engine: change tracking optional for caching mode and item modification

Recording the revision of items during a caching sync is unnecessary
because the next sync will not use the information. For item
modifications, the information was not even recorded.

Now a "don't need changes" mode can be set in SyncSource, which is
done for caching and command line item operations. This is better than
second-guessing the mode in which a source is used.

SyncSourceRevision checks that flag and skips updating the
luid->revision mapping if set.

Individual backends can also check the flag and skip calculating the
revision to begin with.

src/syncevo/Cmdline.cpp

@@ -1328,6 +1328,7 @@ bool Cmdline::run() {
 
         checkSyncPasswords(*context);
         checkSourcePasswords(*context, source->getName(), sourceNodes);
+        source->setNeedChanges(false);
         source->open();
         const SyncSource::Operations &ops = source->getOperations();
         if (m_printItems) {

src/syncevo/SyncContext.cpp

@@ -2198,6 +2198,14 @@ void SyncContext::startSourceAccess(SyncSource *source)
         m_firstSourceAccess = false;
     }
     if (m_serverMode) {
+        // When using the source as cache, change tracking
+        // is not required. Disabling it can make item
+        // changes faster.
+        SyncMode mode = StringToSyncMode(source->getSync());
+        if (mode == SYNC_LOCAL_CACHE_SLOW ||
+            mode == SYNC_LOCAL_CACHE_INCREMENTAL) {
+            source->setNeedChanges(false);
+        }
         // source is active in sync, now open it
         source->open();
     }

src/syncevo/SyncSource.cpp

@@ -196,7 +196,8 @@ SyncSource::SyncSource(const SyncSourceParams &params) :
     m_numDeleted(0),
     m_forceSlowSync(false),
     m_database("", ""),
-    m_name(params.getDisplayName())
+    m_name(params.getDisplayName()),
+    m_needChanges(true)
 {
 }
 
@@ -1174,13 +1175,22 @@ bool SyncSourceRevisions::detectChanges(ConfigNode &trackingNode, ChangeMode mod
     initRevisions();
 
     // Check whether we have valid revision information.  If not, then
-    // we need to do a slow sync. The assumption here is 
-    if (!m_revisions.empty() &&
+    // we need to do a slow sync. The assumption here is that an empty
+    // revision string marks missing information. When we don't need
+    // change information, not having a revision string is okay.
+    if (needChanges() &&
+        !m_revisions.empty() &&
         m_revisions.begin()->second.empty()) {
         forceSlowSync = true;
         mode = CHANGES_SLOW;
     }
 
+    // If we don't need changes, then override the mode so that
+    // we don't compute them below.
+    if (!needChanges()) {
+        mode = CHANGES_SLOW;
+    }
+
     // Delay setProperty calls until after checking all uids.
     // Necessary for MapSyncSource, which shares the revision among
     // several uids. Another advantage is that we can do the "find
@@ -1246,6 +1256,10 @@ void SyncSourceRevisions::updateRevision(ConfigNode &trackingNode,
                                          const std::string &new_luid,
                                          const std::string &revision)
 {
+    if (!needChanges()) {
+        return;
+    }
+
     databaseModified();
     if (old_luid != new_luid) {
         trackingNode.removeProperty(old_luid);
@@ -1259,6 +1273,9 @@ void SyncSourceRevisions::updateRevision(ConfigNode &trackingNode,
 void SyncSourceRevisions::deleteRevision(ConfigNode &trackingNode,
                                          const std::string &luid)
 {
+    if (!needChanges()) {
+        return;
+    }
     databaseModified();
     trackingNode.removeProperty(luid);
 }

src/syncevo/SyncSource.h

@@ -1642,6 +1642,16 @@ class SyncSourceBase {
      */
     virtual void finishItemChanges() {}
 
+    /**
+     * In some usage scenarios, change tracking is not necessary.
+     * This includes local caching (where local data is not expected
+     * to changed outside of sync) or item manipulation.
+     *
+     * The user of a source must explicitly disable change tracking,
+     * see SyncSource.
+     */
+    virtual bool needChanges() { return true; }
+
  protected:
     struct SynthesisInfo {
         /**
@@ -1986,6 +1996,9 @@ class SyncSource : virtual public SyncSourceBase, public SyncSourceConfig, publi
     virtual long getNumDeleted() const { return m_numDeleted; }
     virtual void setNumDeleted(long num) { m_numDeleted = num; }
     virtual void incrementNumDeleted() { m_numDeleted++; }
+    virtual bool needChanges() { return m_needChanges; }
+    void setNeedChanges(bool needChanges) { m_needChanges = needChanges; }
+
 
     /**
      * Set to true in SyncContext::initSAN() when a SyncML server has
@@ -2026,6 +2039,9 @@ class SyncSource : virtual public SyncSourceBase, public SyncSourceConfig, publi
 
     /** actual name of the source */
     std::string m_name;
+
+    /** change tracking enabled? */
+    bool m_needChanges;
 };
 
 /**