server: Add more elaborate startup messages. (#2731)

These messages based on our prep stage during XL
and prints more informative message regarding
drive information.

This change also does a much needed refactoring.

cmd/retry.go

@@ -0,0 +1,109 @@
+/*
+ * Minio Cloud Storage, (C) 2016 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 cmd
+
+import (
+	"math/rand"
+	"sync"
+	"time"
+)
+
+// lockedRandSource provides protected rand source, implements rand.Source interface.
+type lockedRandSource struct {
+	lk  sync.Mutex
+	src rand.Source
+}
+
+// Int63 returns a non-negative pseudo-random 63-bit integer as an
+// int64.
+func (r *lockedRandSource) Int63() (n int64) {
+	r.lk.Lock()
+	n = r.src.Int63()
+	r.lk.Unlock()
+	return
+}
+
+// Seed uses the provided seed value to initialize the generator to a
+// deterministic state.
+func (r *lockedRandSource) Seed(seed int64) {
+	r.lk.Lock()
+	r.src.Seed(seed)
+	r.lk.Unlock()
+}
+
+// MaxRetry is the maximum number of retries before stopping.
+var MaxRetry = 5
+
+// 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
+
+// Global random source for fetching random values.
+var globalRandomSource = rand.New(&lockedRandSource{
+	src: rand.NewSource(time.Now().UTC().UnixNano()),
+})
+
+// newRetryTimer creates a timer with exponentially increasing delays
+// until the maximum retry attempts are reached.
+func newRetryTimer(unit time.Duration, cap time.Duration, jitter float64, doneCh chan struct{}) <-chan struct{} {
+	attemptCh := make(chan struct{})
+
+	// computes the exponential backoff duration according to
+	// https://www.awsarchitectureblog.com/2015/03/backoff.html
+	exponentialBackoffWait := func(attempt int) time.Duration {
+		// normalize jitter to the range [0, 1.0]
+		if jitter < NoJitter {
+			jitter = NoJitter
+		}
+		if jitter > MaxJitter {
+			jitter = MaxJitter
+		}
+
+		//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(globalRandomSource.Float64() * float64(sleep) * jitter)
+		}
+		return sleep
+	}
+
+	go func() {
+		defer close(attemptCh)
+		var nextBackoff int
+		for {
+			select {
+			// Attempts starts.
+			case attemptCh <- struct{}{}:
+				nextBackoff++
+			case <-globalWakeupCh:
+				// Reset nextBackoff to reduce the subsequent wait and re-read
+				// format.json from all disks again.
+				nextBackoff = 0
+			case <-doneCh:
+				// Stop the routine.
+				return
+			}
+			time.Sleep(exponentialBackoffWait(nextBackoff))
+		}
+	}()
+	return attemptCh
+}