From bea7a6e325d9c88674795ea156eea2130c45793b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Istv=C3=A1n=20So=C3=B3s?= <isoos@users.noreply.github.com>
Date: Thu, 29 Aug 2024 21:44:17 +0200
Subject: [PATCH] RuntimeParameters (#352)

---
 CHANGELOG.md                  |  1 +
 lib/src/types/type_codec.dart | 38 +++++++++++++++++++++++++++++++++++
 lib/src/v3/connection.dart    | 22 ++++++++++++--------
 test/v3_test.dart             |  7 +++++++
 4 files changed, 60 insertions(+), 8 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 22b1dcf..fa49915 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@
   - (internal) using `EncodedValue` instead of `EncodeOutput`
 - `TypeCodec` interface (may become public) is used for encoding/decoding value by OIDs.
 - `TypeEncoderFn` type definition for generic Dart -> Postgres object encoders (where type is not specified as parameter).
+- `RuntimeParameters` (accessible through `TypeCodecContext`) to access server-provided parameter status values.
 
 ## 3.3.0
 
diff --git a/lib/src/types/type_codec.dart b/lib/src/types/type_codec.dart
index 95d425c..4710683 100644
--- a/lib/src/types/type_codec.dart
+++ b/lib/src/types/type_codec.dart
@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'dart:collection';
 import 'dart:convert';
 import 'dart:typed_data';
 
@@ -47,12 +48,49 @@ abstract class TypeCodec {
   FutureOr<Object?> decode(TypeCodecContext context, EncodedValue input);
 }
 
+/// The read-only, passive view of the Postgresql's runtime/session parameters.
+///
+/// Postgresql server reports certain parameter values at opening a connection
+/// or whenever their values change. Such parameters may include:
+/// - `application_name`
+/// - `server_version`
+/// - `server_encoding`
+/// - `client_encoding`
+/// - `is_superuser`
+/// - `session_authorization`
+/// - `DateStyle`
+/// - `TimeZone`
+/// - `integer_datetimes`
+///
+/// This class holds the latest parameter values send by the server.
+/// The values are not queried or updated actively.
+///
+/// The available parameters may be discovered following the instructions on these URLs:
+/// - https://www.postgresql.org/docs/current/sql-show.html
+/// - https://www.postgresql.org/docs/current/runtime-config.html
+/// - https://www.postgresql.org/docs/current/libpq-status.html#LIBPQ-PQPARAMETERSTATUS
+class RuntimeParameters {
+  /// The latest values of the runtime parameters.
+  ///
+  /// The backing map may be behind an [UnmodifiableMapView], clients may not
+  /// update these values directly.
+  final Map<String, String> latestValues;
+
+  RuntimeParameters({
+    required this.latestValues,
+  });
+
+  String? get applicationName => latestValues['application_name'];
+}
+
 class TypeCodecContext {
   final Encoding encoding;
+  final RuntimeParameters runtimeParameters;
   final TypeRegistry typeRegistry;
 
   TypeCodecContext({
     required this.encoding,
+    required this.runtimeParameters,
     required this.typeRegistry,
   });
 
diff --git a/lib/src/v3/connection.dart b/lib/src/v3/connection.dart
index 9e5de0d..d643ff7 100644
--- a/lib/src/v3/connection.dart
+++ b/lib/src/v3/connection.dart
@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'dart:collection';
 import 'dart:convert';
 import 'dart:io';
 import 'dart:typed_data';
@@ -343,6 +344,9 @@ class PgConnectionImplementation extends _PgSessionBase implements Connection {
   _TransactionSession? _activeTransaction;
 
   final _parameters = <String, String>{};
+  @internal
+  late final runtimeParameters =
+      RuntimeParameters(latestValues: UnmodifiableMapView(_parameters));
 
   var _statementCounter = 0;
   var _portalCounter = 0;
@@ -533,6 +537,14 @@ class PgConnectionImplementation extends _PgSessionBase implements Connection {
   void _closeAfterError([PgException? cause]) {
     _close(true, cause);
   }
+
+  TypeCodecContext createTypeCodecContext() {
+    return TypeCodecContext(
+      encoding: encoding,
+      runtimeParameters: runtimeParameters,
+      typeRegistry: _settings.typeRegistry,
+    );
+  }
 }
 
 class _PreparedStatement extends Statement {
@@ -631,10 +643,7 @@ class _PgResultStreamSubscription
       connection._pending = this;
 
       final encodedFutures = <Future<EncodedValue?>>[];
-      final context = TypeCodecContext(
-        encoding: connection.encoding,
-        typeRegistry: connection._settings.typeRegistry,
-      );
+      final context = connection.createTypeCodecContext();
       for (final e in statement.parameters) {
         if (e.isSqlNull) {
           encodedFutures.add(Future.value(null));
@@ -755,10 +764,7 @@ class _PgResultStreamSubscription
           final columnCount = message.values.length;
           final futures = <Future>[];
           List<bool>? sqlNulls;
-          final context = TypeCodecContext(
-            encoding: session.encoding,
-            typeRegistry: session._connection._settings.typeRegistry,
-          );
+          final context = session._connection.createTypeCodecContext();
           for (var i = 0; i < message.values.length; i++) {
             final field = schema.columns[i];
             final input = message.values[i];
diff --git a/test/v3_test.dart b/test/v3_test.dart
index 6c46f04..dc04dc7 100644
--- a/test/v3_test.dart
+++ b/test/v3_test.dart
@@ -3,6 +3,7 @@ import 'dart:async';
 import 'package:async/async.dart';
 import 'package:postgres/messages.dart';
 import 'package:postgres/postgres.dart';
+import 'package:postgres/src/v3/connection.dart';
 import 'package:stream_channel/stream_channel.dart';
 import 'package:test/test.dart';
 
@@ -26,6 +27,12 @@ void main() {
 
     tearDown(() => connection.close());
 
+    test('runtime parameters', () async {
+      final c = connection as PgConnectionImplementation;
+      final p = c.runtimeParameters;
+      expect(p.applicationName, 'test_app');
+    });
+
     test('simple queries', () async {
       final rs = await connection.execute("SELECT 'dart', 42, NULL");
       expect(rs, [