[sanitizer] On OS X, exit the forked process gracefully when login_tty fails

[kubamracek]

Summary

We're using forkpty to spawn the atos symbolizer. In some cases, login_tty (part of forkpty) can fail due to security measures (sandboxing). In this case, we should exit with a status code instead of completely crashing the spawned process. Even processing a failed CHECK() is problematic here, because we're post-fork and pre-exec where a lot of things don't work (for multithreaded processes, for OS X GUI apps, etc.).

compiler-rt/trunk/lib/sanitizer_common/sanitizer_mac.cc

int internal_forkpty(int *amaster) {
   int pid = __fork();
   if (pid == -1) {
     close(master);
     close(slave);
     return -1;
   }
   if (pid == 0) {
     close(master);
-    CHECK_EQ(login_tty(slave), 0);
+    if (login_tty(slave) != 0) {
+      // We already forked, there's not much we can do.  Let's quit.
+      Report("login_tty failed (errno %d)\n", errno);
+      internal__exit(1);
+    }
   } else {
     *amaster = master;
     close(slave);
   }
   return pid;
 }
 
 uptr internal_rename(const char *oldpath, const char *newpath) {