profiling: Fix a data race in the circular buffer. (#3254)

internal/profiling/buffer/buffer.go

@@ -18,6 +18,8 @@
  *
  */
 
+// Package buffer provides a high-performant lock free implementation of a
+// circular buffer used by the profiling code.
 package buffer
 
 import (
@@ -42,7 +44,7 @@ type queue struct {
 	// proceeding forwarding with the actual write to arr. This counter is also
 	// used by the Drain operation's drainWait subroutine to wait for all pushes
 	// to complete.
-	acquired uint32
+	acquired uint32 // Accessed atomically.
 	// After the completion of a Push operation, the written counter is
 	// incremented. Also used by drainWait to wait for all pushes to complete.
 	written uint32
@@ -255,8 +257,8 @@ func (cb *CircularBuffer) Drain() []interface{} {
 
 	result := make([]interface{}, 0)
 	for i := 0; i < len(qs); i++ {
-		if qs[i].acquired < qs[i].size {
-			result = dereferenceAppend(result, qs[i].arr, 0, qs[i].acquired)
+		if acquired := atomic.LoadUint32(&qs[i].acquired); acquired < qs[i].size {
+			result = dereferenceAppend(result, qs[i].arr, 0, acquired)
 		} else {
 			result = dereferenceAppend(result, qs[i].arr, 0, qs[i].size)
 		}

internal/profiling/buffer/buffer_appengine.go

@@ -18,21 +18,26 @@
  *
  */
 
-// Appengine does not support stats because of lack of support for unsafe
+package buffer
+
+// CircularBuffer is a no-op implementation for appengine builds.
+//
+// Appengine does not support stats because of lack of the support for unsafe
 // pointers, which are necessary to efficiently store and retrieve things into
 // and from a circular buffer. As a result, Push does not do anything and Drain
 // returns an empty slice.
-package buffer
-
 type CircularBuffer struct{}
 
+// NewCircularBuffer returns a no-op for appengine builds.
 func NewCircularBuffer(size uint32) (*CircularBuffer, error) {
 	return nil, nil
 }
 
+// Push returns a no-op for appengine builds.
 func (cb *CircularBuffer) Push(x interface{}) {
 }
 
+// Drain returns a no-op for appengine builds.
 func (cb *CircularBuffer) Drain() []interface{} {
 	return nil
 }

internal/profiling/profiling.go

@@ -16,7 +16,7 @@
  *
  */
 
-// The profiling package contains two logical components: buffer.go and
+// Package profiling contains two logical components: buffer.go and
 // profiling.go. The former implements a circular buffer (a.k.a. ring buffer)
 // in a lock-free manner using atomics. This ring buffer is used by
 // profiling.go to store various statistics. For example, StreamStats is a
@@ -30,7 +30,6 @@
 // more types of measurements (such as the number of memory allocations) could
 // be measured, which might require a different type of object being pushed
 // into the circular buffer.
-
 package profiling
 
 import (