Enable Feishu session username override with OpenVPN 2.7

backend/internal/service/openvpn/openvpn_test.go

@@ -98,16 +98,47 @@ func TestBuiltInOIDCUsesStableSubjectAsOpenVPNUsername(t *testing.T) {
 	config := map[string]interface{}{
 		"oidc_provider": "builtin", "oidc_issuer": "https://vpn.example.com/id",
 	}
-	generated := oidcConfig(tunnel, config)
+	generated := oidcConfig(tunnel, config, true)
 	if !strings.Contains(generated, `openvpn-username: "string(token.claims.sub)"`) {
 		t.Fatalf("built-in issuer must use verified subject as username: %s", generated)
 	}
+	if !strings.Contains(generated, "override-username: true") {
+		t.Fatal("OpenVPN 2.7 must receive the authenticated subject in its status output")
+	}
+	if generated := oidcConfig(tunnel, config, false); strings.Contains(generated, "override-username:") {
+		t.Fatal("OpenVPN 2.6 must not receive an unsupported username override")
+	}
 	config["oidc_provider"] = "external"
-	if generated := oidcConfig(tunnel, config); strings.Contains(generated, "openvpn-username:") {
+	if generated := oidcConfig(tunnel, config, true); strings.Contains(generated, "openvpn-username:") || strings.Contains(generated, "override-username:") {
 		t.Fatalf("external issuer username behavior must remain unchanged: %s", generated)
 	}
 }
 
+func TestOverrideUsernameRequiresOpenVPN27(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("VPN_DATA_DIR", dir)
+	binDir := filepath.Join(dir, "bin")
+	if err := os.MkdirAll(binDir, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	bin := filepath.Join(binDir, "openvpn")
+	for _, tc := range []struct {
+		version string
+		want    bool
+	}{
+		{"OpenVPN 2.6.19 test\n", false},
+		{"OpenVPN 2.7.8 test\n", true},
+		{"unexpected output\n", false},
+	} {
+		if err := os.WriteFile(bin, []byte("#!/bin/sh\necho '"+strings.TrimSpace(tc.version)+"'\n"), 0o700); err != nil {
+			t.Fatal(err)
+		}
+		if got := supportsOverrideUsername(); got != tc.want {
+			t.Fatalf("version %q: supportsOverrideUsername() = %v, want %v", tc.version, got, tc.want)
+		}
+	}
+}
+
 func TestOIDCRejectsUnsafeURLs(t *testing.T) {
 	base := map[string]interface{}{
 		"auth_mode": "oidc", "oidc_issuer": "https://idp.example.com/realms/a",