[clangd] Add thread priority lowering for MacOS as well

[ilya-biryukov]

Not an expert on MacOS threading APIs, only checked this builds on Mac.
@gribozavr could you confirm using these APIs is the right thing to do on Mac?

clangd/Threading.cpp

126

Could you add a corresponding #include directive on Mac (#include <sys/resource.h>)?

[ilya-biryukov]

clangd/Threading.cpp

125

This points into iPhoneOS docs, might be confusing.
Is there a link for the desktop version of the OS somewhere?
Could also leave it out, I'm sure setpriority is easy to google.

[gribozavr]

macOS supports pthread_setschedparam() that we call on Linux, why not reuse the same code path?

https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man3/pthread_setschedparam.3.html

[kadircet]

This one was used in a few different places in clang, for ex: https://github.com/llvm-mirror/clang/blob/master/tools/libclang/CIndex.cpp#L8713. Therefore I've used the same funcitonality.

Regarding pthread_setschedparam does setting prioirty to PTHREAD_MIN_PRIORITY on either SCHED_FIFO or SCHED_RR policy corresponds to SCHED_IDLE in linux(http://man7.org/linux/man-pages/man7/sched.7.html)?

[kadircet]

• Add required header

[kadircet]

clangd/Threading.cpp

125

It still says This document is a Mac OS X manual page. on top of the page, couldn't find any url that contains something like MacOS instead of iPhoneOS

[gribozavr]

I see, there's no SCHED_IDLE in the macOS SDK. OK then, I trust that people who wrote CIndex.cpp support for macOS probably have got it correctly working for macOS, so it makes sense to do the same in clangd. Please consider factoring a function in libSupport though.

clangd/Threading.cpp

 #if defined(__linux__) && defined(SCHED_IDLE)
   sched_param priority;
   priority.sched_priority = 0;
   pthread_setschedparam(
       pthread_self(),
       Priority == ThreadPriority::Low && !AvoidThreadStarvation ? SCHED_IDLE
                                                                 : SCHED_OTHER,
       &priority);
+#elif defined(__APPLE__)
+  // https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/getpriority.2.html
+  setpriority(PRIO_DARWIN_THREAD, 0,
+              Priority == ThreadPriority::Low && !AvoidThreadStarvation
+                  ? PRIO_DARWIN_BG
+                  : 0);
 #endif
 }
 
 void preventThreadStarvationInTests() { AvoidThreadStarvation = true; }
 
 } // namespace clangd
 } // namespace clang