[Fix] Guard AnalysisResult::m_files with a lock

Summary: For parse-on-demand, we were checking to see if a file had already
been parsed without aquiring a lock. That could result in a crash if files were
inserted during the lookup.

Test Plan: Running the command from the task resulted in a crash once in about
10 tries before the fix. After the fix I ran it 3000 times with no failures.

Reviewers: qigao, myang

Reviewed By: qigao

CC: ps, mwilliams, qigao

Differential Revision: 351734

Task ID: 778331

src/compiler/analysis/analysis_result.cpp

@@ -134,11 +134,14 @@ void AnalysisResult::parseOnDemand(const std::string &name) const {
     if (name.find(root) == 0) {
       rname = name.substr(root.length());
     }
-    if (m_files.find(rname) == m_files.end() &&
-        (m_parseOnDemand || inParseOnDemandDirs(rname)) &&
+    if ((m_parseOnDemand || inParseOnDemandDirs(rname)) &&
         Option::PackageExcludeFiles.find(rname) ==
         Option::PackageExcludeFiles.end() &&
         !Option::IsFileExcluded(rname, Option::PackageExcludePatterns)) {
+      {
+        Locker l(this);
+        if (m_files.find(rname) != m_files.end()) return;
+      }
       m_package->addSourceFile(rname.c_str());
     }
   }