Pull as not paused when the pause cannot be read, and say so once, instead of ending the puller silently

A Load error ended Pull, and the modules run it in a goroutine whose error nobody reads: every
build node would stop pulling when the new pauses state could not be read (or when Pull reached
restore before Serve). A node kept from building is held by capacity 0, not by a pause.

go/buildstep/pull.go

@@ -234,7 +234,9 @@ func (p *Puller) Pull(ctx context.Context) error {
 		return errors.New("buildstep: a puller needs its source, its steps' put and its run")
 	}
 	if err := p.restore(); err != nil {
-		return err // pulling on a pause that cannot be read could build on a node paused on purpose
+		// An unreadable pause degrades to not paused, said once: a puller that stops silently is the worse failure, and a
+		// node kept from building is held by its capacity (0), not by a pause.
+		p.logf("the pause of %s cannot be read: %v; pulling as not paused", p.Node, err)
 	}
 	go p.watchCapacity() // WatchConfigServed waits for Serve, which a module starts after its pull
 	wait := p.PullWait

go/buildstep/pull_test.go

@@ -4,8 +4,10 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -282,6 +284,27 @@ func TestPullPauseOutlivesARestart(t *testing.T) {
 		t.Fatal(err)
 	}
 	waitFor(t, "the ask taken after the resume", func() bool { a, _, _ := src.Fates(); return len(a) == 1 })
+
+	t.Run("a pause that cannot be read is said and pulled as not paused", func(t *testing.T) {
+		src := NewFakeAsks()
+		var said atomic.Value
+		p := &Puller{Source: src, Language: delivery.AskGo, Node: "shanks", Put: steps.put, PausePut: st.put,
+			Load:     func(string) ([]byte, error) { return nil, errors.New("no runtime yet") },
+			PullWait: 10 * time.Millisecond, Capacity: func() int { return 2 }, Run: run,
+			Logf: func(f string, a ...any) {
+				if m := fmt.Sprintf(f, a...); strings.Contains(m, "cannot be read") {
+					said.Store(m)
+				}
+			}}
+		_ = src.PublishAsk(testAsk(delivery.AskDelivery))
+		ctx, cancel := context.WithCancel(context.Background())
+		defer cancel()
+		go p.Pull(ctx)
+		waitFor(t, "the ask taken", func() bool { a, _, _ := src.Fates(); return len(a) == 1 })
+		if m, _ := said.Load().(string); !strings.Contains(m, "pulling as not paused") {
+			t.Fatalf("the unreadable pause was not said: %q", m)
+		}
+	})
 }
 
 // 1887/A2c: a long step tells the bus it is still being worked; a holder that stops mid-step neither ends nor