Give each pending assignment its own condition, and keep a raised row until it clears

Second review of #150: one key per machine and module let a newer failure
be cleared in the tick that raised it, pruning could orphan an open
condition, and a build no longer waited for read as never asked although it
may still run.

internal/link/builds_nats.go

@@ -76,6 +76,11 @@ func (b *natsBuilds) Ask(ctx context.Context, request BuildRequest) error {
 	return nil
 }
 
+// ErrNotHandedOver is a Submit that failed before the build was handed to the build seat: nothing runs. Any
+// other error from Submit came after: the build may still run, and its outcome may still come (novox/hq
+// issue 325).
+var ErrNotHandedOver = errors.New("the build was not handed over")
+
 func (b *natsBuilds) Submit(ctx context.Context, request BuildRequest,
 	wait time.Duration) (BuildResult, error) {
 
@@ -84,23 +89,23 @@ func (b *natsBuilds) Submit(ctx context.Context, request BuildRequest,
 	// the same event on EVENTS, which the controller records.
 	outcomes, err := b.js.Conn().SubscribeSync(BuildOutcomeOf(b.role()))
 	if err != nil {
-		return BuildResult{}, fmt.Errorf("cannot listen for a build's outcome: %w", err)
+		return BuildResult{}, fmt.Errorf("%w: cannot listen for a build's outcome: %w", ErrNotHandedOver, err)
 	}
 	defer func() { _ = outcomes.Unsubscribe() }()
 	if err := b.js.Conn().Flush(); err != nil {
-		return BuildResult{}, err
+		return BuildResult{}, fmt.Errorf("%w: %w", ErrNotHandedOver, err)
 	}
 
 	body, err := json.Marshal(request)
 	if err != nil {
-		return BuildResult{}, err
+		return BuildResult{}, fmt.Errorf("%w: %w", ErrNotHandedOver, err)
 	}
 	// Into the role's work queue and awaited: work the bus never accepted must fail here rather than
 	// be assumed, because nothing else will ever say so.
 	publish, cancel := context.WithTimeout(ctx, 30*time.Second)
 	defer cancel()
 	if _, err := b.js.Context().Publish(BuildWorkOf(b.role()), body, nats.Context(publish)); err != nil {
-		return BuildResult{}, fmt.Errorf("cannot submit a build: %w", err)
+		return BuildResult{}, fmt.Errorf("%w: cannot submit a build: %w", ErrNotHandedOver, err)
 	}
 
 	waiting, cancelWait := context.WithTimeout(ctx, wait)