issue 974: the checks are queued by pull request on two work-queue streams, taken after a delivery build, dropped when judged, stopped when their key moves; the reordering and the forge-asking end are gone

cmd/mesh-builder/main.go

@@ -27,6 +27,7 @@ import (
 	"sort"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"syscall"
 	"time"
 
@@ -290,6 +291,26 @@ func answer(ctx context.Context, publisher builder.Publisher, on, workspace stri
 	if err := work.Began(ctx); err != nil {
 		fmt.Fprintf(os.Stderr, "cannot say a build started: %v\n", err)
 	}
+	// **A running check watches its pull request's key** (novox/hq ADR 0406 §4, issue 974): when the head or the trunk
+	// commit in it moves, the check is for a state nobody wants any more, and ends itself — nothing is asked of the forge
+	// (issue 726). A group's check, a try and a build watch nothing.
+	var superseded atomic.Bool
+	if w, ok := work.(interface {
+		WatchPull(context.Context, func(link.PullKey)) (func(), error)
+	}); ok && request.Check != nil {
+		stopWatching, err := w.WatchPull(building, func(k link.PullKey) {
+			superseded.Store(true)
+			say("superseded", fmt.Sprintf("the pull request's key moved to %.12s on %.12s while this checked %.12s on %.12s: stopped",
+				k.Head, k.Trunk, request.Ref, request.Check.Trunk))
+			cancel()
+		})
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "the key of %s/%s#%d could not be watched, so its check runs on if the key moves: %v\n",
+				request.Check.Owner, request.Check.Repo, request.Check.Number, err)
+		} else {
+			defer stopWatching()
+		}
+	}
 
 	result := link.BuildResult{
 		ID: request.ID, Repository: request.Repository, Path: request.Path,
@@ -382,7 +403,11 @@ func answer(ctx context.Context, publisher builder.Publisher, on, workspace stri
 	if mine != nil {
 		killed = h.returned(mine) && err != nil
 	}
-	if killed {
+	if superseded.Load() && err != nil {
+		// Ended by its own key moving on (ADR 0406 §4): said as the check's failure, as a kill is.
+		result.Failed = link.SupersededByKey
+		say("failed", link.SupersededByKey)
+	} else if killed {
 		// **Killed by hand is the outcome, whatever the build was doing** (novox/hq ADR 0219): the
 		// error it ended with is the kill's consequence, not a fault of the source.
 		result.Failed = link.KilledByHand