From 3c4cd9c7517d22353d083f87474cae37bb5bf876 Mon Sep 17 00:00:00 2001
From: Mitar <mitar.git@tnode.com>
Date: Sun, 12 May 2024 22:17:43 +0200
Subject: [PATCH] Use pgtype.PreallocBytes in LargeObject's Read.

Fixes #1876.
---
 large_objects.go | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/large_objects.go b/large_objects.go
index a3028b638..9d21afdce 100644
--- a/large_objects.go
+++ b/large_objects.go
@@ -4,6 +4,8 @@ import (
 	"context"
 	"errors"
 	"io"
+
+	"github.com/jackc/pgx/v5/pgtype"
 )
 
 // The PostgreSQL wire protocol has a limit of 1 GB - 1 per message. See definition of
@@ -115,9 +117,10 @@ func (o *LargeObject) Read(p []byte) (int, error) {
 			expected = maxLargeObjectMessageLength
 		}
 
-		var res []byte
+		res := pgtype.PreallocBytes(p[nTotal:])
 		err := o.tx.QueryRow(o.ctx, "select loread($1, $2)", o.fd, expected).Scan(&res)
-		copy(p[nTotal:], res)
+		// We compute expected so that it always fits into p, so it should never happen
+		// that PreallocBytes's ScanBytes had to allocate a new slice.
 		nTotal += len(res)
 		if err != nil {
 			return nTotal, err