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

internal/protocol/protocol.go

@@ -16,7 +16,7 @@ import (
 
 // Version is the protocol version this build speaks. Agents send it in
 // Hello. Phase-1 agents do not send the field, which reads as 0.
-const Version = 3
+const Version = 4
 
 // MinJobVersion is the lowest agent protocol version the master sends jobs to.
 const MinJobVersion = 2
@@ -26,6 +26,11 @@ const MinJobVersion = 2
 // with a digest.
 const MinPinnedRefVersion = 3
 
+// MinSelfUpdateVersion is the lowest agent protocol version the master sends a
+// job for the agent's own Compose service to. Older agents neither run the
+// update in a helper container nor resume it after restarting.
+const MinSelfUpdateVersion = 4
+
 // MaxJobOutput caps the job output an agent keeps and the master stores, in
 // bytes. Only the tail is kept. With JSON escaping (worst case 6 bytes per
 // byte) a JobResult carrying this much output still fits the 1 MiB read limit.
@@ -141,6 +146,10 @@ type JobStart struct {
 	SwarmClusterID string `json:"swarm_cluster_id,omitempty"`
 	SwarmService   string `json:"swarm_service,omitempty"`
 	Ref            string `json:"ref"`
+	// Self says the service is the agent's own. The agent runs such a job
+	// after every other queued job; whether it needs a helper container is
+	// decided by the agent itself.
+	Self bool `json:"self,omitempty"`
 }
 
 // JobProgress carries output produced since the previous JobProgress for the