[Abook-devel] RFC: don't rewrite needlessly the database to disk

[Raphaël D. <rap...@gm...>]

Hi,


attached is a tricky patch:
It skip database rewrites to disk when no changes actually happened.


The current issue is that there is no locking/notify mechanism for the
database file and, especially if using autosave=true, multiple abook
instances imply that "latest closed wins".

Reproducer:
- term1 run abook
- term2 run mutt, add an email (trigger abook --add-email)
# here datafile is changed on disk with the new email
- close abook in term1
# here datafile is changed on disk, the email is lost


There are several ways to avoid/warn when this happens, but all of them
first needs that we avoid file rewrites when no change happened, that
means isolating write operations.

Currently:
$ ls -i .abook/addressbook
8535 .abook/addressbook
$ abook
# press "q" to quit
$ ls -i .abook/addressbook
7943 .abook/addressbook


This is what the patch attempts to solve by marking the database "need
save" after (every) "write" operation on items.
That allowed adding a "*" indicator à la emacs to show the "db modified"
status.

The more sensible here is to ensure that *every* possible action
implying modification of the database will really triggers
db_need_save = TRUE.

If some are missing from the patch the related modifications will *not*
be saved to disk. That is to say that this patch has to be *sure*.


I need to think more about the behavior when autosave=false but IHMO
the check for "is db modified" must be activated for all *saving*
vectors, that means that pressing "w" to explicitly save the database to
disk will *not* rewrite if there is no modification.


I tried many (if not all) of the possible operations offered by the UI
and hope to not have missed any. But as shown by the hunk affecting edit.c,
database write operations are not strictly restricted to database.c.
Further testing welcome.



Next I'd see a more robust way to handle concurrent write accesses to
datafile. I can think about file-lock and inotify and I'd rather want
to avoid any need of datafile "merge".



regards


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