lock BufferLogger for reads/writes

With the new addition of the holder background scan, it's possible
for an open holder to write log messages at arbitrary times. The
TestHolder_Open/ErrIndexName test checks the contents of the output
buffer, but those contents could be changing if the background task
happens to run at the right time. Use trivial locking around that
so that this shouldn't happen.

test/logger.go

@@ -18,12 +18,14 @@ import (
 	"bytes"
 	"fmt"
 	"io/ioutil"
+	"sync"
 )
 
 // bufferLogger represents a test Logger that holds log messages
 // in a buffer for review.
 type bufferLogger struct {
 	buf *bytes.Buffer
+	mu  sync.Mutex
 }
 
 // NewBufferLogger returns a new instance of BufferLogger.
@@ -34,6 +36,8 @@ func NewBufferLogger() *bufferLogger {
 }
 
 func (b *bufferLogger) Printf(format string, v ...interface{}) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
 	s := fmt.Sprintf(format, v...)
 	_, err := b.buf.WriteString(s)
 	if err != nil {
@@ -44,5 +48,7 @@ func (b *bufferLogger) Printf(format string, v ...interface{}) {
 func (b *bufferLogger) Debugf(format string, v ...interface{}) {}
 
 func (b *bufferLogger) ReadAll() ([]byte, error) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
 	return ioutil.ReadAll(b.buf)
 }