chown -R on non-empty directory changes mtime

[srd424]

Not sure if this is a bug or an 'unavoidable feature'; fuse-overlayfs 1.12, ext4 on the underlying filesystem. It's causing me a little trouble with some buildah builds!

#! /bin/bash

runtest () {
        local testd=$1
        orig=$(stat -c %Y $testd/testdir)

        sleep 1

        chown -R $USER:$GROUP $testd/testdir

        new=$(stat -c %Y $testd/testdir)

        if [ $new -ne $orig ]; then
                echo "mtime changed from $orig to $new!"
        else
                echo "mtime unchanged"
        fi
}

echo -e "\nRunning test using fuse-overlayfs:\n"

d=`mktemp -d -p $PWD`

mkdir -p $d/upper $d/lower $d/work $d/mnt

mkdir $d/lower/testdir
touch $d/lower/testdir/testfile

fuse-overlayfs -V
fuse-overlayfs -olowerdir=$d/lower,upperdir=$d/upper,workdir=$d/work $d/mnt

echo
runtest $d/mnt

umount $d/mnt

rm -r $d

echo -e "\n\nRunning test WITHOUT fuse-overlayfs:\n"

d=`mktemp -d -p $PWD`
mkdir $d/testdir
touch $d/testdir/testfile

runtest $d
echo

rm -r $d

Yields:

Running test using fuse-overlayfs:

fuse-overlayfs: version 1.12
FUSE library version 3.10.5
using FUSE kernel interface version 7.31
fusermount3 version: 3.10.5

mtime changed from 1691229966 to 1691229967!

Running test WITHOUT fuse-overlayfs:

mtime unchanged