If `FIREBASE_AUTH_EMULATOR_HOST` is set, `VerifyIDToken` calls the Firebase Emulator, but shouldn't

[arzonus]

[REQUIRED] Step 2: Describe your environment

• Operating System version: no matter
• Firebase SDK version: firebase.google.com/go/v4 v4.10.0
• Library version: v4.10.0
• Firebase Product: auth

[REQUIRED] Step 3: Describe the problem

I am working on functionality using AdminSDK to verify ID tokens by the VerifyIDToken function. I would like to write tests including calls of the method, but I found that the SDK tries to call a Firebase Emulator, which host is defined by FIREBASE_AUTH_EMULATOR_HOST variable.

I was surprised because I was sure VerifyIDToken shouldn't call the emulator to check user revocation. I found the place where the problem happens. Could you please describe the reason for this behavior or help me to avoid calls to the emulator?

Relevant Code:

// IDToken returns a valid ID token
func IDToken(uid string) (string, error) {
    const projectID = "project-id"

    var headers = map[string]interface{}{
        "alg": "none",
        "typ": "JWT",
    }

    jsonHeaders, err := json.Marshal(headers)
    if err != nil {
        return "", err
    }
    encodedHeaders := jwt.EncodeSegment(jsonHeaders)

    var claims = map[string]interface{}{
        "aud":       projectID,
        "iss":       "https://securetoken.google.com/" + projectID,
        "iat":       time.Now().Unix() - 100,
        "exp":       time.Now().Unix() + 3600,
        "auth_time": time.Now().Unix() - 100,
        "sub":       "1234567890",
        "uid":       uid,
    }

    jsonClaims, err := json.Marshal(claims)
    if err != nil {
        return "", err
    }
    encodedClaims := jwt.EncodeSegment(jsonClaims)

    return encodedHeaders + "." + encodedClaims + ".", nil
}

func check() {
  ctx := context.Background()
  token, _ := IDToken("uid")
  os.Setenv("FIREBASE_AUTH_EMULATOR_HOST", "localhost:9099")

  app, _ := firebase.NewApp(ctx, nil)
  auth, _ :=  app.Auth(ctx)
  t, err := auth.VerifyIDToken(ctx, token)
  if err != nil {
    fmt.Println(err)
  }
}

I get this error

failed to establish a connection: Post \"http://localhost:9099/identitytoolkit.googleapis.com/v1/projects/project-id/accounts:lookup\": dial tcp [::1]:9099: connect: connection refused"

Thank you!