database/sql: on Tx rollback, retain connection if driver can reset session

Previously the Tx would drop the connection after rolling back from
a context cancel. Now if the driver can reset the session,
keep the connection.

Change-Id: Ie6a3124275632787629844d91a06bb2e70cc060b
Reviewed-on: https://go-review.googlesource.com/c/go/+/216241
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/database/sql/sql.go

@@ -1746,7 +1746,11 @@ func (db *DB) begin(ctx context.Context, opts *TxOptions, strategy connReuseStra
 // beginDC starts a transaction. The provided dc must be valid and ready to use.
 func (db *DB) beginDC(ctx context.Context, dc *driverConn, release func(error), opts *TxOptions) (tx *Tx, err error) {
 	var txi driver.Tx
+	keepConnOnRollback := false
 	withLock(dc, func() {
+		_, hasSessionResetter := dc.ci.(driver.SessionResetter)
+		_, hasConnectionValidator := dc.ci.(driver.Validator)
+		keepConnOnRollback = hasSessionResetter && hasConnectionValidator
 		txi, err = ctxDriverBegin(ctx, opts, dc.ci)
 	})
 	if err != nil {
@@ -1758,12 +1762,13 @@ func (db *DB) beginDC(ctx context.Context, dc *driverConn, release func(error),
 	// The cancel function in Tx will be called after done is set to true.
 	ctx, cancel := context.WithCancel(ctx)
 	tx = &Tx{
-		db:          db,
-		dc:          dc,
-		releaseConn: release,
-		txi:         txi,
-		cancel:      cancel,
-		ctx:         ctx,
+		db:                 db,
+		dc:                 dc,
+		releaseConn:        release,
+		txi:                txi,
+		cancel:             cancel,
+		keepConnOnRollback: keepConnOnRollback,
+		ctx:                ctx,
 	}
 	go tx.awaitDone()
 	return tx, nil
@@ -2025,6 +2030,11 @@ type Tx struct {
 	// Use atomic operations on value when checking value.
 	done int32
 
+	// keepConnOnRollback is true if the driver knows
+	// how to reset the connection's session and if need be discard
+	// the connection.
+	keepConnOnRollback bool
+
 	// All Stmts prepared for this transaction. These will be closed after the
 	// transaction has been committed or rolled back.
 	stmts struct {
@@ -2050,7 +2060,10 @@ func (tx *Tx) awaitDone() {
 	// transaction is closed and the resources are released.  This
 	// rollback does nothing if the transaction has already been
 	// committed or rolled back.
-	tx.rollback(true)
+	// Do not discard the connection if the connection knows
+	// how to reset the session.
+	discardConnection := !tx.keepConnOnRollback
+	tx.rollback(discardConnection)
 }
 
 func (tx *Tx) isDone() bool {

src/database/sql/sql_test.go

@@ -442,6 +442,7 @@ func TestTxContextWait(t *testing.T) {
 		}
 		t.Fatal(err)
 	}
+	tx.keepConnOnRollback = false
 
 	// This will trigger the *fakeConn.Prepare method which will take time
 	// performing the query. The ctxDriverPrepare func will check the context
@@ -454,6 +455,35 @@ func TestTxContextWait(t *testing.T) {
 	waitForFree(t, db, 5*time.Second, 0)
 }
 
+// TestTxContextWaitNoDiscard is the same as TestTxContextWait, but should not discard
+// the final connection.
+func TestTxContextWaitNoDiscard(t *testing.T) {
+	db := newTestDB(t, "people")
+	defer closeDB(t, db)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 15*time.Millisecond)
+	defer cancel()
+
+	tx, err := db.BeginTx(ctx, nil)
+	if err != nil {
+		// Guard against the context being canceled before BeginTx completes.
+		if err == context.DeadlineExceeded {
+			t.Skip("tx context canceled prior to first use")
+		}
+		t.Fatal(err)
+	}
+
+	// This will trigger the *fakeConn.Prepare method which will take time
+	// performing the query. The ctxDriverPrepare func will check the context
+	// after this and close the rows and return an error.
+	_, err = tx.QueryContext(ctx, "WAIT|1s|SELECT|people|age,name|")
+	if err != context.DeadlineExceeded {
+		t.Fatalf("expected QueryContext to error with context deadline exceeded but returned %v", err)
+	}
+
+	waitForFree(t, db, 5*time.Second, 1)
+}
+
 // TestUnsupportedOptions checks that the database fails when a driver that
 // doesn't implement ConnBeginTx is used with non-default options and an
 // un-cancellable context.