db: accept tskey-client- prefix for OAuth client auth

The upstream tailscale client only runs its OAuth client-credentials exchange
for secrets prefixed tskey-client-, so accept it as an alias for hskey-client-.
The prefix is only a label sliced off before lookup, so the same stored client
authenticates under either; lets the official client and GitHub Action mint auth
keys against headscale.

hscontrol/db/oauth.go

@@ -192,7 +192,12 @@ func (hsdb *HSDatabase) AuthenticateOAuthClient(secretStr string) (*types.OAuthC
 	// used directly as an auth key; strip them before parsing.
 	secretStr, _, _ = strings.Cut(secretStr, "?")
 
-	_, rest, found := strings.Cut(secretStr, types.OAuthClientPrefix)
+	// See [types.TailscaleOAuthClientPrefix] for why the alias is accepted.
+	rest, found := strings.CutPrefix(secretStr, types.OAuthClientPrefix)
+	if !found {
+		rest, found = strings.CutPrefix(secretStr, types.TailscaleOAuthClientPrefix)
+	}
+
 	if !found {
 		return nil, ErrOAuthClientFailedToParse
 	}

hscontrol/db/oauth_test.go

@@ -86,6 +86,54 @@ func TestOAuthClientCreateAndAuthenticate(t *testing.T) {
 	require.Error(t, err)
 }
 
+// TestOAuthClientAuthenticateTailscalePrefix asserts the same stored client
+// authenticates under the tskey-client- alias, and that only a leading prefix
+// is recognised.
+func TestOAuthClientAuthenticateTailscalePrefix(t *testing.T) {
+	db, err := newSQLiteTestDB()
+	require.NoError(t, err)
+
+	secret, client, err := db.CreateOAuthClient(
+		[]string{"auth_keys"},
+		[]string{"tag:ci"},
+		"",
+		nil,
+	)
+	require.NoError(t, err)
+
+	rest := strings.TrimPrefix(secret, types.OAuthClientPrefix)
+	tsSecret := types.TailscaleOAuthClientPrefix + rest
+
+	for _, s := range []string{
+		tsSecret,
+		// Callers may pass the raw auth-key form; ?attributes are stripped.
+		tsSecret + "?baseURL=http://127.0.0.1:8080&ephemeral=true",
+	} {
+		got, err := db.AuthenticateOAuthClient(s)
+		require.NoError(t, err, s)
+		assert.Equal(t, client.ClientID, got.ClientID)
+	}
+
+	// A wrong secret under the alias parses but fails verification.
+	_, err = db.AuthenticateOAuthClient(
+		types.TailscaleOAuthClientPrefix + client.ClientID + "-" + strings.Repeat("0", 64),
+	)
+	require.Error(t, err)
+	require.NotErrorIs(t, err, ErrOAuthClientFailedToParse)
+
+	for _, s := range []string{
+		types.TailscaleOAuthClientPrefix,
+		"tskey-auth-" + rest,
+		"tskey-" + rest,
+		"junk-" + tsSecret,
+		"junk-" + secret,
+		types.TailscaleOAuthClientPrefix + secret,
+	} {
+		_, err := db.AuthenticateOAuthClient(s)
+		require.ErrorIs(t, err, ErrOAuthClientFailedToParse, s)
+	}
+}
+
 func TestHashSecretRoundTrip(t *testing.T) {
 	const secret = "a-high-entropy-credential-secret"