posix_spawn should retry upon EINTR

[jfb]

Summary

We've seen cases of bots failing with:

clang: error: unable to execute command: posix_spawn failed: Interrupted system call

Add a small retry loop to posix_spawn in case this happens. Don't retry too much in case there's some systemic problem going on, but retry a few times.
rdar://problem/50181448

[jfb]

[jkorous]

Hmm, we have this utility function RetryAfterSignal() but it doesn't have maxRetries.

http://llvm.org/doxygen/namespacellvm_1_1sys.html#aea8b04d954b2cebd8a26d5d712634312

If you feel like you could make it more generic and use it here.

Anyway, the current form LGTM.

lib/Support/Unix/Program.inc

257

Nit: shouldn't it be <= or s/retries/tries/?

[jfb]

In D61096#1477806, @jkorous wrote:

Hmm, we have this utility function RetryAfterSignal() but it doesn't have maxRetries.

http://llvm.org/doxygen/namespacellvm_1_1sys.html#aea8b04d954b2cebd8a26d5d712634312

That one won't work because posix_spawn doesn't set errno, it returns the error code directly.

lib/Support/Unix/Program.inc

257

Say maxRetries is 1. First we try, it fails, we'll retry because 0 < 1, and then if it fails again we won't retry because 1 < 1 is false. So we got one try and one retry. I think you're counting the initial try as a retry whereas I'm not :-)

lib/Support/Unix/Program.inc

if (MemoryLimit == 0) {
     if (!Envp)
 #if !USE_NSGETENVIRON
       Envp = const_cast<const char **>(environ);
 #else
       // environ is missing in dylibs.
       Envp = const_cast<const char **>(*_NSGetEnviron());
 #endif
 
-    // Explicitly initialized to prevent what appears to be a valgrind false
-    // positive.
-    pid_t PID = 0;
-    int Err = posix_spawn(&PID, Program.str().c_str(), FileActions,
-                          /*attrp*/ nullptr, const_cast<char **>(Argv),
-                          const_cast<char **>(Envp));
+    constexpr int maxRetries = 8;
+    int retries = 0;
+    pid_t PID;
+    int Err;
+    do {
+      PID = 0; // Make Valgrind happy.
+      Err = posix_spawn(&PID, Program.str().c_str(), FileActions,
+    } while (Err == EINTR && ++retries < maxRetries);
 
     if (FileActions)
       posix_spawn_file_actions_destroy(FileActions);
 
     if (Err)
      return !MakeErrMsg(ErrMsg, "posix_spawn failed", Err);
 
     PI.Pid = PID;