return an error in CopyAligned upon premature EOF (#18110)

add a unit-test to capture this corner case

internal/ioutil/ioutil_test.go

@@ -1,4 +1,4 @@
-// Copyright (c) 2015-2021 MinIO, Inc.
+// Copyright (c) 2015-2023 MinIO, Inc.
 //
 // This file is part of MinIO Object Storage stack
 //
@@ -20,8 +20,10 @@ package ioutil
 import (
 	"bytes"
 	"context"
+	"errors"
 	"io"
 	"os"
+	"strings"
 	"testing"
 	"time"
 )
@@ -205,3 +207,36 @@ func TestSameFile(t *testing.T) {
 		t.Fatal("Expected the files not to be same")
 	}
 }
+
+func TestCopyAligned(t *testing.T) {
+	f, err := os.CreateTemp("", "")
+	if err != nil {
+		t.Errorf("Error creating tmp file: %v", err)
+	}
+	defer f.Close()
+	defer os.Remove(f.Name())
+
+	r := strings.NewReader("hello world")
+
+	bufp := ODirectPoolSmall.Get().(*[]byte)
+	defer ODirectPoolSmall.Put(bufp)
+
+	written, err := CopyAligned(f, io.LimitReader(r, 5), *bufp, r.Size(), f)
+	if !errors.Is(err, io.ErrUnexpectedEOF) {
+		t.Errorf("Expected io.ErrUnexpectedEOF, but got %v", err)
+	}
+	if written != 5 {
+		t.Errorf("Expected written to be '5', but got %v", written)
+	}
+
+	f.Seek(0, io.SeekStart)
+	r.Seek(0, io.SeekStart)
+
+	written, err = CopyAligned(f, r, *bufp, r.Size(), f)
+	if !errors.Is(err, nil) {
+		t.Errorf("Expected nil, but got %v", err)
+	}
+	if written != r.Size() {
+		t.Errorf("Expected written to be '%v', but got %v", r.Size(), written)
+	}
+}