stdlib: implement Conn.ResetSession

This prevents closed connections from being returned
by `database.sql.DB.Conn`.

Fixes #974.

stdlib/sql.go

@@ -385,6 +385,13 @@ func (c *Conn) CheckNamedValue(*driver.NamedValue) error {
 	return nil
 }
 
+func (c *Conn) ResetSession(ctx context.Context) error {
+	if c.conn.IsClosed() {
+		return driver.ErrBadConn
+	}
+	return nil
+}
+
 type Stmt struct {
 	sd   *pgconn.StatementDescription
 	conn *Conn

stdlib/sql_test.go

@@ -4,7 +4,6 @@ import (
 	"bytes"
 	"context"
 	"database/sql"
-	"database/sql/driver"
 	"encoding/json"
 	"math"
 	"os"
@@ -719,7 +718,8 @@ func TestConnExecContextSuccess(t *testing.T) {
 
 func TestConnExecContextFailureRetry(t *testing.T) {
 	testWithAndWithoutPreferSimpleProtocol(t, func(t *testing.T, db *sql.DB) {
-		// we get a connection, immediately close it, and then get it back
+		// We get a connection, immediately close it, and then get it back;
+		// DB.Conn along with Conn.ResetSession does the retry for us.
 		{
 			conn, err := stdlib.AcquireConn(db)
 			require.NoError(t, err)
@@ -729,7 +729,7 @@ func TestConnExecContextFailureRetry(t *testing.T) {
 		conn, err := db.Conn(context.Background())
 		require.NoError(t, err)
 		_, err = conn.ExecContext(context.Background(), "select 1")
-		require.EqualValues(t, driver.ErrBadConn, err)
+		require.NoError(t, err)
 	})
 }
 
@@ -749,7 +749,8 @@ func TestConnQueryContextSuccess(t *testing.T) {
 
 func TestConnQueryContextFailureRetry(t *testing.T) {
 	testWithAndWithoutPreferSimpleProtocol(t, func(t *testing.T, db *sql.DB) {
-		// we get a connection, immediately close it, and then get it back
+		// We get a connection, immediately close it, and then get it back;
+		// DB.Conn along with Conn.ResetSession does the retry for us.
 		{
 			conn, err := stdlib.AcquireConn(db)
 			require.NoError(t, err)
@@ -760,7 +761,7 @@ func TestConnQueryContextFailureRetry(t *testing.T) {
 		require.NoError(t, err)
 
 		_, err = conn.QueryContext(context.Background(), "select 1")
-		require.EqualValues(t, driver.ErrBadConn, err)
+		require.NoError(t, err)
 	})
 }