Minor fixes on the MLIR ActionProfiler (NFC)

Ensure the stream flushed to the string before acquiring the mutext.
No need to flush the output stream, the goal of the mutex is to sync
ahead before content is added to the stream.

Author: joker-eph

mlir/include/mlir/Debug/Observers/ActionProfiler.h

@@ -41,7 +41,7 @@ struct ActionProfiler : public ExecutionContext::Observer {
   std::chrono::time_point<std::chrono::steady_clock> startTime;
   bool printComma = false;
 
-  /// A mutex used to guard profiling.
+  /// A mutex used to guard printing from multiple threads.
   std::mutex mutex;
 };
 

mlir/lib/Debug/Observers/ActionProfiler.cpp

@@ -36,6 +36,8 @@ void ActionProfiler::print(const ActionActiveStack *action,
   // Create the event.
   std::string str;
   llvm::raw_string_ostream event(str);
+  if (printComma)
+    event << ",\n";
   event << "{";
   event << R"("name": ")" << action->getAction().getTag() << "\", ";
   event << R"("cat": "PERF", )";
@@ -52,12 +54,11 @@ void ActionProfiler::print(const ActionActiveStack *action,
     event << "\"}";
   }
   event << "}";
+  event.flush();
 
-  // Print the event.
+  // Print the event, guard with a mutex to ensure the stream is correctly
+  // formed.
   std::lock_guard<std::mutex> guard(mutex);
-  if (printComma)
-    os << ",\n";
   printComma = true;
   os << event.str();
-  os.flush();
 }