feat!: Commit/Rollback releases sessions to the pool

If a commit or rollback are succesful, the SpannerTransaction is disposed so that resources are released.

BREAKING CHANGE: After a successfull commit or rollback, the transaction is diposed and may not be used again. See the Google.Cloud.Spanner.Data.SpannerTransaction.DisposeBehavior documentation for more information.

Closes googleapis#10673

apis/Google.Cloud.Spanner.Data/Google.Cloud.Spanner.Data.IntegrationTests/TransactionTests.cs

@@ -86,6 +86,50 @@ private async Task IncrementByOneAsync(SpannerConnection connection, bool orphan
             }
         }
 
+        [Fact]
+        public async Task Commit_ReturnsToPool()
+        {
+            using var connection = new SpannerConnection(_fixture.ConnectionString);
+            await connection.OpenAsync();
+
+            using var transaction = await connection.BeginTransactionAsync();
+            using var command = connection.CreateSelectCommand($"SELECT Int64Value FROM {_fixture.TableName} WHERE K=@k");
+            command.Parameters.Add("k", SpannerDbType.String, _key);
+            command.Transaction = transaction;
+
+            var value = await command.ExecuteScalarAsync();
+
+            transaction.Commit();
+
+            var poolStatistics = connection.GetSessionPoolSegmentStatistics();
+
+            // Because the session is eagerly returned to the pool after a commit, there shouldn't
+            // be any active sessions even before we dispose of the transaction explicitly.
+            Assert.Equal(0, poolStatistics.ActiveSessionCount);
+        }
+
+        [Fact]
+        public async Task Rollback_ReturnsToPool()
+        {
+            using var connection = new SpannerConnection(_fixture.ConnectionString);
+            await connection.OpenAsync();
+
+            using var transaction = await connection.BeginTransactionAsync();
+            using var command = connection.CreateSelectCommand($"SELECT Int64Value FROM {_fixture.TableName} WHERE K=@k");
+            command.Parameters.Add("k", SpannerDbType.String, _key);
+            command.Transaction = transaction;
+
+            var value = await command.ExecuteScalarAsync();
+
+            transaction.Rollback();
+
+            var poolStatistics = connection.GetSessionPoolSegmentStatistics();
+
+            // Because the session is eagerly returned to the pool after a rollback, there shouldn't
+            // be any active sessions even before we dispose of the transaction explicitly.
+            Assert.Equal(0, poolStatistics.ActiveSessionCount);
+        }
+
         [Fact]
         public async Task DetachOnDisposeTransactionIsDetached()
         {

apis/Google.Cloud.Spanner.Data/Google.Cloud.Spanner.Data/SpannerTransaction.cs

@@ -441,6 +441,8 @@ Task<IEnumerable<long>> ISpannerTransaction.ExecuteBatchDmlAsync(ExecuteBatchDml
             {
                 var callSettings = SpannerConnection.CreateCallSettings(settings => settings.CommitSettings, CommitTimeout, cancellationToken);
                 var response = await _session.CommitAsync(request, callSettings).ConfigureAwait(false);
+                // We dispose of the SpannerTransaction to inmediately release the session to the pool when possible.
+                Dispose();
                 if (response.CommitTimestamp == null)
                 {
                     throw new SpannerException(ErrorCode.Internal, "Commit succeeded, but returned a response with no commit timestamp");
@@ -464,17 +466,18 @@ private RequestOptions BuildCommitRequestOptions() =>
         /// </summary>
         /// <param name="cancellationToken">A cancellation token used for this task.</param>
 #if NET462
-        public Task RollbackAsync(CancellationToken cancellationToken = default)
+        public async Task RollbackAsync(CancellationToken cancellationToken = default)
 #else
-        public override Task RollbackAsync(CancellationToken cancellationToken = default)
+        public override async Task RollbackAsync(CancellationToken cancellationToken = default)
 #endif
         {
             CheckNotDisposed();
             GaxPreconditions.CheckState(Mode != TransactionMode.ReadOnly, "You cannot roll back a readonly transaction.");
             var callSettings = SpannerConnection.CreateCallSettings(settings => settings.RollbackSettings, CommitTimeout, cancellationToken);
-            return ExecuteHelper.WithErrorTranslationAndProfiling(
+            await  ExecuteHelper.WithErrorTranslationAndProfiling(
                 () => _session.RollbackAsync(new RollbackRequest(), callSettings),
-                "SpannerTransaction.Rollback", SpannerConnection.Logger);
+                "SpannerTransaction.Rollback", SpannerConnection.Logger).ConfigureAwait(false);
+            Dispose();
         }
 
         /// <summary>