stdlib.ReleaseConn closes connections left in invalid state

If a connection is in a transaction or has an open result set then
close the connection when returning it to database/sql. When next
database/sql attempts to use it the connection will return
driver.ErrBadConn and database/sql will remove it from the pool.

fixes #673

stdlib/sql.go

@@ -570,6 +570,12 @@ func ReleaseConn(db *sql.DB, conn *pgx.Conn) error {
 	var tx *sql.Tx
 	var ok bool
 
+	if conn.PgConn().IsBusy() || conn.PgConn().TxStatus() != 'I' {
+		ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+		defer cancel()
+		conn.Close(ctx)
+	}
+
 	fakeTxMutex.Lock()
 	tx, ok = fakeTxConns[conn]
 	if ok {

stdlib/sql_test.go

@@ -692,6 +692,39 @@ func TestAcquireConn(t *testing.T) {
 	ensureConnValid(t, db)
 }
 
+// https://github.com/jackc/pgx/issues/673
+func TestReleaseConnWithTxInProgress(t *testing.T) {
+	db := openDB(t)
+	defer closeDB(t, db)
+
+	c1, err := stdlib.AcquireConn(db)
+	require.NoError(t, err)
+
+	_, err = c1.Exec(context.Background(), "begin")
+	require.NoError(t, err)
+
+	c1PID := c1.PgConn().PID()
+
+	err = stdlib.ReleaseConn(db, c1)
+	require.NoError(t, err)
+
+	c2, err := stdlib.AcquireConn(db)
+	require.NoError(t, err)
+
+	c2PID := c2.PgConn().PID()
+
+	err = stdlib.ReleaseConn(db, c2)
+	require.NoError(t, err)
+
+	require.NotEqual(t, c1PID, c2PID)
+
+	// Releasing a conn with a tx in progress should close the connection
+	stats := db.Stats()
+	require.Equal(t, 1, stats.OpenConnections)
+
+	ensureConnValid(t, db)
+}
+
 func TestConnPingContextSuccess(t *testing.T) {
 	db := openDB(t)
 	defer closeDB(t, db)