Make InterruptHandler non-blocking for Fuchsia

[aarongreen]

Summary

The initial naive approach to simulate SIGINT on Fuchsia was to getchar and look for ETX. This caused the InterruptHandler thread to lock stdin, preventing musl's exit() from being able to close the stdio descriptors and complete. This change uses select() instead.

[phosek]

lib/fuzzer/FuzzerUtilFuchsia.cpp

53

Do you need to specify the timeout? Why not just block forever?

[aarongreen]

Yeah, I mistakenly thought select might grab the iolock on stdio in musl. Using a null timeout works fine and simplifies things.

[phosek]

LGTM, can you please upload a squashed patch?

[aarongreen]

I believe this should be the squashed diff phosek requested.

lib/fuzzer/FuzzerUtilFuchsia.cpp

 void AlarmHandler(int Seconds) {
   while (true) {
     SleepSeconds(Seconds);
     Fuzzer::StaticAlarmCallback();
   }
 }
 
 void InterruptHandler() {
+  fd_set readfds;
   // Ctrl-C sends ETX in Zircon.
-  while (getchar() != 0x03);
+  do {
+    FD_ZERO(&readfds);
+    FD_SET(STDIN_FILENO, &readfds);
+    select(STDIN_FILENO + 1, &readfds, nullptr, nullptr, nullptr);
+  } while(!FD_ISSET(STDIN_FILENO, &readfds) || getchar() != 0x03);
   Fuzzer::StaticInterruptCallback();
 }
 
 void CrashHandler(zx_handle_t *Port) {
   std::unique_ptr<zx_handle_t> ExceptionPort(Port);
   zx_port_packet_t Packet;
   _zx_port_wait(*ExceptionPort, ZX_TIME_INFINITE, &Packet, 1);
   // Unbind as soon as possible so we don't receive exceptions from this thread.