Made security review, applied recommendations.

docs/storage-transfer.md

@@ -103,17 +103,23 @@ The containers run with the following settings:
 |---|---|---|
 | `imds-emulator` | Image | built from `sts-agent/Dockerfile`: a static Go binary on `gcr.io/distroless/static-debian13` |
 | | Port | `8080`, set by `IMDS_PORT` |
+| | Bind address | all interfaces, set by `IMDS_HOST` |
 | | Mount | `/etc/sts-agent`, read only |
 | | Environment | `GOOGLE_APPLICATION_CREDENTIALS=/etc/sts-agent/cred.json` |
 | | Environment | `GOOGLE_API_CERTIFICATE_CONFIG=/etc/sts-agent/certificate_config.json` |
 | | Environment | `PROJECT_ID`, `PROJECT_NUMBER` |
-| `sts-agent` | Image | `gcr.io/cloud-ingest/tsop-agent:latest` |
+| | Health check | `/imds-emulator -check` every 10 seconds |
+| | Restart policy | `unless-stopped` |
+| | Stop grace period | 30 seconds |
+| `agent` | Image | `gcr.io/cloud-ingest/tsop-agent:latest` |
+| | Start condition | the health check of `imds-emulator` passes |
+| | Restart policy | `unless-stopped` |
 | | Mount | `/mnt/ext-storage`, read only |
 | | Environment | `GCE_METADATA_HOST=imds-emulator:8080`, `GCE_METADATA_IP=imds-emulator:8080` |
 | | Locked memory limit | `64000000` bytes |
 | | Arguments | `--project-id=${PROJECT_ID} --hostname=$(hostname) --agent-pool=${STS_AGENT_POOL}` |
 
-The emulator publishes no port; only the containers of the Compose project reach it. The agent container must not set `GOOGLE_API_CERTIFICATE_CONFIG`, because the metadata token code of the agent parses the certificate it points to with `cryptography`.
+The emulator publishes no port; only the containers of the Compose project reach it. It does not authenticate its clients beyond the `Metadata-Flavor` header, as the Compute Engine metadata server does, so every container of the Compose project can obtain the tokens of `sa-test`. When it runs outside Compose, set `IMDS_HOST=127.0.0.1` to bind the loopback interface only. The agent container must not set `GOOGLE_API_CERTIFICATE_CONFIG`, because the metadata token code of the agent parses the certificate it points to with `cryptography`.
 
 `sts-agent/compose.yaml` holds these settings for Docker Compose. Compose substitutes `PROJECT_ID`, `PROJECT_NUMBER`, `STS_AGENT_POOL` and `HOSTNAME` from `sts-agent/.env`. Create the file with the values of `env.sh` and the host name printed by `hostname`:
 
@@ -134,7 +140,13 @@ docker compose build
 docker compose up -d
 ```
 
-The emulator logs one line per request with the path and the status, never the token:
+The agent starts after the emulator reports healthy. The state is shown by:
+
+```bash
+docker compose ps
+```
+
+The emulator logs one line per request with the path and the status, never the query string or the token:
 
 ```bash
 docker compose logs imds-emulator
@@ -142,9 +154,11 @@ docker compose logs imds-emulator
 
 Requests for paths that the emulator does not serve are logged with the status 404.
 
-The emulator is a static binary without dependencies on files of the system: it embeds the root certificates and the time zone data, and it is the first process (PID 1) of its container. On `SIGTERM` or `SIGINT`, which `docker compose stop` sends, it stops accepting connections, lets the requests in progress finish for up to 5 seconds, and exits with status 0. A second signal makes it exit at once.
+The emulator is a static binary without dependencies on files of the system: it embeds the root certificates and the time zone data, and it is the first process (PID 1) of its container. On `SIGTERM` or `SIGINT`, which `docker compose stop` sends, it stops accepting connections, lets the requests in progress finish for up to 20 seconds, and exits with status 0. A second signal makes it exit with status 1 at once. The stop grace period of the container is 30 seconds, longer than the shutdown of the emulator.
 
-The unit tests of the emulator, `sts-agent/main_test.go`, cover the request checks, the metadata endpoints, the token response, the extraction of the service account email from the credentials file, the shutdown, and a run of the emulator as a process stopped with real signals. They run during the build of the image, so a failing test stops it. To run them on a workstation with Go:
+The emulator keeps at most 16 token providers, one per set of scopes requested, and drops them all when the limit is reached.
+
+The unit tests of the emulator, `sts-agent/main_test.go`, cover the request checks, the metadata endpoints, the scope parsing, the token response, the request log, the extraction of the service account email from the credentials file, the shutdown, the check mode, and runs of the emulator as a process stopped with real signals. They run during the build of the image, so a failing test stops it. To run them on a workstation with Go:
 
 ```bash
 cd sts-agent

sts-agent/.dockerignore

@@ -0,0 +1,5 @@
+*
+!go.mod
+!go.sum
+!main.go
+!main_test.go

sts-agent/Dockerfile

@@ -1,13 +1,14 @@
-FROM golang:1.27-trixie AS build
+FROM golang:1.27.3-trixie AS build
 
+ENV GOTOOLCHAIN=local CGO_ENABLED=0
 WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download
 COPY main.go main_test.go ./
-RUN CGO_ENABLED=0 go test ./... \
+RUN go test ./... \
-    && CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o /imds-emulator .
+    && go build -trimpath -ldflags="-s -w" -o /imds-emulator .
 
-FROM gcr.io/distroless/static-debian13
+FROM gcr.io/distroless/static-debian13:latest
 
 COPY --from=build /imds-emulator /imds-emulator
 

sts-agent/compose.yaml

@@ -2,6 +2,8 @@ name: sts
 services:
   imds-emulator:
     build: .
+    restart: unless-stopped
+    stop_grace_period: 30s
     volumes:
       - /etc/sts-agent:/etc/sts-agent:ro
     environment:
@@ -9,11 +11,20 @@ services:
       GOOGLE_API_CERTIFICATE_CONFIG: /etc/sts-agent/certificate_config.json
       PROJECT_ID: ${PROJECT_ID}
       PROJECT_NUMBER: ${PROJECT_NUMBER}
+    healthcheck:
+      test: ["CMD", "/imds-emulator", "-check"]
+      interval: 10s
+      timeout: 3s
+      start_period: 10s
+      start_interval: 1s
+      retries: 3
 
   agent:
     image: gcr.io/cloud-ingest/tsop-agent:latest
+    restart: unless-stopped
     depends_on:
-      - imds-emulator
+      imds-emulator:
+        condition: service_healthy
     ulimits:
       memlock: 64000000
     volumes:

sts-agent/go.mod

@@ -1,6 +1,6 @@
 module imds-emulator
 
-go 1.27.1
+go 1.27.3
 
 require (
 	cloud.google.com/go/auth v0.24.1

sts-agent/main.go

@@ -6,6 +6,8 @@ import (
 	"cmp"
 	"context"
 	"encoding/json"
+	"flag"
+	"fmt"
 	"io"
 	"log"
 	"net"
@@ -31,8 +33,11 @@ const (
 	accounts       = "instance/service-accounts/"
 	defaultAccount = accounts + "default/"
 
-	// shutdownGrace is shorter than the 10 seconds Docker waits after SIGTERM.
+	// shutdownGrace is shorter than the stop grace period of the container.
-	shutdownGrace = 5 * time.Second
+	shutdownGrace = 20 * time.Second
+
+	// maxProviders bounds the token providers kept for distinct scope sets.
+	maxProviders = 16
 )
 
 // tokenFunc returns an access token for the scopes.
@@ -96,11 +101,7 @@ func (s *metadataServer) handle(w http.ResponseWriter, r *http.Request) int {
 
 	switch {
 	case path == defaultAccount+"token":
-		scopes := []string{defaultScope}
+		token, err := s.token(r.Context(), parseScopes(query.Get("scopes")))
-		if value := query.Get("scopes"); value != "" {
-			scopes = strings.Split(value, ",")
-		}
-		token, err := s.token(r.Context(), scopes)
 		if err != nil {
 			log.Printf("token error: %v", err)
 			return text(w, http.StatusInternalServerError, "Internal Server Error")
@@ -138,11 +139,22 @@ func (s *metadataServer) handle(w http.ResponseWriter, r *http.Request) int {
 	return text(w, http.StatusNotFound, "Not Found")
 }
 
+// parseScopes returns the comma separated scopes of the query, or the default scope.
+func parseScopes(value string) []string {
+	scopes := slices.DeleteFunc(strings.Split(value, ","), func(scope string) bool { return scope == "" })
+	if len(scopes) == 0 {
+		return []string{defaultScope}
+	}
+	return scopes
+}
+
 // text writes a plain text response and returns its status.
 func text(w http.ResponseWriter, status int, body string) int {
 	w.Header().Set("Content-Type", "application/text")
 	w.WriteHeader(status)
-	io.WriteString(w, body)
+	if _, err := io.WriteString(w, body); err != nil {
+		log.Printf("write error: %v", err)
+	}
 	return status
 }
 
@@ -150,10 +162,31 @@ func text(w http.ResponseWriter, status int, body string) int {
 func jsonBody(w http.ResponseWriter, value any) int {
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(http.StatusOK)
-	json.NewEncoder(w).Encode(value)
+	if err := json.NewEncoder(w).Encode(value); err != nil {
+		log.Printf("write error: %v", err)
+	}
 	return http.StatusOK
 }
 
+// ping checks that the emulator at the address answers the metadata ping.
+func ping(address string) error {
+	request, err := http.NewRequest(http.MethodGet, "http://"+address+"/", nil)
+	if err != nil {
+		return err
+	}
+	request.Header.Set("Metadata-Flavor", "Google")
+	client := &http.Client{Timeout: 2 * time.Second}
+	response, err := client.Do(request)
+	if err != nil {
+		return err
+	}
+	defer response.Body.Close()
+	if response.StatusCode != http.StatusOK || response.Header.Get("Metadata-Flavor") != "Google" {
+		return fmt.Errorf("unexpected response: %s", response.Status)
+	}
+	return nil
+}
+
 var impersonationURL = regexp.MustCompile(`/serviceAccounts/([^/:]+):generateAccessToken$`)
 
 // serviceAccountEmail returns the service account of a credentials file, or an empty string.
@@ -193,6 +226,10 @@ func (c *providerCache) get(scopes []string) (auth.TokenProvider, error) {
 	if c.providers == nil {
 		c.providers = map[string]auth.TokenProvider{}
 	}
+	// Drop all providers once the limit is reached, so the cache never grows unbounded.
+	if len(c.providers) >= maxProviders {
+		clear(c.providers)
+	}
 	c.providers[key] = provider
 	return provider, nil
 }
@@ -230,6 +267,18 @@ func serve(ctx context.Context, server *http.Server, listener net.Listener, grac
 }
 
 func main() {
+	check := flag.Bool("check", false, "check that the emulator answers and exit")
+	flag.Parse()
+
+	// IMDS_HOST empty binds all interfaces; the check then uses the loopback address.
+	host, port := os.Getenv("IMDS_HOST"), cmp.Or(os.Getenv("IMDS_PORT"), "8080")
+	if *check {
+		if err := ping(net.JoinHostPort(cmp.Or(host, "127.0.0.1"), port)); err != nil {
+			log.Fatalf("Check failed: %v", err)
+		}
+		return
+	}
+
 	// The first signal stops the server gracefully, the second one exits at once.
 	signals := make(chan os.Signal, 2)
 	signal.Notify(signals, syscall.SIGINT, syscall.SIGTERM)
@@ -279,7 +328,7 @@ func main() {
 		IdleTimeout:       60 * time.Second,
 	}
 
-	listener, err := net.Listen("tcp", ":"+cmp.Or(os.Getenv("IMDS_PORT"), "8080"))
+	listener, err := net.Listen("tcp", net.JoinHostPort(host, port))
 	if err != nil {
 		log.Fatalf("Cannot listen: %v", err)
 	}

sts-agent/main_test.go

@@ -205,10 +205,80 @@ func TestToken(t *testing.T) {
 			if !slices.Equal(scopes, test.wantScopes) {
 				t.Errorf("scopes = %v, want %v", scopes, test.wantScopes)
 			}
+			if got := recorder.Header().Get("Content-Type"); got != "application/json" {
+				t.Errorf("Content-Type = %q, want application/json", got)
+			}
 		})
 	}
 }
 
+func TestParseScopes(t *testing.T) {
+	tests := []struct {
+		value string
+		want  []string
+	}{
+		{"", []string{defaultScope}},
+		{",,", []string{defaultScope}},
+		{"a", []string{"a"}},
+		{"a,,b,", []string{"a", "b"}},
+	}
+	for _, test := range tests {
+		if got := parseScopes(test.value); !slices.Equal(got, test.want) {
+			t.Errorf("parseScopes(%q) = %v, want %v", test.value, got, test.want)
+		}
+	}
+}
+
+func TestRequestLogOmitsQuery(t *testing.T) {
+	var logged bytes.Buffer
+	log.SetOutput(&logged)
+	defer log.SetOutput(io.Discard)
+
+	get(newTestServer(nil), root+"instance/service-accounts/default/token?scopes=secret-scope")
+	if !strings.Contains(logged.String(), "GET "+root+"instance/service-accounts/default/token 200") {
+		t.Errorf("the request was not logged:\n%s", logged.String())
+	}
+	if strings.Contains(logged.String(), "secret-scope") {
+		t.Errorf("the log contains the query:\n%s", logged.String())
+	}
+}
+
+func TestHeadHasNoBody(t *testing.T) {
+	server := httptest.NewServer(newTestServer(nil))
+	defer server.Close()
+
+	request, _ := http.NewRequest(http.MethodHead, server.URL+root+"project/project-id", nil)
+	request.Header.Set("Metadata-Flavor", "Google")
+	response, err := http.DefaultClient.Do(request)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer response.Body.Close()
+	body, _ := io.ReadAll(response.Body)
+	if response.StatusCode != 200 || len(body) != 0 {
+		t.Errorf("got %d with body %q, want 200 without body", response.StatusCode, body)
+	}
+}
+
+func TestPing(t *testing.T) {
+	emulator := httptest.NewServer(newTestServer(nil))
+	defer emulator.Close()
+	other := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {}))
+	defer other.Close()
+	closed := httptest.NewServer(nil)
+	closed.Close()
+
+	if err := ping(strings.TrimPrefix(emulator.URL, "http://")); err != nil {
+		t.Errorf("ping(emulator) = %v, want nil", err)
+	}
+	if err := ping(strings.TrimPrefix(other.URL, "http://")); err == nil {
+		t.Error("ping(other server) = nil, want an error")
+	}
+	if err := ping(strings.TrimPrefix(closed.URL, "http://")); err == nil {
+		t.Error("ping(closed) = nil, want an error")
+	}
+}
+
 func TestTokenExpiry(t *testing.T) {
 	tests := []struct {
 		name     string
@@ -315,6 +385,20 @@ func TestProviderCache(t *testing.T) {
 	}
 }
 
+func TestProviderCacheBound(t *testing.T) {
+	cache := &providerCache{detect: func([]string) (auth.TokenProvider, error) {
+		return tokenProviderFunc(func(context.Context) (*auth.Token, error) { return &auth.Token{Value: "t"}, nil }), nil
+	}}
+	for i := range 10 * maxProviders {
+		if _, err := cache.token(context.Background(), []string{strings.Repeat("s", i+1)}); err != nil {
+			t.Fatal(err)
+		}
+		if len(cache.providers) > maxProviders {
+			t.Fatalf("%d providers cached after %d scope sets, want at most %d", len(cache.providers), i+1, maxProviders)
+		}
+	}
+}
+
 func TestProviderCacheConcurrent(t *testing.T) {
 	var detected atomic.Int32
 	cache := &providerCache{detect: func([]string) (auth.TokenProvider, error) {
@@ -497,6 +581,98 @@ func writeServiceAccountKey(t *testing.T) string {
 	return path
 }
 
+// safeBuffer collects the output of a process while it runs.
+type safeBuffer struct {
+	mu  sync.Mutex
+	buf bytes.Buffer
+}
+
+func (b *safeBuffer) Write(p []byte) (int, error) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	return b.buf.Write(p)
+}
+
+func (b *safeBuffer) String() string {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	return b.buf.String()
+}
+
+// emulator is the test binary running as the emulator process.
+type emulator struct {
+	address string
+	process *os.Process
+	exited  <-chan error
+	output  *safeBuffer
+}
+
+// freePort returns a free TCP port of the loopback interface.
+func freePort(t *testing.T) string {
+	t.Helper()
+	listener, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer listener.Close()
+	_, port, _ := net.SplitHostPort(listener.Addr().String())
+	return port
+}
+
+// runEmulator starts the test binary as the emulator with the arguments and
+// the environment on the loopback interface, and returns it once it answers.
+func runEmulator(t *testing.T, env []string, args ...string) *emulator {
+	t.Helper()
+	port := freePort(t)
+	e := &emulator{address: "127.0.0.1:" + port, output: &safeBuffer{}}
+
+	command := exec.Command(os.Args[0], args...)
+	command.Env = append(os.Environ(), mainEnv+"=1", "GOOGLE_APPLICATION_CREDENTIALS="+writeServiceAccountKey(t), "IMDS_HOST=127.0.0.1", "IMDS_PORT="+port)
+	command.Env = append(command.Env, env...)
+	command.Stderr = e.output
+	if err := command.Start(); err != nil {
+		t.Fatal(err)
+	}
+	exited := make(chan error, 1)
+	go func() { exited <- command.Wait() }()
+	e.process, e.exited = command.Process, exited
+	t.Cleanup(func() { command.Process.Kill() })
+
+	for deadline := time.Now().Add(10 * time.Second); time.Now().Before(deadline); {
+		select {
+		case err := <-exited:
+			t.Fatalf("the emulator exited before it was ready: %v\n%s", err, e.output.String())
+		default:
+		}
+		if ping(e.address) == nil {
+			return e
+		}
+		time.Sleep(50 * time.Millisecond)
+	}
+	t.Fatalf("the emulator did not answer\n%s", e.output.String())
+	return nil
+}
+
+// wait returns the exit error of the emulator, failing the test when it does not stop.
+func (e *emulator) wait(t *testing.T) error {
+	t.Helper()
+	select {
+	case err := <-e.exited:
+		return err
+	case <-time.After(10 * time.Second):
+		t.Fatalf("the emulator did not stop\n%s", e.output.String())
+		return nil
+	}
+}
+
+// runCheck runs the test binary in check mode against the port and returns its exit error.
+func runCheck(t *testing.T, port string) error {
+	t.Helper()
+	command := exec.Command(os.Args[0], "-check")
+	command.Env = append(os.Environ(), mainEnv+"=1", "IMDS_PORT="+port)
+	return command.Run()
+}
+
 // TestSignals runs the emulator as a process and stops it with signals.
 func TestSignals(t *testing.T) {
 	if runtime.GOOS == "windows" {
@@ -504,59 +680,96 @@ func TestSignals(t *testing.T) {
 	}
 	for _, signal := range []syscall.Signal{syscall.SIGTERM, syscall.SIGINT} {
 		t.Run(signal.String(), func(t *testing.T) {
-			listener, err := net.Listen("tcp", "127.0.0.1:0")
+			e := runEmulator(t, nil)
-			if err != nil {
+			if err := e.process.Signal(signal); err != nil {
 				t.Fatal(err)
 			}
-			address := listener.Addr().String()
+			if err := e.wait(t); err != nil {
-			_, port, _ := net.SplitHostPort(address)
+				t.Errorf("the emulator exited with %v, want status 0\n%s", err, e.output.String())
-			listener.Close()
-
-			var output bytes.Buffer
-			command := exec.Command(os.Args[0])
-			command.Env = append(os.Environ(), mainEnv+"=1", "GOOGLE_APPLICATION_CREDENTIALS="+writeServiceAccountKey(t), "IMDS_PORT="+port)
-			command.Stderr = &output
-			if err := command.Start(); err != nil {
-				t.Fatal(err)
 			}
-			exited := make(chan error, 1)
+			if !strings.Contains(e.output.String(), "shutting down") || !strings.Contains(e.output.String(), "Stopped") {
-			go func() { exited <- command.Wait() }()
+				t.Errorf("unexpected log:\n%s", e.output.String())
-			t.Cleanup(func() { command.Process.Kill() })
-
-			// Wait until the emulator answers the ping.
-			ready := false
-			for deadline := time.Now().Add(10 * time.Second); time.Now().Before(deadline) && !ready; {
-				select {
-				case err := <-exited:
-					t.Fatalf("the emulator exited before it was ready: %v\n%s", err, output.String())
-				default:
-				}
-				request, _ := http.NewRequest(http.MethodGet, "http://"+address+"/", nil)
-				request.Header.Set("Metadata-Flavor", "Google")
-				if response, err := http.DefaultClient.Do(request); err == nil {
-					response.Body.Close()
-					ready = response.StatusCode == http.StatusOK
-				}
-				time.Sleep(50 * time.Millisecond)
-			}
-			if !ready {
-				t.Fatalf("the emulator did not answer\n%s", output.String())
-			}
-
-			if err := command.Process.Signal(signal); err != nil {
-				t.Fatal(err)
-			}
-			select {
-			case err := <-exited:
-				if err != nil {
-					t.Errorf("the emulator exited with %v, want status 0\n%s", err, output.String())
-				}
-			case <-time.After(10 * time.Second):
-				t.Fatalf("the emulator did not stop\n%s", output.String())
-			}
-			if !strings.Contains(output.String(), "shutting down") || !strings.Contains(output.String(), "Stopped") {
-				t.Errorf("unexpected log:\n%s", output.String())
 			}
 		})
 	}
+
+	t.Run("second signal", func(t *testing.T) {
+		e := runEmulator(t, nil)
+
+		// A partial request keeps a connection active, so the shutdown waits for it.
+		connection, err := net.Dial("tcp", e.address)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer connection.Close()
+		if _, err := io.WriteString(connection, "GET / HTTP/1.1\r\nHost: imds\r\n"); err != nil {
+			t.Fatal(err)
+		}
+
+		if err := e.process.Signal(syscall.SIGTERM); err != nil {
+			t.Fatal(err)
+		}
+		select {
+		case err := <-e.exited:
+			t.Fatalf("the emulator exited with %v while a connection was active", err)
+		case <-time.After(500 * time.Millisecond):
+		}
+		if err := e.process.Signal(syscall.SIGTERM); err != nil {
+			t.Fatal(err)
+		}
+		var exitError *exec.ExitError
+		if err := e.wait(t); !errors.As(err, &exitError) || exitError.ExitCode() != 1 {
+			t.Errorf("the emulator exited with %v, want status 1\n%s", err, e.output.String())
+		}
+		if !strings.Contains(e.output.String(), "again, exiting") {
+			t.Errorf("unexpected log:\n%s", e.output.String())
+		}
+	})
+}
+
+// TestProcess checks the environment handling and the check mode of the emulator process.
+func TestProcess(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("POSIX signals")
+	}
+	email := func(t *testing.T, address string) string {
+		t.Helper()
+		request, _ := http.NewRequest(http.MethodGet, "http://"+address+root+"instance/service-accounts/default/email", nil)
+		request.Header.Set("Metadata-Flavor", "Google")
+		response, err := http.DefaultClient.Do(request)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer response.Body.Close()
+		body, _ := io.ReadAll(response.Body)
+		return string(body)
+	}
+
+	t.Run("email from credentials", func(t *testing.T) {
+		e := runEmulator(t, nil)
+		if got := email(t, e.address); got != testEmail {
+			t.Errorf("email = %q, want %q", got, testEmail)
+		}
+		if !strings.Contains(e.output.String(), "Serving tokens of "+testEmail+" for project dom-lab on "+e.address) {
+			t.Errorf("unexpected log:\n%s", e.output.String())
+		}
+	})
+
+	t.Run("email override", func(t *testing.T) {
+		e := runEmulator(t, []string{"SERVICE_ACCOUNT_EMAIL=other@dom-lab.iam.gserviceaccount.com"})
+		if got := email(t, e.address); got != "other@dom-lab.iam.gserviceaccount.com" {
+			t.Errorf("email = %q, want the override", got)
+		}
+	})
+
+	t.Run("check", func(t *testing.T) {
+		e := runEmulator(t, nil)
+		_, port, _ := net.SplitHostPort(e.address)
+		if err := runCheck(t, port); err != nil {
+			t.Errorf("check against the emulator = %v, want nil", err)
+		}
+		if err := runCheck(t, freePort(t)); err == nil {
+			t.Error("check against a closed port = nil, want an error")
+		}
+	})
 }