`cmp_ds_prop` is a no-op, 3/4 rsend tests that use it are false positives

[nabijaczleweli]

System information

Type	Version/Name
Distribution Name	all
Architecture	all
OpenZFS Version	trunk

Describe the problem you're observing

If you observe L247 and L248 of cmp_ds_prop:

zfs/tests/zfs-tests/tests/functional/rsend/rsend.kshlib

Lines 219 to 257 in 56a0699

	#
	# Compare the given two dataset properties
	#
	# $1 dataset 1
	# $2 dataset 2
	#
	function cmp_ds_prop
	{
	typeset dtst1=$1
	typeset dtst2=$2
	typeset -a props=("type" "origin" "volblocksize" "acltype" "dnodesize" \
	"atime" "canmount" "checksum" "compression" "copies" "devices" \
	"exec" "quota" "readonly" "recordsize" "reservation" "setuid" \
	"snapdir" "version" "volsize" "xattr" "mountpoint");
	if is_freebsd; then
	props+=("jailed")
	else
	props+=("zoned")
	fi
	for prop in $props;
	do
	zfs get -H -o property,value,source $prop $dtst1 >> \
	$BACKDIR/dtst1
	zfs get -H -o property,value,source $prop $dtst2 >> \
	$BACKDIR/dtst2
	done
	eval sed -e 's:$dtst1:PREFIX:g' < $BACKDIR/dtst1 > $BACKDIR/dtst1
	eval sed -e 's:$dtst2:PREFIX:g' < $BACKDIR/dtst2 > $BACKDIR/dtst2
	diff $BACKDIR/dtst1 $BACKDIR/dtst2
	typeset -i ret=$?
	rm -f $BACKDIR/dtst1 $BACKDIR/dtst2
	return $ret
	}

you'll see that it truncates the intermediate files, then does preprocessing on them; this, for obvious reasons leaves them empty, which means cmp_ds_prop is diff /dev/null /dev/null, or a very convoluted way of alias cmp_ds_prop=:.

Funnily enough, cmp_ds_subs, above, uses a different set of intermediates for before/after.

This makes rsend-group tests send-c_props, rsend_012_pos, and rsend_011_pos (so – all which use it except rsend_008_pos) be false-positives.

I already have this fixed but need an issue number to ref against.