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

Database drivers should be called from a single goroutine to ease
driver's design. If a driver chooses to handle context
cancels internally it may do so.

The sql package violated this agreement when calling Next or
NextResultSet. It was possible for a concurrent rollback
triggered from a context cancel to call a Tx.Rollback (which
takes a driver connection lock) while a Rows.Next is in progress
(which does not tack the driver connection lock).

The current internal design of the sql package is each call takes
roughly two locks: a closemu lock which prevents an disposing of
internal resources (assigning nil or removing from lists)
and a driver connection lock that prevents calling driver code from
multiple goroutines.

Fixes #21117

Change-Id: Ie340dc752a503089c27f57ffd43e191534829360
Reviewed-on: https://go-review.googlesource.com/65731
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/database/sql/sql_test.go

@@ -3127,6 +3127,9 @@ 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)
@@ -3137,7 +3140,7 @@ func TestIssue18429(t *testing.T) {
 
 	const milliWait = 30
 
-	for i := 0; i < 100; i++ {
+	for i := 0; i < 1000; i++ {
 		sem <- true
 		wg.Add(1)
 		go func() {
@@ -3159,6 +3162,9 @@ 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