Move storagedriver package to registry/storage/driver

This change is slightly more complex than previous package maves in that the
package name changed. To address this, we simply always reference the package
driver as storagedriver to avoid compatbility issues with existing code. While
unfortunate, this can be cleaned up over time.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/storage/driver/azure/blockid.go

@@ -0,0 +1,60 @@
+package azure
+
+import (
+	"encoding/base64"
+	"fmt"
+	"math/rand"
+	"sync"
+	"time"
+
+	azure "github.com/MSOpenTech/azure-sdk-for-go/clients/storage"
+)
+
+type blockIDGenerator struct {
+	pool map[string]bool
+	r    *rand.Rand
+	m    sync.Mutex
+}
+
+// Generate returns an unused random block id and adds the generated ID
+// to list of used IDs so that the same block name is not used again.
+func (b *blockIDGenerator) Generate() string {
+	b.m.Lock()
+	defer b.m.Unlock()
+
+	var id string
+	for {
+		id = toBlockID(int(b.r.Int()))
+		if !b.exists(id) {
+			break
+		}
+	}
+	b.pool[id] = true
+	return id
+}
+
+func (b *blockIDGenerator) exists(id string) bool {
+	_, used := b.pool[id]
+	return used
+}
+
+func (b *blockIDGenerator) Feed(blocks azure.BlockListResponse) {
+	b.m.Lock()
+	defer b.m.Unlock()
+
+	for _, bl := range append(blocks.CommittedBlocks, blocks.UncommittedBlocks...) {
+		b.pool[bl.Name] = true
+	}
+}
+
+func newBlockIDGenerator() *blockIDGenerator {
+	return &blockIDGenerator{
+		pool: make(map[string]bool),
+		r:    rand.New(rand.NewSource(time.Now().UnixNano()))}
+}
+
+// toBlockId converts given integer to base64-encoded block ID of a fixed length.
+func toBlockID(i int) string {
+	s := fmt.Sprintf("%029d", i) // add zero padding for same length-blobs
+	return base64.StdEncoding.EncodeToString([]byte(s))
+}