[Dependency Scanning] Synchronize on updating Swift dependency lookup results in-parallel

Resolves rdar://142978967

Author: artemcm

lib/DependencyScan/ModuleDependencyScanner.cpp

@@ -742,13 +742,13 @@ ModuleDependencyScanner::resolveAllClangModuleDependencies(
   // results back to the shared set for future lookups.
   const llvm::DenseSet<clang::tooling::dependencies::ModuleID> seenClangModules =
       cache.getAlreadySeenClangModules();
-  std::mutex cacheAccessLock;
+  std::mutex lookupResultLock;
   auto scanForClangModuleDependency =
       [this, &cache, &moduleLookupResult,
-       &cacheAccessLock, &seenClangModules](Identifier moduleIdentifier) {
+       &lookupResultLock, &seenClangModules](Identifier moduleIdentifier) {
         auto moduleName = moduleIdentifier.str();
         {
-          std::lock_guard<std::mutex> guard(cacheAccessLock);
+          std::lock_guard<std::mutex> guard(lookupResultLock);
           if (cache.hasDependency(moduleName, ModuleDependencyKind::Clang))
             return;
         }
@@ -766,7 +766,7 @@ ModuleDependencyScanner::resolveAllClangModuleDependencies(
         // if looking for a module that was discovered as a transitive dependency
         // in this scan.
         {
-          std::lock_guard<std::mutex> guard(cacheAccessLock);
+          std::lock_guard<std::mutex> guard(lookupResultLock);
           moduleLookupResult.insert_or_assign(moduleName, moduleDependencies);
           if (!moduleDependencies.empty())
             cache.recordDependencies(moduleDependencies);
@@ -926,16 +926,19 @@ void ModuleDependencyScanner::resolveSwiftImportsForModule(
   for (const auto &dependsOn : moduleDependencyInfo.getModuleImports())
     moduleLookupResult.insert(
         std::make_pair(dependsOn.importIdentifier, std::nullopt));
+  std::mutex lookupResultLock;
 
   // A scanning task to query a module by-name. If the module already exists
   // in the cache, do nothing and return.
   auto scanForSwiftModuleDependency =
-      [this, &cache, &moduleLookupResult](Identifier moduleIdentifier,
-                                          bool isTestable) {
+      [this, &cache, &lookupResultLock, &moduleLookupResult](Identifier moduleIdentifier,
+                                                             bool isTestable) {
         auto moduleName = moduleIdentifier.str().str();
-        // If this is already in the cache, no work to do here
-        if (cache.hasSwiftDependency(moduleName))
-          return;
+        {
+          std::lock_guard<std::mutex> guard(lookupResultLock);
+          if (cache.hasSwiftDependency(moduleName))
+            return;
+        }
 
         auto moduleDependencies = withDependencyScanningWorker(
             [&cache, moduleIdentifier,
@@ -944,7 +947,11 @@ void ModuleDependencyScanner::resolveSwiftImportsForModule(
                   moduleIdentifier, cache.getModuleOutputPath(),
                   cache.getScanService().getPrefixMapper(), isTestable);
             });
-        moduleLookupResult.insert_or_assign(moduleName, moduleDependencies);
+
+        {
+          std::lock_guard<std::mutex> guard(lookupResultLock);
+          moduleLookupResult.insert_or_assign(moduleName, moduleDependencies);
+        }
       };
 
   // Enque asynchronous lookup tasks