fix(db): one connect attempt per pooled connection, one reconnect per dropped slot (#1214)

lib/core/database/mysql_connection.dart

@@ -145,6 +145,7 @@ class MysqlConnection {
   String? get connectionString => _connectionString;
 
   MySQLConnection? _conn;
+  Future<void>? _connecting;
   bool _isConnected = false;
   bool _inTransaction = false;
 
@@ -164,7 +165,21 @@ class MysqlConnection {
     return '`${id.replaceAll('`', '``')}`';
   }
 
-  Future<void> connect({int connectTimeoutMs = 10000}) async {
+  /// Single-flight: a caller that arrives while an attempt is in progress waits
+  /// for that attempt. Without this, two callers opened two sockets and two SSH
+  /// tunnels, and a failing attempt dropped the session the other one had made.
+  Future<void> connect({int connectTimeoutMs = 10000}) {
+    if (_isConnected && _conn != null) return Future<void>.value();
+    final inFlight = _connecting;
+    if (inFlight != null) return inFlight;
+    final attempt = _connectOnce(connectTimeoutMs);
+    _connecting = attempt;
+    return attempt.whenComplete(() {
+      if (identical(_connecting, attempt)) _connecting = null;
+    });
+  }
+
+  Future<void> _connectOnce(int connectTimeoutMs) async {
     if (_isConnected && _conn != null) return;
 
     var effectivePassword = _password;

lib/core/database/mysql_connection_pool.dart

@@ -77,10 +77,7 @@ class MysqlConnectionPool {
       entry.idleTimer?.cancel();
       entry.idleTimer = null;
       entry.refs++;
-      if (!entry.connection.isConnected) {
-        await entry.connection.connect();
-        await entry.connection.setSessionReadOnly(mode.isReadOnlySession);
-      }
+      if (!entry.connection.isConnected) await _reconnect(entry, mode);
       return MysqlLease._(this, entry, entry.connection);
     }
 
@@ -113,10 +110,7 @@ class MysqlConnectionPool {
     entry.idleTimer?.cancel();
     entry.idleTimer = null;
     entry.refs++;
-    if (!entry.connection.isConnected) {
-      await entry.connection.connect();
-      await entry.connection.setSessionReadOnly(mode.isReadOnlySession);
-    }
+    if (!entry.connection.isConnected) await _reconnect(entry, mode);
     return MysqlLease._(this, entry, entry.connection);
   }
 
@@ -143,6 +137,21 @@ class MysqlConnectionPool {
   /// Releases one lease on [entry]. A lease taken before an [interrupt] points
   /// at an entry that is no longer in the pool: it must not touch the entry
   /// that replaced it under the same key, so it does nothing.
+  /// Brings a dropped entry back with one reconnect and one session setting,
+  /// even when several callers ask at once; they all wait for the same attempt.
+  Future<void> _reconnect(_PoolEntry entry, MysqlSessionMode mode) {
+    final inFlight = entry.reconnecting;
+    if (inFlight != null) return inFlight;
+    final attempt = () async {
+      await entry.connection.connect();
+      await entry.connection.setSessionReadOnly(mode.isReadOnlySession);
+    }();
+    entry.reconnecting = attempt;
+    return attempt.whenComplete(() {
+      if (identical(entry.reconnecting, attempt)) entry.reconnecting = null;
+    });
+  }
+
   void _release(_PoolEntry entry) {
     if (!identical(_pool[entry.key], entry)) return;
     entry.refs--;
@@ -203,6 +212,7 @@ class _PoolEntry {
 
   final MysqlConnection connection;
   final String key;
+  Future<void>? reconnecting;
   int refs = 0;
   Timer? idleTimer;
   DateTime lastUsed;

lib/core/database/postgres_connection.dart

@@ -143,6 +143,7 @@ class PostgresConnection {
   String? get connectionString => _connectionString;
 
   Connection? _conn;
+  Future<void>? _connecting;
   bool _isConnected = false;
   bool _inTransaction = false;
 
@@ -201,7 +202,21 @@ class PostgresConnection {
   /// [openFromUrl] already parses `sslmode`, `connect_timeout`, `query_timeout`
   /// from the URI. If `sslmode` is omitted, we fall back to [useSSL] so the
   /// form checkbox still applies; otherwise libpq-style URLs drive TLS mode.
-  Future<void> connect() async {
+  /// Single-flight: a caller that arrives while an attempt is in progress waits
+  /// for that attempt. Without this, two callers opened two sockets and two SSH
+  /// tunnels, and a failing attempt dropped the session the other one had made.
+  Future<void> connect() {
+    if (_isConnected && _conn != null) return Future<void>.value();
+    final inFlight = _connecting;
+    if (inFlight != null) return inFlight;
+    final attempt = _connectOnce();
+    _connecting = attempt;
+    return attempt.whenComplete(() {
+      if (identical(_connecting, attempt)) _connecting = null;
+    });
+  }
+
+  Future<void> _connectOnce() async {
     if (_isConnected && _conn != null) return;
 
     var effectivePassword = _password;

lib/core/database/postgres_connection_pool.dart

@@ -89,10 +89,7 @@ class PostgresConnectionPool {
       entry.idleTimer?.cancel();
       entry.idleTimer = null;
       entry.refs++;
-      if (!entry.connection.isConnected) {
-        await entry.connection.connect();
-        await entry.connection.setSessionReadOnly(mode.isReadOnlySession);
-      }
+      if (!entry.connection.isConnected) await _reconnect(entry, mode);
       return PgLease._(this, entry, entry.connection);
     }
 
@@ -125,11 +122,23 @@ class PostgresConnectionPool {
     entry.idleTimer?.cancel();
     entry.idleTimer = null;
     entry.refs++;
-    if (!entry.connection.isConnected) {
+    if (!entry.connection.isConnected) await _reconnect(entry, mode);
+    return PgLease._(this, entry, entry.connection);
+  }
+
+  /// Brings a dropped entry back with one reconnect and one session setting,
+  /// even when several callers ask at once; they all wait for the same attempt.
+  Future<void> _reconnect(_PoolEntry entry, PgSessionMode mode) {
+    final inFlight = entry.reconnecting;
+    if (inFlight != null) return inFlight;
+    final attempt = () async {
       await entry.connection.connect();
       await entry.connection.setSessionReadOnly(mode.isReadOnlySession);
-    }
-    return PgLease._(this, entry, entry.connection);
+    }();
+    entry.reconnecting = attempt;
+    return attempt.whenComplete(() {
+      if (identical(entry.reconnecting, attempt)) entry.reconnecting = null;
+    });
   }
 
   /// Drops idle LRU slots until there is room for one more key.
@@ -222,6 +231,7 @@ class _PoolEntry {
 
   final PostgresConnection connection;
   final String key;
+  Future<void>? reconnecting;
   int refs = 0;
   Timer? idleTimer;
   DateTime lastUsed;

test/core/database/mysql_connection_pool_test.dart

@@ -234,4 +234,36 @@ void main() {
       expect(replacement.disconnectCount, 1);
     });
   });
+
+  group('concurrent reconnect (#1214)', () {
+    test('two callers on a dropped slot share one reconnect and one setting',
+        () async {
+      final pool = MysqlConnectionPool(
+        createAndConnect: (row, {required database, required mode}) async {
+          final c = FakeMysqlConnection();
+          await c.connect();
+          return c;
+        },
+      );
+      final r = _row();
+      final first = await pool.acquire(r,
+          database: 'app', mode: MysqlSessionMode.readOnly);
+      final conn = first.connection as FakeMysqlConnection;
+      await conn.forceClose();
+
+      final leases = await Future.wait([
+        pool.acquire(r, database: 'app', mode: MysqlSessionMode.readOnly),
+        pool.acquire(r, database: 'app', mode: MysqlSessionMode.readOnly),
+      ]);
+
+      expect(conn.connectCount, 2, reason: 'initial connect plus one reconnect');
+      expect(conn.setReadOnlyCount, 1);
+      expect(identical(leases[0].connection, conn), isTrue);
+      expect(identical(leases[1].connection, conn), isTrue);
+      first.release();
+      for (final l in leases) {
+        l.release();
+      }
+    });
+  });
 }

test/core/database/postgres_connection_pool_test.dart

@@ -593,4 +593,38 @@ void main() {
       expect(replacement.disconnectCount, 1);
     });
   });
+
+  group('concurrent reconnect (#1214)', () {
+    test('two callers on a dropped slot share one reconnect and one setting',
+        () async {
+      final created = <FakePostgresConnection>[];
+      final pool = PostgresConnectionPool(
+        createAndConnect: (row, {required database, required mode}) async {
+          final c = FakePostgresConnection();
+          await c.connect();
+          created.add(c);
+          return c;
+        },
+      );
+      final r = _row();
+      final first = await pool.acquire(r,
+          database: 'postgres', mode: PgSessionMode.readOnly);
+      final conn = first.connection as FakePostgresConnection;
+      await conn.forceClose();
+
+      final leases = await Future.wait([
+        pool.acquire(r, database: 'postgres', mode: PgSessionMode.readOnly),
+        pool.acquire(r, database: 'postgres', mode: PgSessionMode.readOnly),
+      ]);
+
+      expect(conn.connectCount, 2, reason: 'initial connect plus one reconnect');
+      expect(conn.setReadOnlyCount, 1);
+      expect(identical(leases[0].connection, conn), isTrue);
+      expect(identical(leases[1].connection, conn), isTrue);
+      first.release();
+      for (final l in leases) {
+        l.release();
+      }
+    });
+  });
 }