fix(lk): say why no user resolves: the default user, or several users

cmd/lk/testdata/script/broken-default-user.txtar

@@ -1,17 +1,12 @@
-# A default user that no longer resolves is a clear error with the way to fix
-# it, never a crash. Today the error says nobody is signed in to the default
-# Public API, while another user is signed in elsewhere and the real problem
-# is the default user.
+# A default user that no longer resolves is a clear error that names it and
+# says how to fix it, never a crash, even with another user signed in
+# elsewhere.
 ! exec lk --user-auth project list
 ! stdout .
-stderr 'run `lk cloud auth --user-auth` to sign in'
-cp stderr lk.stderr
+stderr '^the default user "gone@example.com" is not signed in to https://beta-api.livekit.cloud \(run `lk cloud auth --user-auth` to sign in\)$'
 requests
 ! stdout .
 
-skip 'known bug: the error doesn''t say the default user no longer resolves'
-grep 'gone@example.com' lk.stderr
-
 -- home/.livekit/cli-config.yaml --
 default_user: gone@example.com
 users:

cmd/lk/testdata/script/two-users-same-api.txtar

@@ -15,8 +15,7 @@ exec lk --user-auth --user ops@example.com project list
 requests
 cmp stdout want-requests
 
-skip 'known bug: with two users signed in, the error says no user is signed in instead of asking for --user'
-grep 'pass --user' refused.stderr
+grep '^2 users are signed in to http://publicapi.test and none is the default user; pass --user to pick one$' refused.stderr
 
 -- home/.livekit/cli-config.yaml --
 users:

cmd/lk/user_auth.go

@@ -312,9 +312,23 @@ func resolveSessionUser(conf *config.CLIConfig, userRef string, apiURLSet bool)
 			return nil, fmt.Errorf("user %q is not signed in to %s (run %s to sign in)", userRef, publicAPIURL, signInCommand())
 		}
 	}
-	if users := conf.UsersForAPIURL(publicAPIURL); len(users) == 1 {
+	users := conf.UsersForAPIURL(publicAPIURL)
+	if len(users) == 1 {
 		return users[0], nil
 	}
+	// Here userRef, if set, is the default user: an explicit --user returned
+	// above.
+	switch {
+	case len(users) > 1:
+		return nil, fmt.Errorf("%d users are signed in to %s and none is the default user; pass --user to pick one",
+			len(users), publicAPIURL)
+	case userRef != "":
+		if elsewhere := conf.FindUsers(userRef); len(elsewhere) > 0 {
+			return nil, fmt.Errorf("the default user %q is signed in to %s, not %s (run %s to sign in there)",
+				userRef, cmp.Or(elsewhere[0].APIURL, public.DefaultBaseURL), publicAPIURL, signInCommand())
+		}
+		return nil, fmt.Errorf("the default user %q is not signed in to %s (run %s to sign in)", userRef, publicAPIURL, signInCommand())
+	}
 	return nil, fmt.Errorf("no user is signed in to %s (run %s to sign in)", publicAPIURL, signInCommand())
 }
 

cmd/lk/user_auth_test.go

@@ -168,6 +168,10 @@ func TestRequireUserSessionAPIURL(t *testing.T) {
 		token, _, err = resolve(t, "--experimental-api-url", public.DefaultBaseURL)
 		require.NoError(t, err)
 		assert.Equal(t, "prod", token, "the old --experimental-api-url spelling still sets the URL")
+
+		_, _, err = resolve(t, "--public-api-url", "https://other.example")
+		assert.ErrorContains(t, err, `the default user "usr_stg" is signed in to `+staging+`, not https://other.example`,
+			"an API nobody is signed in to names where the default user is")
 	})
 
 	t.Run("--user", func(t *testing.T) {