Merge pull request #2262 from WIZARD-CXY/master

Fixes go race bug in startup log buffering.

command/agent/gated_writer.go

@@ -29,15 +29,28 @@ func (w *GatedWriter) Flush() {
 }
 
 func (w *GatedWriter) Write(p []byte) (n int, err error) {
-	w.lock.RLock()
-	defer w.lock.RUnlock()
+        // Once we flush we no longer synchronize writers since there's
+        // no use of the internal buffer. This is the happy path.
+        w.lock.RLock()
+        if w.flush {
+                w.lock.RUnlock()
+                return w.Writer.Write(p)
+        }
+        w.lock.RUnlock()
 
-	if w.flush {
-		return w.Writer.Write(p)
-	}
+        // Now take the write lock.
+        w.lock.Lock()
+        defer w.lock.Unlock()
 
-	p2 := make([]byte, len(p))
-	copy(p2, p)
-	w.buf = append(w.buf, p2)
-	return len(p), nil
+        // Things could have changed between the locking operations, so we
+        // have to check one more time.
+        if w.flush {
+                return w.Writer.Write(p)
+        }
+
+        // Buffer up the written data.
+        p2 := make([]byte, len(p))
+        copy(p2, p)
+        w.buf = append(w.buf, p2)
+        return len(p), nil
 }