Answer a panicking verb with an error, say flag errors in the answer, and leave refused logins out of D15

Read verbs now run in the serving process, where a panic would end every
call; refused logins are nobody's reconnect loop (review of hq issue 327).

cmd/mesh-controller/replay327_test.go

@@ -3,10 +3,8 @@ package main
 import (
 	"context"
 	"encoding/json"
-	"os"
 	"os/exec"
 	"path/filepath"
-	"sync"
 	"testing"
 	"time"
 
@@ -70,45 +68,21 @@ func TestReplay327(t *testing.T) {
 	}
 }
 
-// The binary a verb run as a process of its own is, built once for the tests that run one.
-var verbBinary struct {
-	once sync.Once
-	path string
-	err  error
-}
-
-// asAProcess makes a verb that runs as a process of its own run this package's binary, on the bus at url.
+// asAProcess makes a verb that runs as a process of its own run this package's binary, on the bus at url:
+// built into the test's own directory, which goes with the test.
 func asAProcess(t *testing.T, url string) {
 	t.Helper()
-	verbBinary.once.Do(func() {
-		dir, err := os.MkdirTemp("", "mesh-controller-verb-")
-		if err != nil {
-			verbBinary.err = err
-			return
-		}
-		verbBinary.path = filepath.Join(dir, "mesh-controller")
-		out, err := exec.Command("go", "build", "-o", verbBinary.path, ".").CombinedOutput()
-		if err != nil {
-			verbBinary.err = &buildError{out: string(out), err: err}
-		}
-	})
-	if verbBinary.err != nil {
-		t.Fatalf("the controller could not be built to run a verb as its own process: %v", verbBinary.err)
+	path := filepath.Join(t.TempDir(), "mesh-controller")
+	if out, err := exec.Command("go", "build", "-o", path, ".").CombinedOutput(); err != nil {
+		t.Fatalf("the controller could not be built to run a verb as its own process: %v: %s", err, out)
 	}
 	was := ownImage
-	ownImage = func() string { return verbBinary.path }
+	ownImage = func() string { return path }
 	t.Cleanup(func() { ownImage = was })
 	t.Setenv(broker.NATSVar, url)
 	t.Setenv(broker.CertificateVar, "")
 }
 
-type buildError struct {
-	out string
-	err error
-}
-
-func (e *buildError) Error() string { return e.err.Error() + ": " + e.out }
-
 // closedNames are the names of the connections the bus saw closed.
 func closedNames(t *testing.T, bus *server.Server) []string {
 	t.Helper()