feat: self-update of master and agents (#15) (#32)

internal/master/model/job.go

@@ -19,6 +19,10 @@ var (
 	// speaks protocol version 2, which cannot deploy a digest-pinned or
 	// tagless ref. errors.Is(err, ErrAgentTooOld) is true for it as well.
 	ErrAgentTooOldForPinned error = agentTooOldForPinned{}
+	// ErrAgentTooOldForSelfUpdate means the agent that runs the service speaks
+	// a protocol version before 4 and cannot update itself.
+	// errors.Is(err, ErrAgentTooOld) is true for it as well.
+	ErrAgentTooOldForSelfUpdate error = agentTooOldForSelfUpdate{}
 )
 
 type agentTooOldForPinned struct{}
@@ -29,6 +33,12 @@ func (agentTooOldForPinned) Error() string {
 
 func (agentTooOldForPinned) Is(target error) bool { return target == ErrAgentTooOld }
 
+type agentTooOldForSelfUpdate struct{}
+
+func (agentTooOldForSelfUpdate) Error() string { return "agent too old to update itself" }
+
+func (agentTooOldForSelfUpdate) Is(target error) bool { return target == ErrAgentTooOld }
+
 // NotUpdatableError says why a service cannot be updated. Reason is shown
 // in the UI as is.
 type NotUpdatableError struct {
@@ -39,11 +49,13 @@ func (e *NotUpdatableError) Error() string { return "not updatable: " + e.Reason
 
 // Reasons a service cannot be updated.
 const (
-	ReasonSelf      = "updated manually outside the system"
 	ReasonContainer = "plain containers cannot be updated"
 	ReasonNoTag     = "follows no tag"
 	ReasonRemoved   = "service no longer exists"
 	ReasonBuilt     = "built from source (build:)"
+	// ReasonSelfSwarm is for an agent's own Swarm service, which cannot be
+	// updated by the agent that runs in it.
+	ReasonSelfSwarm = "an agent's own Swarm service cannot update itself"
 
 	ReasonDrifted    = "drifted: restore or accept the running version first"
 	ReasonNotDrifted = "runs its desired version"
@@ -57,14 +69,14 @@ type MergeRefusedError struct {
 
 func (e *MergeRefusedError) Error() string { return "cannot merge: " + e.Reason }
 
-// Reasons two services cannot be merged. ReasonSelf is reused for the
-// updater itself.
+// Reasons two services cannot be merged.
 const (
 	ReasonMergeSame      = "a service cannot be merged into itself"
 	ReasonMergeLiveOld   = "the old service still exists"
 	ReasonMergeGoneNew   = "the new service no longer exists"
 	ReasonMergeKind      = "only services of the same kind can be merged"
 	ReasonMergeContainer = "plain containers cannot be merged"
+	ReasonMergeSelf      = "an agent's own service cannot be merged"
 )
 
 // JobStatus is the state of an update job.