WebDAV: use CTag for quick change detection

The CTag mechanism allows to quickly check whether data has changed.
With WebDAV and the recent infrastructure changes in
SyncSourceRevisions/TrackingSyncSource, that is straight-forward.

With CalDAV it is a bit more complicated because the m_cache needs to
be populated for some of the operations to succeed. This is
accomplished via the setAll[Sub]Items() calls.

src/backends/webdav/CalDAVSource.cpp

@@ -130,6 +130,28 @@ int CalDAVSource::appendItem(SubRevisionMap_t &revisions,
     return 0;
 }
 
+void CalDAVSource::setAllSubItems(const SubRevisionMap_t &revisions)
+{
+    if (!m_cache.m_initialized) {
+        // populate our cache (without data) from the information cached
+        // for us
+        BOOST_FOREACH(const SubSyncSource::SubRevisionMap_t::value_type &subentry,
+                      revisions) {
+            const std::string &luid = subentry.first;
+            const std::string &rev = subentry.second.first;
+            boost::shared_ptr<Event> &event = m_cache[luid];
+            event.reset(new Event);
+            event->m_DAVluid = luid;
+            event->m_etag = rev;
+            // We don't know the real UID (may be different from resource name == DAVluid),
+            // sequence and last-modified. This information will have to be filled
+            // in by loadItem() when some operation on this event needs it.
+            event->m_subids = subentry.second.second;
+        }
+        m_cache.m_initialized = true;
+    }
+}
+
 SubSyncSource::SubItemResult CalDAVSource::insertSubItem(const std::string &luid, const std::string &callerSubID,
                                                          const std::string &item)
 {
@@ -179,7 +201,10 @@ SubSyncSource::SubItemResult CalDAVSource::insertSubItem(const std::string &luid
     }
     std::string subid = *newEvent->m_subids.begin();
 
-    // determine whether we already know the merged item even though our caller didn't
+    // Determine whether we already know the merged item even though
+    // our caller didn't. That additional safe guard will fail if
+    // setAllSubItems() was used to fill the cache, because then the m_UID
+    // values in the cache will not be set yet. That should be okay.
     std::string davLUID = luid;
     std::string knownSubID = callerSubID;
     if (davLUID.empty()) {
@@ -706,11 +731,17 @@ CalDAVSource::Event &CalDAVSource::loadItem(Event &event)
         Event::unescapeRecurrenceID(item);
         event.m_calendar.set(icalcomponent_new_from_string((char *)item.c_str()), // hack for old libical
                              "parsing iCalendar 2.0");
-        // sequence number might have been increased by last save,
-        // so check it again
+
+        // Sequence number/last-modified might have been increased by last save.
+        // Or the cache was populated by setAllSubItems(), which doesn't give
+        // us the information. In that case, UID might also still be unknown.
+        // Either way, check it again.
         for (icalcomponent *comp = icalcomponent_get_first_component(event.m_calendar, ICAL_VEVENT_COMPONENT);
              comp;
              comp = icalcomponent_get_next_component(event.m_calendar, ICAL_VEVENT_COMPONENT)) {
+            if (event.m_UID.empty()) {
+                event.m_UID = Event::getUID(comp);
+            }
             long sequence = Event::getSequence(comp);
             if (sequence > event.m_sequence) {
                 event.m_sequence = sequence;

src/backends/webdav/CalDAVSource.h

@@ -34,7 +34,9 @@ class CalDAVSource : public WebDAVSource,
     /* implementation of SubSyncSource interface */
     virtual void begin() { contactServer(); }
     virtual void endSubSync(bool success) { if (success) { storeServerInfos(); } }
+    virtual std::string subDatabaseRevision() { return databaseRevision(); }
     virtual void listAllSubItems(SubRevisionMap_t &revisions);
+    virtual void setAllSubItems(const SubRevisionMap_t &revisions);
     virtual SubItemResult insertSubItem(const std::string &uid, const std::string &subid,
                                         const std::string &item);
     virtual void readSubItem(const std::string &uid, const std::string &subid, std::string &item);

src/backends/webdav/WebDAVSource.cpp

@@ -881,6 +881,29 @@ void WebDAVSource::storeServerInfos()
     }
 }
 
+/**
+ * See https://trac.calendarserver.org/browser/CalendarServer/trunk/doc/Extensions/caldav-ctag.txt
+ */
+static const ne_propname getctag[] = {
+    { "http://calendarserver.org/ns/", "getctag" },
+    { NULL, NULL }
+};
+
+std::string WebDAVSource::databaseRevision()
+{
+    Timespec deadline = createDeadline();
+    Neon::Session::PropfindPropCallback_t callback =
+        boost::bind(&WebDAVSource::openPropCallback,
+                    this, _1, _2, _3, _4);
+    m_davProps[m_calendar.m_path]["http://calendarserver.org/ns/:getctag"] = "";
+    m_session->propfindProp(m_calendar.m_path, 0, getctag, callback, deadline);
+    // Fatal communication problems will be reported via exceptions.
+    // Once we get here, invalid or incomplete results can be
+    // treated as "don't have revision string".
+    string ctag = m_davProps[m_calendar.m_path]["http://calendarserver.org/ns/:getctag"];
+    return ctag;
+}
+
 
 static const ne_propname getetag[] = {
     { "DAV:", "getetag" },

src/backends/webdav/WebDAVSource.h

@@ -79,6 +79,7 @@ class WebDAVSource : public TrackingSyncSource, private boost::noncopyable
     }
 
     /* implementation of TrackingSyncSource interface */
+    virtual std::string databaseRevision();
     virtual void listAllItems(RevisionMap_t &revisions);
     virtual InsertItemResult insertItem(const string &luid, const std::string &item, bool raw);
     void readItem(const std::string &luid, std::string &item, bool raw);