CP-10986: Return read caching status on VDI.attach

Returning the status of read caching during VDI Attach is a bit tricky.
This is because the SM only works out whether or not O_DIRECT should be
used during VDI Activate (which happens after attach). This is determined by a
number of elements: licensing, override parameters or VDI Type, etc.

This patch isolates this check in a separate (private) method and
caches it. Both VDI attach and activate use this method to work out whether
tapdisk will be enabling read caching or not.

We also add this information to the return struct in VDI.attach as
requested by the ticket.

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Acked-by: Felipe Franciosi <felipe@paradoxo.org>

Github: closes #242 on xapi-project/sm

Author: siddharthv

drivers/blktap2.py

@@ -977,8 +977,29 @@ def __init__(self, uuid, target, driver_info):
         self._vdi_uuid   = uuid
         self._session    = target.session
         self.xenstore_data = scsiutil.update_XS_SCSIdata(uuid,scsiutil.gen_synthetic_page_data(uuid))
+        self.__o_direct  = None
         self.lock        = Lock("vdi", uuid)
 
+    def get_o_direct_capability(self, options = {}):
+        """Returns True/False based on licensing and caching_params"""
+        if self.__o_direct is not None:
+            return self.__o_direct
+
+        if not util.read_caching_is_restricted(self._session):
+            self.__o_direct = options.get(self.CONF_KEY_O_DIRECT)
+            if self.__o_direct is not None:
+                return self.__o_direct
+            if (self.target.vdi.sr.handles("nfs") or
+                self.target.vdi.sr.handles("ext")):
+                from FileSR import FileVDI
+                if vhdutil.getParent(self.target.vdi.path, FileVDI.extractUuid):
+                    self.__o_direct = False
+
+        if self.__o_direct is None:
+            self.__o_direct = True
+
+        return self.__o_direct
+
     @classmethod
     def from_cli(cls, uuid):
         import VDI as sm
@@ -1469,6 +1490,7 @@ def attach(self, sr_uuid, vdi_uuid, writable, activate = False):
         # Return backend/ link
         back_path = self.BackendLink.from_uuid(sr_uuid, vdi_uuid).path()
         struct = { 'params': back_path,
+                   'o_direct': self.get_o_direct_capability(),
                    'xenstore_data': self.xenstore_data}
         util.SMlog('result: %s' % struct)
 
@@ -1562,12 +1584,7 @@ def _activate(self, sr_uuid, vdi_uuid, options):
             # Maybe launch a tapdisk on the physical link
             if self.tap_wanted():
                 vdi_type = self.target.get_vdi_type()
-                if util.read_caching_is_restricted(self._session):
-                    options["o_direct"] = True
-                else:
-                    options["o_direct"] = options.get(self.CONF_KEY_O_DIRECT)
-                    if options["o_direct"] is None:
-                        options["o_direct"] = True
+                options["o_direct"] = self.get_o_direct_capability(options)
                 dev_path = self._tap_activate(phy_path, vdi_type, sr_uuid,
                         options,
                         self._get_pool_config(sr_uuid).get("mem-pool-size"))