Persist the quota size for btrfs so that daemon restart keeps quota

This commit is an extension of fix for 29325 based on the review comment.
In this commit, the quota size for btrfs is kept in `/var/lib/docker/btrfs/quotas`
so that a daemon restart keeps quota.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

daemon/graphdriver/btrfs/btrfs.go

@@ -16,10 +16,12 @@ import "C"
 
 import (
 	"fmt"
+	"io/ioutil"
 	"math"
 	"os"
 	"path"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"sync"
 	"syscall"
@@ -477,6 +479,14 @@ func (d *Driver) subvolumesDirID(id string) string {
 	return path.Join(d.subvolumesDir(), id)
 }
 
+func (d *Driver) quotasDir() string {
+	return path.Join(d.home, "quotas")
+}
+
+func (d *Driver) quotasDirID(id string) string {
+	return path.Join(d.quotasDir(), id)
+}
+
 // CreateReadWrite creates a layer that is writable for use as a container
 // file system.
 func (d *Driver) CreateReadWrite(id, parent string, opts *graphdriver.CreateOpts) error {
@@ -485,6 +495,7 @@ func (d *Driver) CreateReadWrite(id, parent string, opts *graphdriver.CreateOpts
 
 // Create the filesystem with given id.
 func (d *Driver) Create(id, parent string, opts *graphdriver.CreateOpts) error {
+	quotas := path.Join(d.home, "quotas")
 	subvolumes := path.Join(d.home, "subvolumes")
 	rootUID, rootGID, err := idtools.GetRootUIDGID(d.uidMaps, d.gidMaps)
 	if err != nil {
@@ -521,9 +532,16 @@ func (d *Driver) Create(id, parent string, opts *graphdriver.CreateOpts) error {
 		if err := d.parseStorageOpt(storageOpt, driver); err != nil {
 			return err
 		}
+
 		if err := d.setStorageSize(path.Join(subvolumes, id), driver); err != nil {
 			return err
 		}
+		if err := idtools.MkdirAllAs(quotas, 0700, rootUID, rootGID); err != nil {
+			return err
+		}
+		if err := ioutil.WriteFile(path.Join(quotas, id), []byte(fmt.Sprint(driver.options.size)), 0644); err != nil {
+			return err
+		}
 	}
 
 	// if we have a remapped root (user namespaces enabled), change the created snapshot
@@ -588,6 +606,14 @@ func (d *Driver) Remove(id string) error {
 	if _, err := os.Stat(dir); err != nil {
 		return err
 	}
+	quotasDir := d.quotasDirID(id)
+	if _, err := os.Stat(quotasDir); err == nil {
+		if err := os.Remove(quotasDir); err != nil {
+			return err
+		}
+	} else if !os.IsNotExist(err) {
+		return err
+	}
 
 	// Call updateQuotaStatus() to invoke status update
 	d.updateQuotaStatus()
@@ -616,6 +642,17 @@ func (d *Driver) Get(id, mountLabel string) (string, error) {
 		return "", fmt.Errorf("%s: not a directory", dir)
 	}
 
+	if quota, err := ioutil.ReadFile(d.quotasDirID(id)); err == nil {
+		if size, err := strconv.ParseUint(string(quota), 10, 64); err == nil && size >= d.options.minSpace {
+			if err := d.subvolEnableQuota(); err != nil {
+				return "", err
+			}
+			if err := subvolLimitQgroup(dir, size); err != nil {
+				return "", err
+			}
+		}
+	}
+
 	return dir, nil
 }