database/sql: add Conn.Raw to expose the driver Conn safely

Exposing the underlying driver conn will allow the use of the
standard connection pool while still able to run special function
directly on the driver.

Fixes #29835

Change-Id: Ib6d3b9535e730f008916805ae3bf76e4494c88f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/174182
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/database/sql/sql_test.go

@@ -1339,6 +1339,54 @@ func TestConnQuery(t *testing.T) {
 	}
 }
 
+func TestConnRaw(t *testing.T) {
+	db := newTestDB(t, "people")
+	defer closeDB(t, db)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	conn, err := db.Conn(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	conn.dc.ci.(*fakeConn).skipDirtySession = true
+	defer conn.Close()
+
+	sawFunc := false
+	err = conn.Raw(func(dc interface{}) error {
+		sawFunc = true
+		if _, ok := dc.(*fakeConn); !ok {
+			return fmt.Errorf("got %T want *fakeConn", dc)
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !sawFunc {
+		t.Fatal("Raw func not called")
+	}
+
+	func() {
+		defer func() {
+			x := recover()
+			if x == nil {
+				t.Fatal("expected panic")
+			}
+			conn.closemu.Lock()
+			closed := conn.dc == nil
+			conn.closemu.Unlock()
+			if !closed {
+				t.Fatal("expected connection to be closed after panic")
+			}
+		}()
+		err = conn.Raw(func(dc interface{}) error {
+			panic("Conn.Raw panic should return an error")
+		})
+		t.Fatal("expected panic from Raw func")
+	}()
+}
+
 func TestCursorFake(t *testing.T) {
 	db := newTestDB(t, "people")
 	defer closeDB(t, db)