libsyncevolution: load backends in function instead of constructor

Previously, backends were loaded in a constructor. At some point (*), that
loading crashed with a segfault in the dynamic linker. To make debugging a bit
easier and rule out non-determinism as the root cause of that crash, loading
backends was moved into SyncContect::initMain().

(*) the crash happened when there were two backends which couldn't
be loaded due to missing libraries and was related to error strings.
A simpler test program did not trigger the problem, and now SyncEvolution
has suitable backends for (hopefully?!) all platforms, so it no longer
occurs, at least not during automated testing.

src/syncevo/SyncContext.cpp

@@ -3257,6 +3257,9 @@ void SyncContext::initMain(const char *appname)
         SySync_ConsolePrintf = nopPrintf;
     }
 
+    // Load backends.
+    SyncSource::backendsInit();
+
     // invoke optional init parts, for example KDE KApplication init
     // in KDE backend
     GetInitMainSignal()(appname);

src/syncevo/SyncSource.cpp

@@ -332,7 +332,7 @@ RegisterSyncSourceTest::RegisterSyncSourceTest(const string &configName, const s
 
 static class ScannedModules {
 public:
-    ScannedModules() {
+    void init() {
 #ifdef ENABLE_MODULES
         list<pair <string, boost::shared_ptr<ReadDir> > > dirs;
         /* If enviroment variable SYNCEVOLUTION_BACKEND_DIR is set, will search
@@ -429,6 +429,10 @@ public:
     std::ostringstream debug, info;
 } scannedModules;
 
+void SyncSource::backendsInit() {
+    scannedModules.init();
+}
+
 string SyncSource::backendsInfo() {
     return scannedModules.info.str();
 }

src/syncevo/SyncSource.h

@@ -2124,6 +2124,13 @@ class SyncSource : virtual public SyncSourceBase, public SyncSourceConfig, publi
     static SyncSource *createTestingSource(const string &name, const string &type, bool error,
                                            const char *prefix = getenv("CLIENT_TEST_EVOLUTION_PREFIX"));
 
+    /**
+     * Initialize and/or load backends. No longer
+     * done automatically to give libsyncevolution
+     * better control over when backends get loaded.
+     */
+    static void backendsInit();
+
     /**
      * Some information about available backends.
      * Multiple lines, formatted for users of the