Begin the shadow composer again with a backoff when an input cannot be watched yet, and pin mesh-sdk's fixed head, so a node answers candidates for the whole link (issue 1126, review of L2b round 1)

internal/selfcompose/shadow.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"sort"
 	"sync"
+	"time"
 
 	"git.novox.be/novox/mesh-sdk/go/compose/frombus"
 	"git.novox.be/novox/mesh-sdk/go/delivery"
@@ -44,6 +45,37 @@ func NewShadow(env Env, answer func(ctx context.Context, answers map[string]deli
 		answers: map[string]delivery.CandidateAnswer{}}
 }
 
+// Keep runs the shadow until ctx ends, beginning again after backoff(try) whenever it stops: an input it cannot watch
+// yet (the candidates' state not made, or the node's grant of it not loaded at a delivery) or a watch that ended is
+// tried again rather than leaving the node answering nothing for the rest of the link (review of L2b, round 1, finding
+// 2). Each distinct reason is said once.
+func (s *Shadow) Keep(ctx context.Context, backoff func(try int) time.Duration) {
+	said := ""
+	for try := 1; ctx.Err() == nil; try++ {
+		began := time.Now()
+		err := s.Run(ctx)
+		if ctx.Err() != nil {
+			return
+		}
+		if time.Since(began) > time.Minute {
+			try = 1 // it ran: a later stop begins the waits again
+		}
+		why := "it stopped"
+		if err != nil {
+			why = err.Error()
+		}
+		if why != said {
+			s.c.env.Say("this node answers no placement candidate for now, and tries again: " + why)
+			said = why
+		}
+		select {
+		case <-ctx.Done():
+			return
+		case <-time.After(backoff(try)):
+		}
+	}
+}
+
 // Run answers until ctx ends or an input's watch ends.
 func (s *Shadow) Run(ctx context.Context) error {
 	c := s.c