performance/md-cache: Do not skip caching of null character xattr values

Null character string is a valid xattr value in file system. But for
those xattrs processed by md-cache, it does not update its entries if
value is null('\0'). This results in ENODATA when those xattrs are
queried afterwards via getxattr() causing failures in basic operations
like create, copy etc in a specially configured Samba setup for Mac OS
clients.

On the other side snapview-server is internally setting empty string("")
as value for xattrs received as part of listxattr() and are not intended
to be cached. Therefore we try to maintain that behaviour using an
additional dictionary key to prevent updation of entries in getxattr()
and fgetxattr() callbacks in md-cache.

Credits: Poornima G <pgurusid@redhat.com>

Change-Id: I7859cbad0a06ca6d788420c2a495e658699c6ff7
Fixes: bz#1785228
Signed-off-by: Anoop C S <anoopcs@redhat.com>
(cherry picked from commit b4b683736367d93daad08a5ee6ca95778c07c5a4)

2 files changed, 23 insertions, 20 deletions

diff --git a/xlators/features/snapview-server/src/snapview-server.c b/xlators/features/snapview-server/src/snapview-server.c
index ad5c2baad12..76cccae5914 100644
--- a/xlators/features/snapview-server/src/snapview-server.c
+++ b/xlators/features/snapview-server/src/snapview-server.c
@@ -828,7 +828,8 @@ out:
  * back into the dict. But to get the values for those xattrs it has to do the
  * getxattr operation on each xattr which might turn out to be a costly
  * operation. So for each of the xattrs present in the list, a 0 byte value
- * ("") is set into the dict before unwinding. This can be treated as an
+ * ("") is set into the dict before unwinding. Since ("") is also a valid xattr
+ * value(in a file system) we use an extra key in the same dictionary as an
  * indicator to other xlators which want to cache the xattrs (as of now,
  * md-cache which caches acl and selinux related xattrs) to not to cache the
  * values of the xattrs present in the dict.
@@ -871,6 +872,15 @@ svs_add_xattrs_to_dict(xlator_t *this, dict_t *dict, char *list, ssize_t size)
         list_offset += strlen(keybuffer) + 1;
     } /* while (remaining_size > 0) */
 
+    /* Add an additional key to indicate that we don't need to cache these
+     * xattrs(with value "") */
+    ret = dict_set_str(dict, "glusterfs.skip-cache", "");
+    if (ret < 0) {
+        gf_msg(this->name, GF_LOG_ERROR, 0, SVS_MSG_DICT_SET_FAILED,
+               "dict set operation for the key glusterfs.skip-cache failed.");
+        goto out;
+    }
+
     ret = 0;
 
 out:
diff --git a/xlators/performance/md-cache/src/md-cache.c b/xlators/performance/md-cache/src/md-cache.c
index ae7f013c36b..e3b44c5de7d 100644
--- a/xlators/performance/md-cache/src/md-cache.c
+++ b/xlators/performance/md-cache/src/md-cache.c
@@ -714,25 +714,6 @@ updatefn(dict_t *dict, char *key, data_t *value, void *data)
             }
         }
 
-        /* posix xlator as part of listxattr will send both names
-         * and values of the xattrs in the dict. But as per man page
-         * listxattr is mainly supposed to send names of the all the
-         * xattrs. gfapi, as of now will put all the keys it obtained
-         * in the dict (sent by posix) into a buffer provided by the
-         * caller (thus the values of those xattrs are lost). If some
-         * xlator makes gfapi based calls (ex: snapview-server), then
-         * it has to unwind the calls by putting those names it got
-         * in the buffer again into the dict. But now it would not be
-         * having the values for those xattrs. So it might just put
-         * a 0 byte value ("") into the dict for each xattr and unwind
-         * the call. So the xlators which cache the xattrs (as of now
-         * md-cache caches the acl and selinux related xattrs), should
-         * not update their cache if the value of a xattr is a 0 byte
-         * data (i.e. "").
-         */
-        if (value->len == 1 && value->data[0] == '\0')
-            return 0;
-
         if (dict_set(u->dict, key, value) < 0) {
             u->ret = -1;
             return -1;
@@ -2421,6 +2402,12 @@ mdc_getxattr_cbk(call_frame_t *frame, void *cookie, xlator_t *this,
         goto out;
     }
 
+    if (dict_get(xattr, "glusterfs.skip-cache")) {
+        gf_msg(this->name, GF_LOG_DEBUG, 0, 0,
+               "Skipping xattr update due to empty value");
+        goto out;
+    }
+
     mdc_inode_xatt_set(this, local->loc.inode, xdata);
 
 out:
@@ -2503,6 +2490,12 @@ mdc_fgetxattr_cbk(call_frame_t *frame, void *cookie, xlator_t *this,
         goto out;
     }
 
+    if (dict_get(xattr, "glusterfs.skip-cache")) {
+        gf_msg(this->name, GF_LOG_DEBUG, 0, 0,
+               "Skipping xattr update due to empty value");
+        goto out;
+    }
+
     mdc_inode_xatt_set(this, local->fd->inode, xdata);
 
 out: