perf: cache the selective sync list in the journal

getSelectiveSyncList() runs a full query and materializes the whole list on
every call, and Folder::warnOnNewExcludedItem() calls it once per file added to
the sync folder. The list only changes when the user edits the selective sync
configuration, so keep it in memory and drop it in setSelectiveSyncList(), the
only writer of the table, and on close().

Signed-off-by: Felipe Tumonis <ftumonis@gmail.com>

Author: ftapps

src/common/syncjournaldb.cpp

@@ -703,6 +703,7 @@ void SyncJournalDb::close()
 
     _db.close();
     clearEtagStorageFilter();
+    _selectiveSyncListCache.clear();
     _metadataTableIsEmpty = false;
 }
 
@@ -2380,6 +2381,12 @@ QStringList SyncJournalDb::getSelectiveSyncList(SyncJournalDb::SelectiveSyncList
         return result;
     }
 
+    const auto cached = _selectiveSyncListCache.constFind(int(type));
+    if (cached != _selectiveSyncListCache.constEnd()) {
+        *ok = true;
+        return *cached;
+    }
+
     const auto query = _queryManager.get(PreparedSqlQueryManager::GetSelectiveSyncListQuery, QByteArrayLiteral("SELECT path FROM selectivesync WHERE type=?1"), _db);
     if (!query) {
         qCWarning(lcDb) << "database error:" << query->error();
@@ -2408,6 +2415,8 @@ QStringList SyncJournalDb::getSelectiveSyncList(SyncJournalDb::SelectiveSyncList
     }
     *ok = true;
 
+    _selectiveSyncListCache.insert(int(type), result);
+
     return result;
 }
 
@@ -2418,6 +2427,10 @@ void SyncJournalDb::setSelectiveSyncList(SyncJournalDb::SelectiveSyncListType ty
         return;
     }
 
+    // Dropped rather than replaced with `list`: getSelectiveSyncList() hands out entries with a
+    // trailing slash, which is added on read, not on write.
+    _selectiveSyncListCache.remove(int(type));
+
     startTransaction();
 
     //first, delete all entries of this type

src/common/syncjournaldb.h

@@ -431,6 +431,15 @@ public slots:
     int _transaction = 0;
     bool _metadataTableIsEmpty = false;
 
+    /* Cache for getSelectiveSyncList(), keyed by SelectiveSyncListType.
+     *
+     * The list is read once per file in some hot paths (see Folder::warnOnNewExcludedItem) while
+     * changing only when the user edits the selective sync configuration, so it is kept here
+     * rather than re-queried. Invalidated by setSelectiveSyncList() -- the only writer of the
+     * selectivesync table -- and on close().
+     */
+    QHash<int, QStringList> _selectiveSyncListCache;
+
     /* Storing etags to these folders, or their parent folders, is filtered out.
      *
      * When schedulePathForRemoteDiscovery() is called some etags to _invalid_ in the