[RFC] Split notmuch_database_close into two functions

[Austin Clements]

Quoth Mark Walters on Apr 17 at  9:42 am:
> On Thu, 12 Apr 2012, Austin Clements <amdragon at MIT.EDU> wrote:
> > Quoth Justus Winter on Apr 12 at 11:05 am:
> >> Quoting Austin Clements (2012-04-01 05:23:23)
> >> >Quoth Justus Winter on Mar 21 at  1:55 am:
> >> >> I propose to split the function notmuch_database_close into
> >> >> notmuch_database_close and notmuch_database_destroy so that long
> >> >> running processes like alot can close the database while still using
> >> >> data obtained from queries to that database.
> >> >
> >> >Is this actually safe?  My understanding of Xapian::Database::close is
> >> >that, once you've closed the database, basically anything can throw a
> >> >Xapian exception.  A lot of data is retrieved lazily, both by notmuch
> >> >and by Xapian, so simply having, say, a notmuch_message_t object isn't
> >> >enough to guarantee that you'll be able to get data out of it after
> >> >closing the database.  Hence, I don't see how this interface could be
> >> >used correctly.
> >> 
> >> I do not know how, but both alot and afew (and occasionally the
> >> notmuch binary) are somehow safely using this interface on my box for
> >> the last three weeks.
> >
> > I see.  TL;DR: This isn't safe, but that's okay if we document it.
> >
> > The bug report [0] you pointed to was quite informative.  At its core,
> > this is really a memory management issue.  To sum up for the record
> > (and to check my own thinking): It sounds like alot is careful not to
> > use any notmuch objects after closing the database.  The problem is
> > that, currently, closing the database also talloc_free's it, which
> > recursively free's everything derived from it.  Python later GCs the
> > wrapper objects, which *also* try to free their underlying objects,
> > resulting in a double free.
> >
> > Before the change to expose notmuch_database_close, the Python
> > bindings would only talloc_free from destructors.  Furthermore, they
> > prevented the library from recursively freeing things at other times
> > by internally maintaining a reverse reference for every library talloc
> > reference (e.g., message is a sub-allocation of query, so the bindings
> > keep a reference from each message to its query to ensure the query
> > doesn't get freed).  The ability to explicitly talloc_free the
> > database subverts this mechanism.
> >
> >
> > So, I've come around to thinking that splitting notmuch_database_close
> > and _destroy is okay.  It certainly parallels the rest of the API
> > better.  However, notmuch_database_close needs a big warning similar
> > to Xapian::Database::close's warning that retrieving information from
> > objects derived from this database may not work after calling close.
> > notmuch_database_close is really a specialty interface, and about the
> > only thing you can guarantee after closing the database is that you
> > can destroy other objects.  This is also going to require a SONAME
> > major version bump, as mentioned by others.  Which, to be fair, would
> > be a good opportunity to fix some other issues, too, like how
> > notmuch_database_open can't return errors and how
> > notmuch_database_get_directory is broken on read-only databases.  The
> > actual bump should be done at release time, but maybe we should drop a
> > note somewhere (NEWS?) so we don't forget.
> 
> Can I just check that there is no way to reopen the Xapian database
> readonly? (I may be using the wrong term: I mean is there a way of
> switching an open read-write database to read-only without losing the
> attached structures/messages/threads etc) If I understand it this would
> be sufficient as it would free the lock, but could be more generally
> useful for long lived notmuch processes.

That would be handy and perfect for this situation, but no (I
double-checked with Olly on IRC, which you probably saw).  We might be
able to lobby for this capability if it seems more generally useful.
On the other hand, I think it would probably mix poorly with Xapian's
optimistic snapshot isolation if we tried to use it for anything
non-trivial (combined with real snapshot isolation it would be
awesome).