util/log: use sync.Pool for formatting buffer

This was suggested by @petermattis: the free list code predates the
introduction of `sync.Pool` in the Go standard library. Using
`sync.Pool` ensures that the code becomes less sensitive to a missing
call to `putBuffer()`.

Release note: None

pkg/util/log/clog.go

@@ -17,6 +17,7 @@ import (
 	"fmt"
 	"os"
 	"runtime/debug"
+	"sync"
 	"sync/atomic"
 	"time"
 
@@ -38,13 +39,8 @@ type loggingT struct {
 	// Level flag for output to stderr. Handled atomically.
 	stderrThreshold Severity
 
-	freeList struct {
-		syncutil.Mutex
-
-		// head is a list of byte buffers, maintained under
-		// freeList.Lock().
-		head *buffer
-	}
+	// pool for entry formatting buffers.
+	bufPool sync.Pool
 
 	// interceptor is the configured InterceptorFn callback, if any.
 	interceptor atomic.Value
@@ -113,6 +109,7 @@ type loggerT struct {
 }
 
 func init() {
+	logging.bufPool.New = newBuffer
 	logging.mu.fatalCh = make(chan struct{})
 	// Default stderrThreshold and fileThreshold to log everything.
 	// This will be the default in tests unless overridden; the CLI

pkg/util/log/log_buffer.go

@@ -10,11 +10,7 @@
 
 package log
 
-import (
-	"bytes"
-
-	"github.com/knz/shakespeare/pkg/crdb/syncutil"
-)
+import "bytes"
 
 // buffer holds a byte Buffer for reuse while constructing log lines
 // prior to sending them to a sync buffer or log stream.
@@ -26,26 +22,13 @@ type buffer struct {
 	next *buffer
 }
 
-var freeList struct {
-	syncutil.Mutex
-	// head is the head of a list of byte buffers, maintained under the mutex.
-	head *buffer
-}
+// newBuffer is the constructor for the sync.Pool.
+func newBuffer() interface{} { return new(buffer) }
 
 // getBuffer returns a new, ready-to-use buffer.
 func getBuffer() *buffer {
-	freeList.Lock()
-	b := freeList.head
-	if b != nil {
-		freeList.head = b.next
-	}
-	freeList.Unlock()
-	if b == nil {
-		b = new(buffer)
-	} else {
-		b.next = nil
-		b.Reset()
-	}
+	b := logging.bufPool.Get().(*buffer)
+	b.Reset()
 	return b
 }
 
@@ -55,10 +38,7 @@ func putBuffer(b *buffer) {
 		// Let big buffers die a natural death.
 		return
 	}
-	freeList.Lock()
-	b.next = freeList.head
-	freeList.head = b
-	freeList.Unlock()
+	logging.bufPool.Put(b)
 }
 
 // Some custom tiny helper functions to print the log header efficiently.