Pin the scratch store session to UTC in live tests

Darling/Darling.Tests/ScratchPostgres.cs

@@ -23,8 +23,15 @@ internal sealed class ScratchPostgres : IAsyncDisposable
 
     public string ConnectionString { get; }
 
-    private ScratchPostgres(string adminConnectionString, string databaseName, string connectionString)
+    /// <summary>
+    /// The same store WITHOUT the session time-zone pin, so a session opened through it reports the database's own
+    /// default zone. It exists for tests that must observe that default (e.g. to prove the product pin overrides it).
+    /// </summary>
+    public string UnpinnedConnectionString { get; }
+
+    private ScratchPostgres(string adminConnectionString, string databaseName, string connectionString, string unpinnedConnectionString)
     {
+        UnpinnedConnectionString = unpinnedConnectionString;
         _adminConnectionString = adminConnectionString;
         DatabaseName = databaseName;
         ConnectionString = connectionString;
@@ -48,7 +55,13 @@ public static async Task<ScratchPostgres> CreateAsync(string baseConnectionStrin
             Pooling = false,
         };
 
-        return new ScratchPostgres(baseConnectionString, databaseName, builder.ConnectionString);
+        /* Pinned to UTC the way every product store connection is, so a cluster whose default zone is behind UTC
+           cannot skew timestamp round trips (a chunk's range_end read back through a ::timestamp cast). */
+        return new ScratchPostgres(
+            baseConnectionString,
+            databaseName,
+            PerformanceMonitor.Darling.Storage.DarlingStoreConnection.PinSessionTimeZoneUtc(builder.ConnectionString),
+            builder.ConnectionString);
     }
 
     public async ValueTask DisposeAsync()

Darling/Darling.Tests/StoreSessionTimeZoneLiveTests.cs

@@ -64,22 +64,22 @@ shares DARLING_TEST_PG's own database. */
 
         /* ALTER DATABASE ... SET only takes effect for sessions opened AFTER it commits, so migrate through a
            fresh connection rather than the admin one above. */
-        await using (var migrate = new NpgsqlConnection(scratch.ConnectionString))
+        await using (var migrate = new NpgsqlConnection(scratch.UnpinnedConnectionString))
         {
             await migrate.OpenAsync(ct);
             await PgMigrations.MigrateAsync(migrate, ct);
         }
 
         /* The database's own default, UNPINNED — confirms the fixture set up a genuinely non-UTC store
            before asserting that the product's pin overrides it. */
-        await using (var unpinned = new NpgsqlConnection(scratch.ConnectionString))
+        await using (var unpinned = new NpgsqlConnection(scratch.UnpinnedConnectionString))
         {
             await unpinned.OpenAsync(ct);
             Assert.Equal("America/New_York", await ShowTimeZoneAsync(unpinned, ct));
         }
 
         /* The way the product opens a STORE connection: PinSessionTimeZoneUtc first, then the connection. */
-        var pinnedConnectionString = DarlingStoreConnection.PinSessionTimeZoneUtc(scratch.ConnectionString);
+        var pinnedConnectionString = DarlingStoreConnection.PinSessionTimeZoneUtc(scratch.UnpinnedConnectionString);
         await using var connection = new NpgsqlConnection(pinnedConnectionString);
         await connection.OpenAsync(ct);