[#LU-11107] getxattr() returns 0 length values for nonexistent xattrs (with xattr

When the xattr cache is disabled (or when we bypass it as in ~~LU-11074~~), getxattr() will return a zero length value for any nonexistent attribute:

o:~# lctl set_param llite.*.xattr_cache=0
llite.lustre-ffff8c4ac8a60000.xattr_cache=0
o:~# cd /mnt/lustre
o:lustre# touch f0
o:lustre# getfattr -d -m- f0
# file: f0
lustre.lov=0s0AvRCwEAAAABAAAAAAAAAAEEAAACAAAAAAAQAAEAAAACAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
trusted.link=0s3/HqEQEAAAAsAAAAAAAAAAAAAAAAAAAAABQAAAACAAAABwAAAAEAAAAAZjA=
trusted.lma=0sAAAAAAAAAAABBAAAAgAAAAEAAAAAAAAA
trusted.lov=0s0AvRCwEAAAABAAAAAAAAAAEEAAACAAAAAAAQAAEAAAACAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
trusted.version=0sAQAAAAEAAAA=

o:lustre# getfattr -d -n user.blah f0
# file: f0
user.blah

o:lustre# strace getfattr -d -n user.blah f0
...
lstat("f0", {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
getxattr("f0", "user.blah", NULL, 0)    = 0
getxattr("f0", "user.blah", NULL, 0)    = 0
...

After reenabling the xattr cache getxattr() works as expected:

o:lustre# lctl set_param llite.*.xattr_cache=1
llite.lustre-ffff8c4ac8a60000.xattr_cache=1
o:lustre# getfattr -d -n user.blah f0
f0: user.blah: No such attribute

This is because in mdt_getxattr_pack_reply() if the xattr doesn't exist then we return 0. And in mdt_getxattr() we have the following:

        easize = mdt_getxattr_pack_reply(info);
        if (easize < 0)
                GOTO(out, rc = err_serious(easize));

        repbody = req_capsule_server_get(info->mti_pill, &RMF_MDT_BODY);
        LASSERT(repbody != NULL);

        /* No need further getxattr. */
        if (easize == 0 || reqbody->mbo_eadatasize == 0)
                GOTO(out, rc = easize);

[LU-11107] getxattr() returns 0 length values for nonexistent xattrs (with xattr_cache=0) Created: 29/Jun/18 Updated: 03/Aug/18 Resolved: 24/Jul/18
Status:	Resolved
Project:	Lustre
Component/s:	None
Affects Version/s:	None
Fix Version/s:	Lustre 2.12.0, Lustre 2.10.5

[LU-11107] getxattr() returns 0 length values for nonexistent xattrs (with xattr_cache=0) Created: 29/Jun/18 Updated: 03/Aug/18 Resolved: 24/Jul/18