Merge pull request #2262 from WIZARD-CXY/master

Fixes go race bug in startup log buffering.

command/agent/gated_writer.go

@@ -29,13 +29,26 @@ func (w *GatedWriter) Flush() {
 }
 
 func (w *GatedWriter) Write(p []byte) (n int, err error) {
+        // 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()
-	defer w.lock.RUnlock()
+        if w.flush {
+                w.lock.RUnlock()
+                return w.Writer.Write(p)
+        }
+        w.lock.RUnlock()
 
+        // Now take the write lock.
+        w.lock.Lock()
+        defer w.lock.Unlock()
+
+        // 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)