rbd: add functions to parse json in mirroring status description field

Add UnmarshalDescriptionJSON method to SiteMirrorImageStatus
Add DescriptionReplayStatus method to SiteMirrorImageStatus

Both these functions are meant to help extract the JSON object that is
some times appended to the Description field in the
SiteMirrorImageStatus struct. I have only seen the JSON appear after
"replaying, " but there's little docs for this and I skimmed to code to
even find out what fields the JSON might include.  As such, the
UnmarshalDescriptionJSON tries to unmarshal the appended JSON into an
object the caller provides in case skimming the code was insufficient or
the caller has other needs. For most cases, DescriptionReplayStatus can
be used to parse the JSON into a struct containing known fields.

Signed-off-by: John Mulligan <jmulligan@redhat.com>

rbd/mirror_desc_status.go

@@ -0,0 +1,71 @@
+//go:build !nautilus && ceph_preview
+// +build !nautilus,ceph_preview
+
+package rbd
+
+// #cgo LDFLAGS: -lrbd
+// #include <stdlib.h>
+// #include <rbd/librbd.h>
+import "C"
+
+import (
+	"encoding/json"
+	"strings"
+)
+
+// MirrorDescriptionReplayStatus contains information pertaining to the status
+// of snapshot based RBD image mirroring.
+type MirrorDescriptionReplayStatus struct {
+	ReplayState              string  `json:"replay_state"`
+	RemoteSnapshotTimestamp  int64   `json:"remote_snapshot_timestamp"`
+	LocalSnapshotTimestamp   int64   `json:"local_snapshot_timestamp"`
+	SyncingSnapshotTimestamp int64   `json:"syncing_snapshot_timestamp"`
+	SyncingPercent           int     `json:"syncing_percent"`
+	BytesPerSecond           float64 `json:"bytes_per_second"`
+	BytesPerSnapshot         float64 `json:"bytes_per_snapshot"`
+	LastSnapshotSyncSeconds  int64   `json:"last_snapshot_sync_seconds"`
+	LastSnapshotBytes        int64   `json:"last_snapshot_bytes"`
+}
+
+// extractDescriptionJSON will extract one string containing a JSON object from
+// the description if one can be found.
+func (s *SiteMirrorImageStatus) extractDescriptionJSON() (string, error) {
+	start := strings.Index(s.Description, "{")
+	if start == -1 {
+		return "", ErrNotExist
+	}
+	end := strings.LastIndex(s.Description, "}")
+	if end == -1 {
+		return "", ErrNotExist
+	}
+	if start >= end {
+		return "", ErrNotExist
+	}
+	return s.Description[start : end+1], nil
+}
+
+// UnmarshalDescriptionJSON parses an embedded JSON string that may be found in
+// the description of the SiteMirrorImageStatus. It will store the result in
+// the value pointed to by v.  If no embedded JSON string is found an
+// ErrNotExist error is returned. An error may also be returned if the contents
+// can not be parsed.
+func (s *SiteMirrorImageStatus) UnmarshalDescriptionJSON(v interface{}) error {
+	desc, err := s.extractDescriptionJSON()
+	if err != nil {
+		return err
+	}
+	return json.Unmarshal([]byte(desc), v)
+}
+
+// DescriptionReplayStatus parses a MirrorDescriptionReplayStatus result out of
+// the image status description field if available. If the embedded status JSON
+// is not found or fails to parse and error will be returned.
+func (s *SiteMirrorImageStatus) DescriptionReplayStatus() (
+	*MirrorDescriptionReplayStatus, error) {
+	// ---
+	mdrs := MirrorDescriptionReplayStatus{}
+	if err := s.UnmarshalDescriptionJSON(&mdrs); err != nil {
+		return nil, err
+	}
+	return &mdrs, nil
+}