a test to show that atimes are always updating on empty dirs in read-only snapshots which were subvol parents, unlike plain empty dirs, regardless of mounting with noatime

btrfs-unstable-atime-bug.sh

#!/bin/sh
set -e

get_atime() {
    ls -lad --time=atime --time-style='+%Y-%m-%dT%H:%M:%S%z' "$1" | tail -n+1 \
        | cut -d ' ' -f6 -
}

FAIL=0
test_atime_stable() {
    DIR="$1"
    FIRST=$(get_atime "$DIR")
    echo "Testing that '$DIR' has repeatable atime of"
    echo "    ${FIRST}..."
    for i in 1 2; do
        sleep 2
        TEST=$(get_atime "$DIR")
        echo "$i:  $TEST"
        if [ "$FIRST" != "$TEST" ]; then FAIL=1; fi
    done
    if [ "$FAIL" = "1" ]; then
        echo "    FAIL $DIR atime is unstable :("
    else
        echo "    pass $DIR atime is stable :)"
    fi
}

test_send_receive() {
    DIR="$1"
    SRC="$2"
    DST_PARENT="$DIR/snapshots"
    DST="$DST_PARENT/1"
    mkdir -p "$DST_PARENT"
    btrfs send "$SRC" | btrfs receive "$DST_PARENT/"
    NUM_SRC=$(find "$SRC" -type d |wc -l)
    NUM_DST=$(find "$DST" -type d |wc -l)

    if [ "$NUM_SRC" = "$NUM_DST" ]; then
        echo "    pass number of dirs in '$SRC'='$DST' ($NUM_SRC=$NUM_DST)"
    else
        echo "    FAIL number of dirs in '$SRC'!='$DST' ($NUM_SRC!=$NUM_DST)"
        FAIL=1
    fi
}

IMG=$(mktemp)
MNT=$(mktemp -d)
truncate -s 20M "$IMG"
mkfs.btrfs "$IMG"
modprobe loop
mount -v -o noatime "$IMG" "$MNT"
echo "Showing mount:"
mount |grep "$IMG"
btrfs subvolume create "$MNT/subvol"
mkdir -v "$MNT/snapshots"
mkdir -v "$MNT/empty_dir"
btrfs subvolume snapshot -r "$MNT" "$MNT/snapshots/1"
echo "Testing that the subvol dir has stable atime on original parent FS:"
test_atime_stable "$MNT/subvol"
echo "Testing that a normal empty dir has stable atime on the snapshot:"
test_atime_stable "$MNT/snapshots/1/empty_dir"
echo "Testing that the subvol dir has stable atime on snapshot of parent FS:"
test_atime_stable "$MNT/snapshots/1/subvol"
echo "Testing that the same number of directories is received from btrfs send:"
test_send_receive "$MNT/received" "$MNT/snapshots/1"
if [ "$1" = "nocleanup" ]; then
    echo "'$0 nocleanup' specified so leaving our mess behind:"
    echo "    btrfs filesystem image file: $IMG"
    echo "    btrfs filesystem mountpoint: $MNT"
else
    echo "'$0 nocleanup' not specified so cleaning up our mess:"
    umount -v "$MNT"
    rmdir -v "$MNT"
    rm -v "$IMG"
fi

exit "$FAIL"

# on Debian Linux sid 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt9-1 (2015-04-08) x86_64 GNU/Linux
# SMALL VOLUME: forcing mixed metadata/data groups
# Btrfs v3.17
# See http://btrfs.wiki.kernel.org for more information.
# 
# Turning ON incompat feature 'mixed-bg': mixed data and metadata block groups
# Turning ON incompat feature 'extref': increased hardlink limit per file to 65536
# Created a data/metadata chunk of size 2097152
# ERROR: device scan failed '/tmp/tmp.vX7LBQhMRn' - Block device required
# fs created label (null) on /tmp/tmp.vX7LBQhMRn
#         nodesize 4096 leafsize 4096 sectorsize 4096 size 20.00MiB
# mount: /dev/loop3 mounted on /tmp/tmp.vkQaOOgI0T.
# Showing mount:
# /tmp/tmp.vX7LBQhMRn on /tmp/tmp.vkQaOOgI0T type btrfs (rw,noatime,space_cache)
# Create subvolume '/tmp/tmp.vkQaOOgI0T/subvol'
# mkdir: created directory ‘/tmp/tmp.vkQaOOgI0T/snapshots’
# mkdir: created directory ‘/tmp/tmp.vkQaOOgI0T/empty_dir’
# Create a readonly snapshot of '/tmp/tmp.vkQaOOgI0T' in '/tmp/tmp.vkQaOOgI0T/snapshots/1'
# Testing that the subvol dir has stable atime on original parent FS:
# Testing that '/tmp/tmp.vkQaOOgI0T/subvol' has repeatable atime of
#     2015-04-23T13:58:45+1000...
# 1:  2015-04-23T13:58:45+1000
# 2:  2015-04-23T13:58:45+1000
#     pass /tmp/tmp.vkQaOOgI0T/subvol atime is stable :)
# Testing that a normal empty dir has stable atime on the snapshot:
# Testing that '/tmp/tmp.vkQaOOgI0T/snapshots/1/empty_dir' has repeatable atime of
#     2015-04-23T13:58:45+1000...
# 1:  2015-04-23T13:58:45+1000
# 2:  2015-04-23T13:58:45+1000
#     pass /tmp/tmp.vkQaOOgI0T/snapshots/1/empty_dir atime is stable :)
# Testing that the subvol dir has stable atime on snapshot of parent FS:
# Testing that '/tmp/tmp.vkQaOOgI0T/snapshots/1/subvol' has repeatable atime of
#     2015-04-23T13:58:53+1000...
# 1:  2015-04-23T13:58:55+1000
# 2:  2015-04-23T13:58:57+1000
#     FAIL /tmp/tmp.vkQaOOgI0T/snapshots/1/subvol atime is unstable :(
# Testing that the same number of directories is received from btrfs send:
# At subvol /tmp/tmp.vkQaOOgI0T/snapshots/1
# At subvol 1
#     FAIL number of dirs in '/tmp/tmp.vkQaOOgI0T/snapshots/1'!='/tmp/tmp.vkQaOOgI0T/received/snapshots/1' (4!=3)
# './btrfs-unstable-atime-bug.sh nocleanup' specified so leaving our mess behind:
#     btrfs filesystem image file: /tmp/tmp.vX7LBQhMRn
#     btrfs filesystem mountpoint: /tmp/tmp.vkQaOOgI0T