[clang-doc] Protect Index with mutex during reducing and generation stage

[DiegoAstiazaran]

Summary

Idx in ClangDocContext instance was being modified by multiple threads causing a seg fault.
A mutex is added to avoid this.

[juliehockett]

LGTM

clang-tools-extra/trunk/clang-doc/tool/ClangDocMain.cpp

 #include "clang/Tooling/AllTUsExecution.h"
 #include "clang/Tooling/CommonOptionsParser.h"
 #include "clang/Tooling/Execution.h"
 #include "clang/Tooling/Tooling.h"
 #include "llvm/ADT/APFloat.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/FileSystem.h"
+#include "llvm/Support/Mutex.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/Process.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/ThreadPool.h"
 #include "llvm/Support/raw_ostream.h"
 #include <atomic>
 #include <string>
 
Exec->get()->getToolResults()->forEachResult(
         auto R = USRToBitcode.try_emplace(Key, std::vector<StringRef>());
         R.first->second.emplace_back(Value);
       });
 
   // First reducing phase (reduce all decls into one info per decl).
   llvm::outs() << "Reducing " << USRToBitcode.size() << " infos...\n";
   std::atomic<bool> Error;
   Error = false;
+  llvm::sys::Mutex IndexMutex;
   // ExecutorConcurrency is a flag exposed by AllTUsExecution.h
   llvm::ThreadPool Pool(ExecutorConcurrency == 0 ? llvm::hardware_concurrency()
                                                  : ExecutorConcurrency);
   for (auto &Group : USRToBitcode) {
     Pool.async([&]() {
       std::vector<std::unique_ptr<doc::Info>> Infos;
 
       for (auto &Bitcode : Group.getValue()) {
Pool.async([&]() {
       llvm::raw_fd_ostream InfoOS(InfoPath.get(), FileErr,
                                   llvm::sys::fs::F_None);
       if (FileErr != OK) {
         llvm::errs() << "Error opening info file: " << FileErr.message()
                      << "\n";
         return;
       }
 
+      IndexMutex.lock();
       // Add a reference to this Info in the Index
       clang::doc::Generator::addInfoToIndex(CDCtx.Idx, I);
+      IndexMutex.unlock();
 
       if (auto Err = G->get()->generateDocForInfo(I, InfoOS, CDCtx))
         llvm::errs() << toString(std::move(Err)) << "\n";
     });
   }
 
   Pool.wait();