[Support] Avoid concurrency hazard in signal handler registration

[bruno]

Summary

Several static functions from the signal API can be invoked simultaneously; RemoveFileOnSignal for instance can be called indirectly by multiple parallel loadModule() invocations, which might lead to the assertion:

Assertion failed: (NumRegisteredSignals < array_lengthof(RegisteredSignalInfo) && "Out of space for signal handlers!"),

function RegisterHandler, file /llvm/lib/Support/Unix/Signals.inc, line 105.

RemoveFileOnSignal calls RegisterHandlers(), which isn't currently mutex protected, leading to the behavior above. Since there are a few potentially affected static functions, this patch fix RegisterHandlers to use a mutex.

[beanz]

LGTM.

[bruno]

Committed r298871

lib/Support/Unix/Signals.inc

static void CreateSigAltStack() {
   if (sigaltstack(&AltStack, &OldAltStack) != 0)
     free(AltStack.ss_sp);
 }
 #else
 static void CreateSigAltStack() {}
 #endif
 
 static void RegisterHandlers() {
-  // We need to dereference the signals mutex during handler registration so
-  // that we force its construction. This is to prevent the first use being
-  // during handling an actual signal because you can't safely call new in a
-  // signal handler.
-  *SignalsMutex;
+  sys::SmartScopedLock<true> Guard(*SignalsMutex);
 
   // If the handlers are already registered, we're done.
   if (NumRegisteredSignals != 0) return;
 
   // Create an alternate stack for signal handling. This is necessary for us to
   // be able to reliably handle signals due to stack overflow.
   CreateSigAltStack();