PARQUET-775: Make TrackingAllocator thread-safe

Mutex is only locked around the statistics as the other parts are
thread-safe and should run in parallel if needed/possible.

Author: Uwe L. Korn <uwelk@xhochy.com>

Closes apache#190 from xhochy/PARQUET-775 and squashes the following commits:

18fabb7 [Uwe L. Korn] PARQUET-775: Make TrackingAllocator thread-safe

Change-Id: I73294821335a339879dfd6590e42ad45400a0645

cpp/src/parquet/util/mem-allocator.cc

@@ -30,17 +30,23 @@ uint8_t* TrackingAllocator::Malloc(int64_t size) {
 
   uint8_t* p = static_cast<uint8_t*>(std::malloc(size));
   if (!p) { throw ParquetException("OOM: memory allocation failed"); }
-  total_memory_ += size;
-  if (total_memory_ > max_memory_) { max_memory_ = total_memory_; }
+  {
+    std::lock_guard<std::mutex> lock(stats_mutex_);
+    total_memory_ += size;
+    if (total_memory_ > max_memory_) { max_memory_ = total_memory_; }
+  }
   return p;
 }
 
 void TrackingAllocator::Free(uint8_t* p, int64_t size) {
   if (nullptr != p && size > 0) {
-    if (total_memory_ < size) {
-      throw ParquetException("Attempting to free too much memory");
+    {
+      std::lock_guard<std::mutex> lock(stats_mutex_);
+      if (total_memory_ < size) {
+        throw ParquetException("Attempting to free too much memory");
+      }
+      total_memory_ -= size;
     }
-    total_memory_ -= size;
     std::free(p);
   }
 }

cpp/src/parquet/util/mem-allocator.h

@@ -19,6 +19,7 @@
 #define PARQUET_UTIL_MEMORY_POOL_H
 
 #include <cstdint>
+#include <mutex>
 
 #include "parquet/util/visibility.h"
 
@@ -48,6 +49,7 @@ class PARQUET_EXPORT TrackingAllocator : public MemoryAllocator {
   int64_t MaxMemory() { return max_memory_; }
 
  private:
+  std::mutex stats_mutex_;
   int64_t total_memory_;
   int64_t max_memory_;
 };