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

internal/master/poller/poller.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"errors"
 	"log/slog"
+	"slices"
 	"strings"
 	"sync"
 	"time"
@@ -209,14 +210,29 @@ func (p *Poller) record(ctx context.Context, l lookup, svcs []model.Service, sta
 	}
 	stats.ok++
 	p.setPollError(ctx, svcs, "", stats, func(s model.Service) bool { return s.LastPollError != "" })
+	var auto []model.Service
 	for _, svc := range svcs {
 		if !p.suggest(ctx, svc, l, stats) {
 			continue
 		}
 		if autoWanted(svc, l.ref, l.digest) {
-			p.startAuto(ctx, svc.ID, l.ref, l.digest, stats)
+			auto = append(auto, svc)
 		}
 	}
+	// An agent's own service goes last: jobs that reach an agent while it
+	// replaces itself are lost, and messages to one agent arrive in order.
+	slices.SortStableFunc(auto, func(a, b model.Service) int { return selfRank(a) - selfRank(b) })
+	for _, svc := range auto {
+		p.startAuto(ctx, svc.ID, l.ref, l.digest, stats)
+	}
+}
+
+// selfRank sorts an agent's own service after all others.
+func selfRank(svc model.Service) int {
+	if svc.Self {
+		return 1
+	}
+	return 0
 }
 
 // suggest marks l.digest as the available version of svc unless the service