--- 9.0.3	2024-12-12 08:06:13.000000000 +0100
+++ aneksajo	2024-12-16 08:23:15.000000000 +0100
@@ -5,9 +5,11 @@
 package tests
 
 import (
+	"bytes"
 	"context"
 	"database/sql"
 	"fmt"
+	"io"
 	"os"
 	"path"
 	"path/filepath"
@@ -450,3 +452,80 @@
 
 	return CreateDeclarativeRepoWithOptions(t, owner, opts)
 }
+
+// Decide if two files have the same contents or not.
+// chunkSize is the size of the blocks to scan by; pass 0 to get a sensible default.
+// *Follows* symlinks.
+//
+// May return an error if something else goes wrong; in this case, you should ignore the value of 'same'.
+//
+// derived from https://stackoverflow.com/a/30038571
+// under CC-BY-SA-4.0 by several contributors
+func FileCmp(file1, file2 string, chunkSize int) (same bool, err error) {
+	if chunkSize == 0 {
+		chunkSize = 4 * 1024
+	}
+
+	// shortcuts: check file metadata
+	stat1, err := os.Stat(file1)
+	if err != nil {
+		return false, err
+	}
+
+	stat2, err := os.Stat(file2)
+	if err != nil {
+		return false, err
+	}
+
+	// are inputs are literally the same file?
+	if os.SameFile(stat1, stat2) {
+		return true, nil
+	}
+
+	// do inputs at least have the same size?
+	if stat1.Size() != stat2.Size() {
+		return false, nil
+	}
+
+	// long way: compare contents
+	f1, err := os.Open(file1)
+	if err != nil {
+		return false, err
+	}
+	defer f1.Close()
+
+	f2, err := os.Open(file2)
+	if err != nil {
+		return false, err
+	}
+	defer f2.Close()
+
+	b1 := make([]byte, chunkSize)
+	b2 := make([]byte, chunkSize)
+	for {
+		n1, err1 := io.ReadFull(f1, b1)
+		n2, err2 := io.ReadFull(f2, b2)
+
+		// https://pkg.go.dev/io#Reader
+		// > Callers should always process the n > 0 bytes returned
+		// > before considering the error err. Doing so correctly
+		// > handles I/O errors that happen after reading some bytes
+		// > and also both of the allowed EOF behaviors.
+
+		if !bytes.Equal(b1[:n1], b2[:n2]) {
+			return false, nil
+		}
+
+		if (err1 == io.EOF && err2 == io.EOF) || (err1 == io.ErrUnexpectedEOF && err2 == io.ErrUnexpectedEOF) {
+			return true, nil
+		}
+
+		// some other error, like a dropped network connection or a bad transfer
+		if err1 != nil {
+			return false, err1
+		}
+		if err2 != nil {
+			return false, err2
+		}
+	}
+}