Fix unconstrained data value slices

See https://github.com/jackc/pgx/issues/859

go.mod

@@ -7,7 +7,7 @@ require (
 	github.com/jackc/pgio v1.0.0
 	github.com/jackc/pgmock v0.0.0-20190831213851-13a1b77aafa2
 	github.com/jackc/pgpassfile v1.0.0
-	github.com/jackc/pgproto3/v2 v2.0.5
+	github.com/jackc/pgproto3/v2 v2.0.6
 	github.com/jackc/pgservicefile v0.0.0-20200714003250-2b9c44734f2b
 	github.com/stretchr/testify v1.5.1
 	golang.org/x/crypto v0.0.0-20200622213623-75b288015ac9

go.sum

@@ -38,6 +38,8 @@ github.com/jackc/pgproto3/v2 v2.0.4 h1:RHkX5ZUD9bl/kn0f9dYUWs1N7Nwvo1wwUYvKiR26Z
 github.com/jackc/pgproto3/v2 v2.0.4/go.mod h1:WfJCnwN3HIg9Ish/j3sgWXnAfK8A9Y0bwXYU5xKaEdA=
 github.com/jackc/pgproto3/v2 v2.0.5 h1:NUbEWPmCQZbMmYlTjVoNPhc0CfnYyz2bfUAh6A5ZVJM=
 github.com/jackc/pgproto3/v2 v2.0.5/go.mod h1:WfJCnwN3HIg9Ish/j3sgWXnAfK8A9Y0bwXYU5xKaEdA=
+github.com/jackc/pgproto3/v2 v2.0.6 h1:b1105ZGEMFe7aCvrT1Cca3VoVb4ZFMaFJLJcg/3zD+8=
+github.com/jackc/pgproto3/v2 v2.0.6/go.mod h1:WfJCnwN3HIg9Ish/j3sgWXnAfK8A9Y0bwXYU5xKaEdA=
 github.com/jackc/pgservicefile v0.0.0-20200307190119-3430c5407db8 h1:Q3tB+ExeflWUW7AFcAhXqk40s9mnNYLk1nOkKNZ5GnU=
 github.com/jackc/pgservicefile v0.0.0-20200307190119-3430c5407db8/go.mod h1:vsD4gTJCa9TptPL8sPkXrLZ+hDuNrZCnj29CQpr4X1E=
 github.com/jackc/pgservicefile v0.0.0-20200714003250-2b9c44734f2b h1:C8S2+VttkHFdOOCXJe+YGfa4vHYwlt4Zx+IVXQ97jYg=

pgconn_test.go

@@ -758,6 +758,32 @@ func TestConnExecParamsEmptySQL(t *testing.T) {
 	ensureConnValid(t, pgConn)
 }
 
+// https://github.com/jackc/pgx/issues/859
+func TestResultReaderValuesHaveSameCapacityAsLength(t *testing.T) {
+	t.Parallel()
+
+	pgConn, err := pgconn.Connect(context.Background(), os.Getenv("PGX_TEST_CONN_STRING"))
+	require.NoError(t, err)
+	defer closeConn(t, pgConn)
+
+	result := pgConn.ExecParams(context.Background(), "select $1::text as msg", [][]byte{[]byte("Hello, world")}, nil, nil, nil)
+	require.Len(t, result.FieldDescriptions(), 1)
+	assert.Equal(t, []byte("msg"), result.FieldDescriptions()[0].Name)
+
+	rowCount := 0
+	for result.NextRow() {
+		rowCount += 1
+		assert.Equal(t, "Hello, world", string(result.Values()[0]))
+		assert.Equal(t, len(result.Values()[0]), cap(result.Values()[0]))
+	}
+	assert.Equal(t, 1, rowCount)
+	commandTag, err := result.Close()
+	assert.Equal(t, "SELECT 1", string(commandTag))
+	assert.NoError(t, err)
+
+	ensureConnValid(t, pgConn)
+}
+
 func TestConnExecPrepared(t *testing.T) {
 	t.Parallel()