For CrashRecoveryContext::RunSafelyOnThread, propagate Darwin's PRIO_…

…DARWIN_BG to the new thread if it is

set on the calling thread.

This allows libclang's indexing threads to propagate their priority to the clang module building threads.

rdar://17459872


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211747 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/CrashRecoveryContext.h

@@ -87,6 +87,9 @@ class CrashRecoveryContext {
   /// requested stack size).
   ///
   /// See RunSafely() and llvm_execute_on_thread().
+  ///
+  /// On Darwin, if PRIO_DARWIN_BG is set on the calling thread, it will be
+  /// propagated to the new thread as well.
   bool RunSafelyOnThread(function_ref<void()>, unsigned RequestedStackSize = 0);
   bool RunSafelyOnThread(void (*Fn)(void*), void *UserData,
                          unsigned RequestedStackSize = 0) {

lib/Support/CrashRecoveryContext.cpp

@@ -332,24 +332,43 @@ const std::string &CrashRecoveryContext::getBacktrace() const {
   return CRC->Backtrace;
 }
 
-//
+// FIXME: Portability.
+static void setThreadBackgroundPriority() {
+#ifdef __APPLE__
+  setpriority(PRIO_DARWIN_THREAD, 0, PRIO_DARWIN_BG);
+#endif
+}
+
+static bool hasThreadBackgroundPriority() {
+#ifdef __APPLE__
+  return getpriority(PRIO_DARWIN_THREAD, 0) == 1;
+#else
+  return false;
+#endif
+}
 
 namespace {
 struct RunSafelyOnThreadInfo {
   function_ref<void()> Fn;
   CrashRecoveryContext *CRC;
+  bool UseBackgroundPriority;
   bool Result;
 };
 }
 
 static void RunSafelyOnThread_Dispatch(void *UserData) {
   RunSafelyOnThreadInfo *Info =
     reinterpret_cast<RunSafelyOnThreadInfo*>(UserData);
+
+  if (Info->UseBackgroundPriority)
+    setThreadBackgroundPriority();
+
   Info->Result = Info->CRC->RunSafely(Info->Fn);
 }
 bool CrashRecoveryContext::RunSafelyOnThread(function_ref<void()> Fn,
                                              unsigned RequestedStackSize) {
-  RunSafelyOnThreadInfo Info = { Fn, this, false };
+  bool UseBackgroundPriority = hasThreadBackgroundPriority();
+  RunSafelyOnThreadInfo Info = { Fn, this, UseBackgroundPriority, false };
   llvm_execute_on_thread(RunSafelyOnThread_Dispatch, &Info, RequestedStackSize);
   if (CrashRecoveryContextImpl *CRC = (CrashRecoveryContextImpl *)Impl)
     CRC->setSwitchedThread();