fix: avoid timer leaks in dsync/lsync (#9781)

At a customer setup with lots of concurrent calls
it can be observed that in newRetryTimer there
were lots of tiny alloations which are not
relinquished upon retries, in this codepath
we were only interested in re-using the timer
and use it wisely for each locker.

```
(pprof) top
Showing nodes accounting for 8.68TB, 97.02% of 8.95TB total
Dropped 1198 nodes (cum <= 0.04TB)
Showing top 10 nodes out of 79
      flat  flat%   sum%        cum   cum%
    5.95TB 66.50% 66.50%     5.95TB 66.50%  time.NewTimer
    1.16TB 13.02% 79.51%     1.16TB 13.02%  github.com/ncw/directio.AlignedBlock
    0.67TB  7.53% 87.04%     0.70TB  7.78%  github.com/minio/minio/cmd.xlObjects.putObject
    0.21TB  2.36% 89.40%     0.21TB  2.36%  github.com/minio/minio/cmd.(*posix).Walk
    0.19TB  2.08% 91.49%     0.27TB  2.99%  os.statNolog
    0.14TB  1.59% 93.08%     0.14TB  1.60%  os.(*File).readdirnames
    0.10TB  1.09% 94.17%     0.11TB  1.25%  github.com/minio/minio/cmd.readDirN
    0.10TB  1.07% 95.23%     0.10TB  1.07%  syscall.ByteSliceFromString
    0.09TB  1.03% 96.27%     0.09TB  1.03%  strings.(*Builder).grow
    0.07TB  0.75% 97.02%     0.07TB  0.75%  path.(*lazybuf).append
```

pkg/retry/retry.go

@@ -0,0 +1,133 @@
+/*
+ * Minio Cloud Storage, (C) 2020 MinIO, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package retry
+
+import (
+	"context"
+	"math"
+	"math/rand"
+	"time"
+)
+
+// MaxJitter will randomize over the full exponential backoff time
+const MaxJitter = 1.0
+
+// NoJitter disables the use of jitter for randomizing the
+// exponential backoff time
+const NoJitter = 0.0
+
+// defaultTimer implements Timer interface using time.Timer
+type defaultTimer struct {
+	timer *time.Timer
+}
+
+// C returns the timers channel which receives the current time when the timer fires.
+func (t *defaultTimer) C() <-chan time.Time {
+	return t.timer.C
+}
+
+// Start starts the timer to fire after the given duration
+// don't use this code concurrently.
+func (t *defaultTimer) Start(duration time.Duration) {
+	if t.timer == nil {
+		t.timer = time.NewTimer(duration)
+	} else {
+		t.timer.Reset(duration)
+	}
+}
+
+// Stop is called when the timer is not used anymore and resources may be freed.
+func (t *defaultTimer) Stop() {
+	if t.timer != nil {
+		t.timer.Stop()
+	}
+}
+
+// NewTimerWithJitter creates a timer with exponentially increasing delays
+// until the maximum retry attempts are reached. - this function is a fully
+// configurable version, meant for only advanced use cases. For the most part
+// one should use newRetryTimerSimple and newRetryTimer.
+func NewTimerWithJitter(ctx context.Context, unit time.Duration, cap time.Duration, jitter float64) <-chan int {
+	attemptCh := make(chan int)
+
+	// normalize jitter to the range [0, 1.0]
+	jitter = math.Max(NoJitter, math.Min(MaxJitter, jitter))
+
+	// computes the exponential backoff duration according to
+	// https://www.awsarchitectureblog.com/2015/03/backoff.html
+	exponentialBackoffWait := func(attempt int) time.Duration {
+		// 1<<uint(attempt) below could overflow, so limit the value of attempt
+		const maxAttempt = 30
+		if attempt > maxAttempt {
+			attempt = maxAttempt
+		}
+		//sleep = random_between(0, min(cap, base * 2 ** attempt))
+		sleep := unit * time.Duration(1<<uint(attempt))
+		if sleep > cap {
+			sleep = cap
+		}
+		if jitter != NoJitter {
+			sleep -= time.Duration(rand.Float64() * float64(sleep) * jitter)
+		}
+		return sleep
+	}
+
+	go func() {
+		nextBackoff := 0
+		t := &defaultTimer{}
+
+		defer func() {
+			t.Stop()
+		}()
+
+		defer close(attemptCh)
+
+		// Channel used to signal after the expiry of backoff wait seconds.
+		for {
+			select {
+			case attemptCh <- nextBackoff:
+				nextBackoff++
+			case <-ctx.Done():
+				return
+			}
+
+			t.Start(exponentialBackoffWait(nextBackoff))
+
+			select {
+			case <-ctx.Done():
+				return
+			case <-t.C():
+			}
+		}
+	}()
+
+	// Start reading..
+	return attemptCh
+}
+
+// Default retry constants.
+const (
+	defaultRetryUnit = 50 * time.Millisecond  // 50 millisecond.
+	defaultRetryCap  = 500 * time.Millisecond // 500 millisecond.
+)
+
+// NewTimer creates a timer with exponentially increasing delays
+// until the maximum retry attempts are reached. - this function is a
+// simpler version with all default values.
+func NewTimer(ctx context.Context) <-chan int {
+	return NewTimerWithJitter(ctx, defaultRetryUnit, defaultRetryCap, MaxJitter)
+}

pkg/retry/retry_test.go

@@ -0,0 +1,85 @@
+/*
+ * Minio Cloud Storage, (C) 2020 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package retry
+
+import (
+	"context"
+	"testing"
+	"time"
+)
+
+// Tests for retry timer.
+func TestRetryTimerSimple(t *testing.T) {
+	retryCtx, cancel := context.WithCancel(context.Background())
+	attemptCh := NewTimer(retryCtx)
+	i := <-attemptCh
+	if i != 0 {
+		cancel()
+		t.Fatalf("Invalid attempt counter returned should be 0, found %d instead", i)
+	}
+	i = <-attemptCh
+	if i <= 0 {
+		cancel()
+		t.Fatalf("Invalid attempt counter returned should be greater than 0, found %d instead", i)
+	}
+	cancel()
+	_, ok := <-attemptCh
+	if ok {
+		t.Fatal("Attempt counter should be closed")
+	}
+}
+
+// Test retry time with no jitter.
+func TestRetryTimerWithNoJitter(t *testing.T) {
+	retryCtx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	// No jitter
+	attemptCh := NewTimerWithJitter(retryCtx, time.Millisecond, 5*time.Millisecond, NoJitter)
+	i := <-attemptCh
+	if i != 0 {
+		cancel()
+		t.Fatalf("Invalid attempt counter returned should be 0, found %d instead", i)
+	}
+	// Loop through the maximum possible attempt.
+	for i = range attemptCh {
+		if i == 30 {
+			cancel()
+		}
+	}
+	_, ok := <-attemptCh
+	if ok {
+		t.Fatal("Attempt counter should be closed")
+	}
+}
+
+// Test retry time with Jitter greater than MaxJitter.
+func TestRetryTimerWithJitter(t *testing.T) {
+	retryCtx, cancel := context.WithCancel(context.Background())
+	// Jitter will be set back to 1.0
+	attemptCh := NewTimerWithJitter(retryCtx, time.Second, 30*time.Second, 2.0)
+	i := <-attemptCh
+	if i != 0 {
+		cancel()
+		t.Fatalf("Invalid attempt counter returned should be 0, found %d instead", i)
+	}
+	cancel()
+	_, ok := <-attemptCh
+	if ok {
+		t.Fatal("Attempt counter should be closed")
+	}
+}