database/sql: fix double decrement of numOpen count; test for connection leaks

Add a check at the end of every test to make sure
there are no leaked connections after running a test.

Avoid incorrectly decrementing the number of open connections
when the driver connection ends up it a bad state (numOpen was
decremented twice).

Prevent leaking a Rows struct (which ends up leaking a
connection) in Row.Scan() when a *RawBytes destination is
improperly used.

Close the Rows struct in TestRowsColumns.

Update #6593

R=golang-dev, bradfitz, dave
CC=golang-dev
https://golang.org/cl/14642044

src/pkg/database/sql/sql_test.go

@@ -94,6 +94,12 @@ func closeDB(t testing.TB, db *DB) {
 	if err != nil {
 		t.Fatalf("error closing DB: %v", err)
 	}
+	db.mu.Lock()
+	count := db.numOpen
+	db.mu.Unlock()
+	if count != 0 {
+		t.Fatalf("%d connections still open after closing DB", db.numOpen)
+	}
 }
 
 // numPrepares assumes that db has exactly 1 idle conn and returns
@@ -246,6 +252,9 @@ func TestRowsColumns(t *testing.T) {
 	if !reflect.DeepEqual(cols, want) {
 		t.Errorf("got %#v; want %#v", cols, want)
 	}
+	if err := rows.Close(); err != nil {
+		t.Errorf("error closing rows: %s", err)
+	}
 }
 
 func TestQueryRow(t *testing.T) {