synapse: add smarter defaults for database configuration

synapse-module/default.nix

@@ -286,6 +286,57 @@ in
             ];
           };
 
+          database.name = mkOption {
+            type = types.enum [
+              "sqlite3"
+              "psycopg2"
+            ];
+            default = "psycopg2";
+            description = ''
+              The database engine name. Can be sqlite3 or psycopg2.
+            '';
+          };
+
+          database.args.database = mkOption {
+            type = types.str;
+            default =
+              {
+                sqlite3 = "${cfg.dataDir}/homeserver.db";
+                psycopg2 = "matrix-synapse";
+              }
+              .${cfg.settings.database.name};
+            defaultText = literalExpression ''
+              {
+                sqlite3 = "''${${cfgText}.dataDir}/homeserver.db";
+                psycopg2 = "matrix-synapse";
+              }.''${${cfgText}.settings.database.name};
+            '';
+            description = ''
+              Name of the database when using the psycopg2 backend,
+              path to the database location when using sqlite3.
+            '';
+          };
+
+          database.args.user = mkOption {
+            type = types.nullOr types.str;
+            default =
+              {
+                sqlite3 = null;
+                psycopg2 = "matrix-synapse";
+              }
+              .${cfg.settings.database.name};
+            defaultText = lib.literalExpression ''
+              {
+                sqlite3 = null;
+                psycopg2 = "matrix-synapse";
+              }.''${${cfgText}.settings.database.name};
+            '';
+            description = ''
+              Username to connect with psycopg2, set to null
+              when using sqlite3.
+            '';
+          };
+
           federation_ip_range_blacklist = mkOption {
             type = types.listOf types.str;
             description = ''