database/sql: fail on unsupported options when context is un-cancellable

Currently, the check for `ctx.Done() == context.Background().Done()`
comes before the check to see if we are ignoring any options.  That
check should be done earlier, so that the options are not silently
ignored.

Fixes #21350

Change-Id: I3704e4209854c7d99f3f92498bae831cabc7e419
Reviewed-on: https://go-review.googlesource.com/53970Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/database/sql/ctxutil.go

@@ -107,10 +107,6 @@ func ctxDriverBegin(ctx context.Context, opts *TxOptions, ci driver.Conn) (drive
 		return ciCtx.BeginTx(ctx, dopts)
 	}
 
-	if ctx.Done() == context.Background().Done() {
-		return ci.Begin()
-	}
-
 	if opts != nil {
 		// Check the transaction level. If the transaction level is non-default
 		// then return an error here as the BeginTx driver value is not supported.
@@ -125,6 +121,10 @@ func ctxDriverBegin(ctx context.Context, opts *TxOptions, ci driver.Conn) (drive
 		}
 	}
 
+	if ctx.Done() == context.Background().Done() {
+		return ci.Begin()
+	}
+
 	txi, err := ci.Begin()
 	if err == nil {
 		select {

src/database/sql/sql_test.go

@@ -439,6 +439,20 @@ func TestTxContextWait(t *testing.T) {
 	waitForFree(t, db, 5*time.Second, 0)
 }
 
+// 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.
+func TestUnsupportedOptions(t *testing.T) {
+	db := newTestDB(t, "people")
+	defer closeDB(t, db)
+	_, err := db.BeginTx(context.Background(), &TxOptions{
+		Isolation: LevelSerializable, ReadOnly: true,
+	})
+	if err == nil {
+		t.Fatal("expected error when using unsupported options, got nil")
+	}
+}
+
 func TestMultiResultSetQuery(t *testing.T) {
 	db := newTestDB(t, "people")
 	defer closeDB(t, db)