Run a verb from the controller's own running image, and refuse what it cannot run as a handover (hq issue 289)

The witness moves a running build aside into a directory the controller's user
cannot enter, then deletes it; a verb exec'd from os.Executable() in that window
failed with permission denied. /proc/self/exe stays valid while the process lives.
A verb that still cannot start, or arrives while the controller stops, is refused
with link.ErrHandingOver and marked retry: handing-over.

cmd/mesh-controller/seatverbs.go

@@ -740,12 +740,18 @@ func isWhyFlag(word string) bool {
 }
 
 // runVerb runs this binary with the given command line and gathers what it said.
+//
+// **This binary is the image this process runs, never the file at the path it started from**
+// (novox/hq issue 289): the node-engine's witness moves a running build aside when it places the next
+// one, into a directory only it may enter, and deletes it once the next is proved — while this process
+// may still be serving. A verb run from the path then failed with "permission denied" for the seconds
+// the old controller still answered. selfCommand runs what this process is running, wherever its file
+// went; a verb it still cannot start is refused as a handover, which the caller asks again.
 func runVerb(ctx context.Context, argv []string) (verbAnswer, error) {
-	self, err := os.Executable()
-	if err != nil {
-		return verbAnswer{}, err
+	if handingOver.Load() {
+		return verbAnswer{}, fmt.Errorf("%w: this controller is stopping and runs no new command", link.ErrHandingOver)
 	}
-	cmd := exec.CommandContext(ctx, self, argv...)
+	cmd := selfCommand(ctx, argv)
 	// The same environment: the stores' credentials, the bus, the broker — everything a command run
 	// from a shell in this container would have, because it is that.
 	cmd.Env = os.Environ()
@@ -773,6 +779,12 @@ func runVerb(ctx context.Context, argv []string) (verbAnswer, error) {
 	var exit *exec.ExitError
 	if runErr != nil && !errors.As(runErr, &exit) {
 		// Not the command refusing — the command not running at all, which is this process's fault.
+		if errors.Is(runErr, os.ErrPermission) || errors.Is(runErr, os.ErrNotExist) {
+			// Its own image unreachable: a build replaced under a process that has not yet stopped.
+			// Refused as a handover, so the caller asks the controller that follows.
+			return answer, fmt.Errorf("%w: could not run %s from this controller's own build: %v",
+				link.ErrHandingOver, strings.Join(argv, " "), runErr)
+		}
 		return answer, fmt.Errorf("could not run %s: %w", strings.Join(argv, " "), runErr)
 	}
 	return answer, nil