Set indexer threads' priority to ThreadPriority::Background (MaskRay#538

)

src/messages/initialize.cc

@@ -12,6 +12,7 @@
 #include "working_files.hh"
 
 #include <llvm/ADT/Twine.h>
+#include <llvm/Config/llvm-config.h>
 #include <llvm/Support/Threading.h>
 
 #include <rapidjson/document.h>
@@ -255,6 +256,12 @@ void *indexer(void *arg_) {
   delete arg;
   std::string name = "indexer" + std::to_string(idx);
   set_thread_name(name.c_str());
+  // Don't lower priority on __APPLE__. getpriority(2) says "When setting a
+  // thread into background state the scheduling priority is set to lowest
+  // value, disk and network IO are throttled."
+#if LLVM_ENABLE_THREADS && LLVM_VERSION_MAJOR >= 9 && !defined(__APPLE__)
+  set_thread_priority(ThreadPriority::Background);
+#endif
   pipeline::indexer_Main(h->manager, h->vfs, h->project, h->wfiles);
   pipeline::threadLeave();
   return nullptr;