cephfs: added support for snapshot-backed volumes

This commit implements most of
docs/design/proposals/cephfs-snapshot-shallow-ro-vol.md design document;
specifically (de-)provisioning of snapshot-backed volumes, mounting such
volumes as well as mounting pre-provisioned snapshot-backed volumes.

Signed-off-by: Robert Vasek <robert.vasek@cern.ch>

internal/cephfs/store/backingsnapshot.go

@@ -0,0 +1,168 @@
+/*
+Copyright 2022 The Ceph-CSI Authors.
+
+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 store
+
+import (
+	"context"
+	"fmt"
+
+	fsutil "github.com/ceph/ceph-csi/internal/cephfs/util"
+	"github.com/ceph/ceph-csi/internal/util/log"
+	"github.com/ceph/ceph-csi/internal/util/reftracker"
+	"github.com/ceph/ceph-csi/internal/util/reftracker/radoswrapper"
+	"github.com/ceph/ceph-csi/internal/util/reftracker/reftype"
+)
+
+func fmtBackingSnapshotReftrackerName(backingSnapID string) string {
+	return fmt.Sprintf("rt-backingsnapshot-%s", backingSnapID)
+}
+
+func AddSnapshotBackedVolumeRef(
+	ctx context.Context,
+	volOptions *VolumeOptions,
+) error {
+	ioctx, err := volOptions.conn.GetIoctx(volOptions.MetadataPool)
+	if err != nil {
+		log.ErrorLog(ctx, "failed to create RADOS ioctx: %s", err)
+
+		return err
+	}
+	defer ioctx.Destroy()
+
+	ioctx.SetNamespace(fsutil.RadosNamespace)
+
+	var (
+		backingSnapID = volOptions.BackingSnapshotID
+		ioctxW        = radoswrapper.NewIOContext(ioctx)
+	)
+
+	created, err := reftracker.Add(
+		ioctxW,
+		fmtBackingSnapshotReftrackerName(backingSnapID),
+		map[string]struct{}{
+			backingSnapID:    {},
+			volOptions.VolID: {},
+		},
+	)
+	if err != nil {
+		log.ErrorLog(ctx, "failed to add refs for backing snapshot %s: %v",
+			backingSnapID, err)
+
+		return err
+	}
+
+	defer func() {
+		if err == nil {
+			return
+		}
+
+		// Clean up after failure.
+
+		var deleted bool
+		deleted, err = reftracker.Remove(
+			ioctxW,
+			fmtBackingSnapshotReftrackerName(backingSnapID),
+			map[string]reftype.RefType{
+				backingSnapID:    reftype.Normal,
+				volOptions.VolID: reftype.Normal,
+			},
+		)
+		if err != nil {
+			log.ErrorLog(ctx, "failed to remove refs in cleanup procedure for backing snapshot %s: %v",
+				backingSnapID, err)
+		}
+
+		if created && !deleted {
+			log.ErrorLog(ctx, "orphaned reftracker object %s (pool %s, namespace %s)",
+				backingSnapID, volOptions.MetadataPool, fsutil.RadosNamespace)
+		}
+	}()
+
+	// There may have been a race between adding a ref to the reftracker and
+	// deleting the backing snapshot. Make sure the snapshot still exists by
+	// trying to retrieve it again.
+	_, _, _, err = NewSnapshotOptionsFromID(ctx, volOptions.BackingSnapshotID, volOptions.conn.Creds)
+	if err != nil {
+		log.ErrorLog(ctx, "failed to get backing snapshot %s: %v", volOptions.BackingSnapshotID, err)
+	}
+
+	return err
+}
+
+func UnrefSnapshotBackedVolume(
+	ctx context.Context,
+	volOptions *VolumeOptions,
+) (bool, error) {
+	ioctx, err := volOptions.conn.GetIoctx(volOptions.MetadataPool)
+	if err != nil {
+		log.ErrorLog(ctx, "failed to create RADOS ioctx: %s", err)
+
+		return false, err
+	}
+	defer ioctx.Destroy()
+
+	ioctx.SetNamespace(fsutil.RadosNamespace)
+
+	var (
+		backingSnapID = volOptions.BackingSnapshotID
+		ioctxW        = radoswrapper.NewIOContext(ioctx)
+	)
+
+	deleted, err := reftracker.Remove(
+		ioctxW,
+		fmtBackingSnapshotReftrackerName(backingSnapID),
+		map[string]reftype.RefType{
+			volOptions.VolID: reftype.Normal,
+		},
+	)
+	if err != nil {
+		log.ErrorLog(ctx, "failed to remove refs for backing snapshot %s: %v",
+			backingSnapID, err)
+
+		return false, err
+	}
+
+	return deleted, err
+}
+
+// UnrefSelfInSnapshotBackedVolumes removes (masks) snapshot ID in the
+// reftracker for volumes backed by this snapshot. The returned boolean
+// value signals whether the snapshot is not referenced by any such volumes
+// and needs to be removed.
+func UnrefSelfInSnapshotBackedVolumes(
+	ctx context.Context,
+	snapParentVolOptions *VolumeOptions,
+	snapshotID string,
+) (bool, error) {
+	ioctx, err := snapParentVolOptions.conn.GetIoctx(snapParentVolOptions.MetadataPool)
+	if err != nil {
+		log.ErrorLog(ctx, "failed to create RADOS ioctx: %s", err)
+
+		return false, err
+	}
+	defer ioctx.Destroy()
+
+	ioctx.SetNamespace(fsutil.RadosNamespace)
+
+	return reftracker.Remove(
+		radoswrapper.NewIOContext(ioctx),
+		fmtBackingSnapshotReftrackerName(snapshotID),
+		map[string]reftype.RefType{
+			snapshotID: reftype.Mask,
+		},
+	)
+}