Model recovery from an event log (recovery 3/3)

[amolenaar]

This PR builds on #3319 and #3322

PR Type

What kind of change does this PR introduce?

• Bug fix
• Feature
• Chore (refactoring, formatting, local variables, other cleanup)
• Documentation content changes

What is the current behavior?

Models are not automatically saved.

Issue Number: #2831

What is the new behavior?

In this PR, I want to pivot the idea of an event log. The event log contains all (model related) events. This log can be stored on disk and replayed. The goal is to provide a feature in Gaphor that prevents users from losing their work in case Gaphor crashes and the user has not saved his work in time.

The idea is that we log all transactions to a file. This file is based on the hash of the absolute filename of the model , similar to the per model user properties.
If such a log exists after a model is loaded, the log is applied and a notification is shown to the user.

The log will replay all user transactions, so the user should be able to continue his work and undo any step he's done previously. The model will show as modified.

When a model is saved, the event log (on disk) is truncated/reset.
If we have an event log for an unsaved model, we display that as first entry in the "Recent Models" list on the greeter window.

New models should get a unique event log file, so that you can have multiple new models open at any time.
The Greeter window should show any new, unsaved model (with a date?) so a user can re-open it and continue working.

Extra

• Move diagram update to sanitizer service, so they're also performed if no diagrams UI is opened yet
• move get_config_dir() and get_cache_dir() to gaphor.settings
• better cache events in main window, so notifications are displayed as soon as the window shows.
• Clean undo/redo stacks on model flush, instead of model-ready, so it does not interfere with recovery updates.

To do

• Load saved model if recovery fails -> undo changes and remove event log?
• What to do if the model changed in the meantime (e.g. due to git pull)? -> Add a checksum/hash at the beginning of the event log file.
• Clear event log when a model is reloaded (model changed banner) -> checksum/hash in event log preamble does not match.
• Do not replay events when the same model is opened twice. -> Second time, open model is "forced". In that case, we do not store the model file name in the Recovery service and no event log can be created.
• How to deal with recording if the same model is opened twice? -> Same as above.
• How does a user know if a recovery log is created?

Future ideas

• Compact recovery records: reduce matrix updates and handle position changes.
• If the user exits the application, we can just quit, any changes not saved can be re-applied when Gaphor is started again. Gaphor can then always start with the models that were open the last time Gaphor was quit.
• Instead of basing the event log file on the model loaded (filename), it could be a unique identifier (a session id if you like). In the Greeter we can show all progress sessions with unsaved data (at the top, before the recent files). This gives users the option to either continue where they left off, or just open a model. This will also work for for new models that have not been saved to file yet.

Does this PR introduce a breaking change?

• Yes
• No

Other information

An alternative is to actually save the model every so many minutes. However, saving can take a bit of time, especially for big models. With the event log approach we can restore the session to the point where Gaphor was shut down (or crashed).

[danyeaw]

@amolenaar These changes look really great!

One thing I noticed is if I create a model, save it, edit it, and then force close Gaphor, when it comes back up it recovers the changes when I open the model which is great. However, I am not then able to save the model during that session from the menu or by hitting Ctrl+s. For example, I can hit Ctrl+s, the unsaved changes shows in the titlebar, and it will ask if I want to save when I go to close the model even though I just tried to save them.

[amolenaar]

I see that the "edited" dot remains after saving. Saving itself works with me.

[amolenaar]

@danyeaw The modified indicator works again.

I also changed how the modified indicator is set when changes are done, so it will always show modified after modification. Now, if you do edit, save, undo, it will show as modified.

[danyeaw]

Nice one @amolenaar!