrError(int error_code) {
   static const int kBufferSize = 1024;
   char* error = static_cast<char*>(malloc(kBufferSize));
   error[0] = '\0';
   strerror_r(error_code, error, kBufferSize);
   return error;
 }