Return deferred errors

Deferred errors are sent after the CommandComplete message. They could
be silently dropped depending on the context in which it occurred.

fixes #570

batch.go

@@ -268,6 +268,23 @@ func (b *Batch) Close() (err error) {
 		}
 	}
 
+	for b.conn.pendingReadyForQueryCount > 0 {
+		msg, err := b.conn.rxMsg()
+		if err != nil {
+			return err
+		}
+
+		switch msg := msg.(type) {
+		case *pgproto3.ErrorResponse:
+			return b.conn.rxErrorResponse(msg)
+		default:
+			err = b.conn.processContextFreeMsg(msg)
+			if err != nil {
+				return err
+			}
+		}
+	}
+
 	if err = b.conn.ensureConnectionReadyForQuery(); err != nil {
 		return err
 	}

batch_test.go

@@ -701,3 +701,55 @@ func TestTxBeginBatchRollback(t *testing.T) {
 
 	ensureConnValid(t, conn)
 }
+
+func TestConnBeginBatchDeferredError(t *testing.T) {
+	t.Parallel()
+
+	conn := mustConnect(t, *defaultConnConfig)
+	defer closeConn(t, conn)
+
+	mustExec(t, conn, `create temporary table t (
+		id text primary key,
+		n int not null,
+		unique (n) deferrable initially deferred
+	);
+
+	insert into t (id, n) values ('a', 1), ('b', 2), ('c', 3);`)
+
+	batch := conn.BeginBatch()
+	batch.Queue(`update t set n=n+1 where id='b' returning *`,
+		nil,
+		nil,
+		[]int16{pgx.BinaryFormatCode},
+	)
+
+	err := batch.Send(context.Background(), nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	rows, err := batch.QueryResults()
+	if err != nil {
+		t.Error(err)
+	}
+
+	for rows.Next() {
+		var id string
+		var n int32
+		err = rows.Scan(&id, &n)
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	err = batch.Close()
+	if err == nil {
+		t.Fatal("expected error 23505 but got none")
+	}
+
+	if err, ok := err.(pgx.PgError); !ok || err.Code != "23505" {
+		t.Fatalf("expected error 23505, got %v", err)
+	}
+
+	ensureConnValid(t, conn)
+}

conn_test.go

@@ -1370,6 +1370,32 @@ func TestExecFailure(t *testing.T) {
 	}
 }
 
+func TestExecDeferredError(t *testing.T) {
+	t.Parallel()
+
+	conn := mustConnect(t, *defaultConnConfig)
+	defer closeConn(t, conn)
+
+	mustExec(t, conn, `create temporary table t (
+	id text primary key,
+	n int not null,
+	unique (n) deferrable initially deferred
+);
+
+insert into t (id, n) values ('a', 1), ('b', 2), ('c', 3);`)
+
+	_, err := conn.Exec(`update t set n=n+1 where id='b'`)
+	if err == nil {
+		t.Fatal("expected error 23505 but got none")
+	}
+
+	if err, ok := err.(pgx.PgError); !ok || err.Code != "23505" {
+		t.Fatalf("expected error 23505, got %v", err)
+	}
+
+	ensureConnValid(t, conn)
+}
+
 func TestExecFailureWithArguments(t *testing.T) {
 	t.Parallel()
 

query.go

@@ -69,6 +69,25 @@ func (rows *Rows) Close() {
 		return
 	}
 
+	// If there is no error and a batch operation is not in progress read until we get the ReadyForQuery message or the
+	// ErrorResponse. This is necessary to detect a deferred constraint violation where the ErrorResponse is sent after
+	// CommandComplete.
+	if rows.err == nil && rows.batch == nil && rows.conn.pendingReadyForQueryCount == 1 {
+		for rows.conn.pendingReadyForQueryCount > 0 {
+			msg, err := rows.conn.rxMsg()
+			if err != nil {
+				rows.err = err
+				break
+			}
+
+			err = rows.conn.processContextFreeMsg(msg)
+			if err != nil {
+				rows.err = err
+				break
+			}
+		}
+	}
+
 	if rows.unlockConn {
 		rows.conn.unlock()
 		rows.unlockConn = false

query_test.go

@@ -14,7 +14,7 @@ import (
 	"github.com/jackc/pgx"
 	"github.com/jackc/pgx/pgtype"
 	satori "github.com/jackc/pgx/pgtype/ext/satori-uuid"
-	"github.com/satori/go.uuid"
+	uuid "github.com/satori/go.uuid"
 	"github.com/shopspring/decimal"
 )
 
@@ -424,6 +424,47 @@ func TestConnQueryErrorWhileReturningRows(t *testing.T) {
 
 }
 
+// https://github.com/jackc/pgx/issues/570
+func TestConnQueryDeferredError(t *testing.T) {
+	t.Parallel()
+
+	conn := mustConnect(t, *defaultConnConfig)
+	defer closeConn(t, conn)
+
+	mustExec(t, conn, `create temporary table t (
+	id text primary key,
+	n int not null,
+	unique (n) deferrable initially deferred
+);
+
+insert into t (id, n) values ('a', 1), ('b', 2), ('c', 3);`)
+
+	rows, err := conn.Query(`update t set n=n+1 where id='b' returning *`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer rows.Close()
+
+	for rows.Next() {
+		var id string
+		var n int32
+		err = rows.Scan(&id, &n)
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	if rows.Err() == nil {
+		t.Fatal("expected error 23505 but got none")
+	}
+
+	if err, ok := rows.Err().(pgx.PgError); !ok || err.Code != "23505" {
+		t.Fatalf("expected error 23505, got %v", err)
+	}
+
+	ensureConnValid(t, conn)
+}
+
 func TestQueryEncodeError(t *testing.T) {
 	t.Parallel()