Add an assertion that parallelForEach doesn't nest.

[ruiu]

Summary

Add an assertion that parallelForEach doesn't nest.

[MaskRay]

Duplicate of D60758?

lld/Common/Threads.cpp

 //===- Threads.cpp --------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "lld/Common/Threads.h"
+#include <thread>
 
 bool lld::ThreadsEnabled = true;
+thread_local bool lld::ParallelForEachRunning;

lld/include/lld/Common/Threads.h

 #define LLD_COMMON_THREADS_H
 
 #include "llvm/Support/Parallel.h"
 #include <functional>
 
 namespace lld {
 
 extern bool ThreadsEnabled;
+extern thread_local bool ParallelForEachRunning;
 
 template <typename R, class FuncTy> void parallelForEach(R &&Range, FuncTy Fn) {
+  // for_each is not reentrant, so guard against it.
+  assert(!ParallelForEachRunning);
+  ParallelForEachRunning = true;
+
   if (ThreadsEnabled)
     for_each(llvm::parallel::par, std::begin(Range), std::end(Range), Fn);
   else
     for_each(llvm::parallel::seq, std::begin(Range), std::end(Range), Fn);
+
+  ParallelForEachRunning = false;
 }
 
 inline void parallelForEachN(size_t Begin, size_t End,
                              llvm::function_ref<void(size_t)> Fn) {
+  assert(!ParallelForEachRunning);
+  ParallelForEachRunning = true;
+
   if (ThreadsEnabled)
     for_each_n(llvm::parallel::par, Begin, End, Fn);
   else
     for_each_n(llvm::parallel::seq, Begin, End, Fn);
+
+  ParallelForEachRunning = false;
 }
 
 template <typename R, class FuncTy> void parallelSort(R &&Range, FuncTy Fn) {
   if (ThreadsEnabled)
     sort(llvm::parallel::par, std::begin(Range), std::end(Range), Fn);
   else
     sort(llvm::parallel::seq, std::begin(Range), std::end(Range), Fn);
 }
 
 } // namespace lld
 
 #endif