feat(store): drift state, move candidates and service merge (#10)

Adds services.awaiting_inventory, Service.Drifted/RunsRef/RunningRef, AcceptRunning, MoveCandidates, MergeSources and MergeService, and maps a job for a deleted service to ErrNotFound.

internal/master/model/model.go

@@ -68,9 +68,13 @@ type Service struct {
 	// not start another job for the same digest, so a rollback sticks until the
 	// registry shows a different one.
 	LastTagDigest string
-	FirstSeenAt   time.Time
-	UpdatedAt     time.Time
-	RemovedAt     *time.Time
+	// AwaitingInventory is true from a successful job until the next inventory
+	// write for the row. Until then DesiredRef is newer than ActualRef without
+	// the service having drifted.
+	AwaitingInventory bool
+	FirstSeenAt       time.Time
+	UpdatedAt         time.Time
+	RemovedAt         *time.Time
 }
 
 // Update modes of a service.
@@ -113,6 +117,63 @@ func (v ImageVersion) Ref() string {
 	return imageref.Ref{Repo: v.Repo, Tag: v.Tag, Digest: v.Digest}.String()
 }
 
+// Drifted reports whether the service runs something other than the version
+// the last job deployed. A service the updater never touched, a removed one
+// and one whose inventory has not caught up with the last job do not drift.
+func (s Service) Drifted() bool {
+	return s.DesiredRef != "" && s.RemovedAt == nil && !s.AwaitingInventory &&
+		!RunsRef(s.DesiredRef, s.ActualRef, s.ActualDigest)
+}
+
+// RunsRef reports whether a service reporting actualRef and actualDigest runs
+// the desired ref. The digest decides when both sides know one, so
+// "repo:tag@D" is run by "repo:tag" reporting digest D, and by "repo@D".
+// Otherwise the repo (compared exactly, without registry normalisation) and
+// the followed tag must be equal.
+func RunsRef(desired, actualRef, actualDigest string) bool {
+	d, a := imageref.Parse(desired), imageref.Parse(actualRef)
+	if d.Repo != a.Repo {
+		return false
+	}
+	ad := a.Digest
+	if ad == "" {
+		ad = actualDigest
+	}
+	if d.Digest != "" && ad != "" {
+		return d.Digest == ad
+	}
+	return followedTag(d) == followedTag(a)
+}
+
+// followedTag is the tag a ref follows: a bare image follows "latest" and a
+// ref with a digest but no tag follows nothing.
+func followedTag(r imageref.Ref) string {
+	if r.Tag == "" && r.Digest == "" {
+		return "latest"
+	}
+	return r.Tag
+}
+
+// RunningRef is the ref to record as the desired one when the running version
+// is accepted: actualRef itself if it carries a digest, "repo:tag@digest" if
+// only actualDigest is known (a bare image is "latest"), else actualRef.
+func RunningRef(actualRef, actualDigest string) string {
+	r := imageref.Parse(actualRef)
+	if r.Digest != "" || !imageref.IsDigest(actualDigest) {
+		return actualRef
+	}
+	r.Tag, r.Digest = followedTag(r), actualDigest
+	return r.String()
+}
+
+// Move says that the removed service FromID has probably moved to the live
+// service ToID on another agent. The names are the agents' own; a revoked
+// agent keeps its name.
+type Move struct {
+	FromID, ToID       int64
+	FromAgent, ToAgent string
+}
+
 // DisplayName is the human-facing name of the service.
 func (s Service) DisplayName() string {
 	switch s.Kind {