update AsyncLogWriter::performIO() to take the vector by reference

Summary:
Update the `AsyncLogWriter::performIO()` API to take the vector of messages by
const reference, rather than by modifiable pointer.  This parameter is purely
an input parameter, and shouldn't be modified by the `performIO()` code, so it
makes more sense to take this as const reference.

Differential Revision: D21367425

fbshipit-source-id: 4bbf3a58dd48bb620101ec3982b189bea656b301

folly/logging/AsyncFileWriter.cpp

@@ -36,7 +36,7 @@ bool AsyncFileWriter::ttyOutput() const {
 }
 
 void AsyncFileWriter::writeToFile(
-    std::vector<std::string>* ioQueue,
+    const std::vector<std::string>& ioQueue,
     size_t numDiscarded) {
   // kNumIovecs controls the maximum number of strings we write at once in a
   // single writev() call.
@@ -44,10 +44,10 @@ void AsyncFileWriter::writeToFile(
   std::array<iovec, kNumIovecs> iovecs;
 
   size_t idx = 0;
-  while (idx < ioQueue->size()) {
+  while (idx < ioQueue.size()) {
     int numIovecs = 0;
-    while (numIovecs < kNumIovecs && idx < ioQueue->size()) {
-      const auto& str = (*ioQueue)[idx];
+    while (numIovecs < kNumIovecs && idx < ioQueue.size()) {
+      const auto& str = ioQueue[idx];
       iovecs[numIovecs].iov_base = const_cast<char*>(str.data());
       iovecs[numIovecs].iov_len = str.size();
       ++numIovecs;
@@ -70,7 +70,7 @@ void AsyncFileWriter::writeToFile(
 }
 
 void AsyncFileWriter::performIO(
-    std::vector<std::string>* ioQueue,
+    const std::vector<std::string>& ioQueue,
     size_t numDiscarded) {
   try {
     writeToFile(ioQueue, numDiscarded);

folly/logging/AsyncFileWriter.h

@@ -55,9 +55,11 @@ class AsyncFileWriter : public AsyncLogWriter {
   }
 
  private:
-  void writeToFile(std::vector<std::string>* ioQueue, size_t numDiscarded);
+  void writeToFile(
+      const std::vector<std::string>& ioQueue,
+      size_t numDiscarded);
 
-  void performIO(std::vector<std::string>* ioQueue, size_t numDiscarded)
+  void performIO(const std::vector<std::string>& ioQueue, size_t numDiscarded)
       override;
 
   std::string getNumDiscardedMsg(size_t numDiscarded);

folly/logging/AsyncLogWriter.cpp

@@ -78,7 +78,7 @@ void AsyncLogWriter::cleanup() {
 
   // If there are still any pending messages, flush them now.
   if (!ioQueue->empty()) {
-    performIO(ioQueue, numDiscarded);
+    performIO(*ioQueue, numDiscarded);
   }
 }
 
@@ -169,7 +169,7 @@ void AsyncLogWriter::ioThread() {
     ioCV_.notify_all();
 
     // Write the log messages now that we have released the lock
-    performIO(ioQueue, numDiscarded);
+    performIO(*ioQueue, numDiscarded);
 
     // clear() empties the vector, but the allocated capacity remains so we can
     // just reuse it without having to re-allocate in most cases.

folly/logging/AsyncLogWriter.h

@@ -136,7 +136,7 @@ class AsyncLogWriter : public LogWriter {
    * messages. This method will be called in a separate IO thread.
    */
   virtual void performIO(
-      std::vector<std::string>* logs,
+      const std::vector<std::string>& logs,
       size_t numDiscarded) = 0;
 
   void ioThread();

folly/logging/test/AsyncLogWriterTest.cpp

@@ -38,7 +38,7 @@ void handleLoggingError(
 }
 
 class NoCleanUpLogWriter : public AsyncLogWriter {
-  void performIO(std::vector<std::string>*, size_t) override {}
+  void performIO(const std::vector<std::string>&, size_t) override {}
 
   bool ttyOutput() const override {
     return false;