User:Rkent/Folder Data Persistence

Intro

The point of this page is to record notes on understanding data persistence of information associated with folders. This is complicated because the information is frequently stored in multiple in-memory objects, as well as in two different database: dbFolderInfo which is a table of the main folder message summary database, as well as panacea.dat which is a cache of the same information.

These notes were prepared to understand proposals in bug 1032360: "nsMsgLocalMailFolder::GetSizeOnDisk seems to return wrong value for maildir store" to persistently store the folder size, but should be applicable to other issues of folder persistence.

Notes

Folder Cache (panacea.dat)

The folder cache file, named "panacea.dat", is a profile-wide file that contains summary information for each folder. Its main purpose is to allow the folder pane to display the folder tree and associated folder information (such as unread count) without having to open the mork summary file for each folder to get that information, which would take both excessive time and memory. The canonical source for folder metadata though is the dbFolderInfo object that is stored within the mork summary file, so this plan creates a sync issue between these files, as well as with associated memory objects owner by the folder.

(The naming of panacea.dat is in nsMailDirProvider.cpp for the leaf name, but NS_APP_MESSENGER_FOLDER_CACHE_50_FILE is used to access the full path to that file).

panacea.dat is accessed using nsIMsgFolderCacheElement, which uses a string key to access data for a folder. The string key is defined in nsMsgDBFolder::GetFolderCacheKey using the persistent path of the .msf mork file for the folder (for non root-folders) or the main folder (for the root folder). Examples:

C:\\Users\\Kent\\AppData\\Roaming\\Thunderbird\\Profiles\\3gtxygma.default\\ImapMail\\mail.caspia.com\\Sent.msf

C:\\Users\\Kent\\AppData\\Roaming\\Thunderbird\\Profiles\\3gtxygma.default\\ImapMail\\mail.caspia.com

panacea.dat is owned by nsMsgAccountManager, which has methods to acquire the folder cache object and write to it.

Updating the Folder Cache

Here is an example of how the Folder Cache gets updated (along with dbFolderInfo) in a sample operation. The sample used is copying an unread message from an IMAP folder to a local folder, which will require updating the unread count and the total count for the local folder.

Stack to write to local folder cache element (using esr31):

	xul.dll!nsMsgLocalMailFolder::WriteToFolderCacheElem(nsIMsgFolderCacheElement * element) Line 1088
 	xul.dll!nsMsgDBFolder::WriteToFolderCache(nsIMsgFolderCache * folderCache, bool deep) Line 1386
 	xul.dll!nsMsgDBFolder::FlushToFolderCache() Line 1362
 	xul.dll!nsMsgDBFolder::UpdateSummaryTotals(bool force) Line 4178
 	xul.dll!nsMsgDBFolder::EnableNotifications(int notificationType, bool enable, bool dbBatching) Line 5085
 	xul.dll!nsMsgLocalMailFolder::EndMove(bool moveSucceeded) Line 2548
 	xul.dll!nsCopyMessageStreamListener::EndCopy(nsISupports * url, tag_nsresult aStatus) Line 134
 	xul.dll!nsCopyMessageStreamListener::OnStopRequest(nsIRequest * request, nsISupports * ctxt, tag_nsresult aStatus) Line 144

The method UpdateSummaryTotals is key to keeping the folder cache current, but the way information is managed there is quite convoluted. It does the following things:

• Call ReadDBFolderInfo(force), which makes sure that the folder object member variables have been initialized from the cache. If force == true, or the initialization from the cache fails, then the variables are initialized from dbFolderInfo instead. So this method is badly misnamed, it should be something like "InitializeFolderMetadata". Because force==true will cause the folder db to be opened, it is important that force==true is only used in cases where the db is already open, or we expect it to be opened. If force==false, except at initialization ReadDBFolderInfo is a noop.
• Sends OnItemIntPropertyChanged notifications to nsIFolderListener objects for the unread count and total count for the folder
• Writes updated information to the folder cache

If force==true, then things are really confusing and convoluted. In ReadDBFolderInfo, certain folder metadata (mNumTotalMessages, mNumUnreadMessages, mExpungedBytes, mName, mCharset, mCharsetOverride, nsMsgFolderFlags::GotNew) are read from dbFolderInfo (overwriting any local values in the folder object). Cache element metadata mNumPendingUnreadMessages, mNumPendingTotalMessages, mFolderSize, mFlags is not. The handling of pending counts is particularly confusing. They seem to be mostly managed through the method ChangeNumPending... which updates mNumPendingUnreadMessages and then updates dbFolderInfo but not the cache, as well as notifies. (This seems like a performance issue. You should not have to open a folder database to record that there are messages pending that have not been downloaded).

How to understand all of this? The philosophy seems to be the following:

1) Any time that folder metadata is changed, that change needs to be written immediately to dbFolderInfo.