SyncSource: allow StartDataRead/beginSync() to force a slow sync

This is needed for ActiveSync, which can detect inside beginSync()
that the sync key became invalid and then has to fall back to a slow
sync.

It is done by asking the engine to call StartDataRead earlier (via
<plugin_earlystartdataread>) and documenting the special 508 error
code. Depends on support for both in libsynthesis, thus the bumped
version number of it.

Normally SyncEvolution is meant to never throw an exception unless
something fatal happens, so it would have been better to add a return
code to beginSync(). But that is a fairly intrusive change, so instead
the simpler "use exception" solution is used. The change can still be
made later on.

3 files changed, 9 insertions, 1 deletions

diff --git a/src/syncevo/SyncML.h b/src/syncevo/SyncML.h
index 97ed30fe..b61fcce2 100644
--- a/src/syncevo/SyncML.h
+++ b/src/syncevo/SyncML.h
@@ -120,6 +120,8 @@ enum SyncMLStatus {
     STATUS_ALREADY_EXISTS = 418,
     /** command failed / fatal DB error */
     STATUS_FATAL = 500,
+    /** in Synthesis StartDataRead: slow sync forced by backend */
+    STATUS_SLOW_SYNC_508 = 508,
     /** general DB error */
     STATUS_DATASTORE_FAILURE = 510,
     /** database / memory full error */
diff --git a/src/syncevo/SyncSource.cpp b/src/syncevo/SyncSource.cpp
index dd881239..de3fd244 100644
--- a/src/syncevo/SyncSource.cpp
+++ b/src/syncevo/SyncSource.cpp
@@ -97,6 +97,7 @@ void SyncSourceBase::getDatastoreXML(string &xml, XMLConfigFragments &fragments)
 
     xmlstream <<
         "      <plugin_module>SyncEvolution</plugin_module>\n"
+        "      <plugin_earlystartdataread>yes</plugin_earlystartdataread>\n"
         "      <plugin_datastoreadmin>" <<
         (serverModeEnabled() ? "yes" : "no") <<
         "</plugin_datastoreadmin>\n"
diff --git a/src/syncevo/SyncSource.h b/src/syncevo/SyncSource.h
index f05dcded..e63bedeb 100644
--- a/src/syncevo/SyncSource.h
+++ b/src/syncevo/SyncSource.h
@@ -1322,7 +1322,12 @@ class SyncSourceSession : virtual public SyncSourceBase {
     /**
      * called before Synthesis engine starts to ask for changes and item data
      *
-     * See BeingDataRead for details.
+     * May throw a STATUS_SLOW_SYNC_508 StatusException if an
+     * incremental sync is not possible. In that case, preparations
+     * for a slow sync must have completed successfully inside the
+     * beginSync() call. It is not going to get called again.
+     *
+     * See StartDataRead for details.
      *
      * @param lastToken     identifies the last completed sync
      * @param resumeToken   identifies a more recent sync which needs to be resumed;