[libFuzzer] Port executing system commands to OS X

[kubamracek]

Summary

1. nproc doesn't exist on OS X, let's use sysctl instead.
2. running system() on OS X takes a lock, so we can't do that on multiple threads.

[kcc]

Yay, thanks for doing this!

lib/Fuzzer/FuzzerIO.cpp

97

This code is gone, you should not need this chunk any more,

lib/Fuzzer/FuzzerUtil.cpp

65

I would like to minimize the number of ifdefs in the code.
Can we use sysctl on Linux? (If yes, just do it)

Also, if we need a simple if, something like this is better:
// in FuzzerInternal.h:
#ifdef APPLE
#define LIBFUZZER_APPLE 1
#else
#define LIBFUZZER_APPLE 0
#endif

// in the code:
command = LIBFUZZER_APPLE ? "foo_mac" : "foo_linux".

If we absolutely have to use ifdefs, make them lool like

#ifdef APPLE
int Foo() {
...
}
#else
int Foo() {
...
}

#endif

78

Assuming this code works on any Posix OS, let's just use your code and not have extra ifdefs

lib/Fuzzer/FuzzerIO.cpp

 }
 
 std::string DirPlusFile(const std::string &DirPath,
                         const std::string &FileName) {
   return DirPath + "/" + FileName;
 }
 
 void PrintFileAsBase64(const std::string &Path) {
-  std::string Cmd = "base64 -w 0 < " + Path + "; echo";
+#ifdef __APPLE__
+  std::string Cmd = "base64";
+#else
+  std::string Cmd = "base64 -w 0";
+#endif
+  Cmd += " < " + Path + "; echo";
   ExecuteCommand(Cmd);
 }
 
 void Printf(const char *Fmt, ...) {
   va_list ap;
   va_start(ap, Fmt);
   vfprintf(stderr, Fmt, ap);
   va_end(ap);
 }
 
 }  // namespace fuzzer

lib/Fuzzer/FuzzerUtil.cpp

void SetTimer(int Seconds) {
   struct sigaction sigact;
   memset(&sigact, 0, sizeof(sigact));
   sigact.sa_sigaction = AlarmHandler;
   Res = sigaction(SIGALRM, &sigact, 0);
   assert(Res == 0);
 }
 
 int NumberOfCpuCores() {
-  FILE *F = popen("nproc", "r");
+#ifdef __APPLE__
+  const char *command = "sysctl -n hw.ncpu";
+#else
+  const char *command = "nproc";
+#endif
+  FILE *F = popen(command, "r");
   int N = 0;
   fscanf(F, "%d", &N);
   fclose(F);
   return N;
 }
 
 int ExecuteCommand(const std::string &Command) {
+#ifdef __APPLE__
+  pid_t pid = fork();
+  int status = 0;
+  if (pid == -1)
+    _exit(EXIT_FAILURE);   // exec never returns
+  else if (pid > 0)
+    waitpid(pid, &status, 0);
+  else {
+    // we are the child
+    execl("/bin/bash", "/bin/bash", "-c", Command.c_str(), nullptr);
+    _exit(EXIT_FAILURE);   // exec never returns
+  }
+  return status;
+#else
   return system(Command.c_str());
+#endif
 }
 
 bool ToASCII(Unit &U) {
   bool Changed = false;
   for (auto &X : U) {
     auto NewX = X;
     NewX &= 127;
     if (!isspace(NewX) && !isprint(NewX))