Revert "database/sql: prevent race in driver by locking dc in Next"

This reverts commit 897080d5.

Reason for revert: Fails to fix all the locking issues.

Updates #21117

Change-Id: I6fc9cb7897244d6e1af78c089a2bf383258ec049
Reviewed-on: https://go-review.googlesource.com/71450Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/database/sql/fakedb_test.go

@@ -943,7 +943,6 @@ type rowsCursor struct {
 }
 
 func (rc *rowsCursor) touchMem() {
-	rc.parentMem.touchMem()
 	rc.line++
 }
 

src/database/sql/sql.go

@@ -2491,12 +2491,6 @@ func (rs *Rows) nextLocked() (doClose, ok bool) {
 	if rs.lastcols == nil {
 		rs.lastcols = make([]driver.Value, len(rs.rowsi.Columns()))
 	}
-
-	// Lock the driver connection before calling the driver interface
-	// rowsi to prevent a Tx from rolling back the connection at the same time.
-	rs.dc.Lock()
-	defer rs.dc.Unlock()
-
 	rs.lasterr = rs.rowsi.Next(rs.lastcols)
 	if rs.lasterr != nil {
 		// Close the connection if there is a driver error.
@@ -2546,12 +2540,6 @@ func (rs *Rows) NextResultSet() bool {
 		doClose = true
 		return false
 	}
-
-	// Lock the driver connection before calling the driver interface
-	// rowsi to prevent a Tx from rolling back the connection at the same time.
-	rs.dc.Lock()
-	defer rs.dc.Unlock()
-
 	rs.lasterr = nextResultSet.NextResultSet()
 	if rs.lasterr != nil {
 		doClose = true

src/database/sql/sql_test.go

@@ -3127,9 +3127,6 @@ func TestIssue6081(t *testing.T) {
 // In the test, a context is canceled while the query is in process so
 // the internal rollback will run concurrently with the explicitly called
 // Tx.Rollback.
-//
-// The addition of calling rows.Next also tests
-// Issue 21117.
 func TestIssue18429(t *testing.T) {
 	db := newTestDB(t, "people")
 	defer closeDB(t, db)
@@ -3140,7 +3137,7 @@ func TestIssue18429(t *testing.T) {
 
 	const milliWait = 30
 
-	for i := 0; i < 1000; i++ {
+	for i := 0; i < 100; i++ {
 		sem <- true
 		wg.Add(1)
 		go func() {
@@ -3162,9 +3159,6 @@ func TestIssue18429(t *testing.T) {
 			// reported.
 			rows, _ := tx.QueryContext(ctx, "WAIT|"+qwait+"|SELECT|people|name|")
 			if rows != nil {
-				// Call Next to test Issue 21117 and check for races.
-				for rows.Next() {
-				}
 				rows.Close()
 			}
 			// This call will race with the context cancel rollback to complete