Address bryancall's review comments

Fix sync retry logic: revert last_sync to its previous value on
syncMap failure instead of setting it to last_write. The old code
made the slot appear clean, preventing retries until a new Insert
bumped last_write again.

Author: zwoop

doc/developer-guide/cripts/cripts-misc.en.rst

@@ -415,6 +415,8 @@ Debug logging uses the same format string syntax as ``fmt::format()`` in ``libfm
     The default debug tag for Cripts is the name of the Cript itself, either
     the Cript source file, or the compiled plugin name.
 
+.. _cripts-misc-cache-groups:
+
 Cache Groups
 ============
 

src/cripts/CacheGroup.cc

@@ -292,12 +292,13 @@ Cache::Group::WriteToDisk()
 
   for (size_t ix = 0; ix < _slots.size(); ++ix) {
     if (_slots[ix].last_write > _slots[ix].last_sync) {
+      auto old_sync        = _slots[ix].last_sync;
       any_dirty            = true;
       _slots[ix].last_sync = now;
       if (syncMap(ix)) {
         _last_sync = now;
       } else {
-        _slots[ix].last_sync = _slots[ix].last_write; // revert so next call retries
+        _slots[ix].last_sync = old_sync; // revert so next call retries
         all_synced           = false;
       }
     }