Add synchronization to ProtobufSchemas.

src/Formats/ProtobufSchemas.cpp

@@ -73,6 +73,7 @@ ProtobufSchemas::~ProtobufSchemas() = default;
 
 const google::protobuf::Descriptor * ProtobufSchemas::getMessageTypeForFormatSchema(const FormatSchemaInfo & info)
 {
+    std::lock_guard lock(mutex);
     auto it = importers.find(info.schemaDirectory());
     if (it == importers.end())
         it = importers.emplace(info.schemaDirectory(), std::make_unique<ImporterWithSourceTree>(info.schemaDirectory())).first;

src/Formats/ProtobufSchemas.h

@@ -4,6 +4,7 @@
 #if USE_PROTOBUF
 
 #include <memory>
+#include <mutex>
 #include <unordered_map>
 #include <base/types.h>
 #include <boost/noncopyable.hpp>
@@ -39,6 +40,7 @@ public:
 private:
     class ImporterWithSourceTree;
     std::unordered_map<String, std::unique_ptr<ImporterWithSourceTree>> importers;
+    std::mutex mutex;
 };
 
 }