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+From: Mark Walters <markwalters1009@gmail.com>\r
+To: Austin Clements <amdragon@MIT.EDU>,\r
+ Justus Winter <4winter@informatik.uni-hamburg.de>\r
+Subject: Re: [RFC] Split notmuch_database_close into two functions\r
+In-Reply-To: <20120412165744.GF13549@mit.edu>\r
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+\r
+On Thu, 12 Apr 2012, Austin Clements <amdragon@MIT.EDU> wrote:\r
+> Quoth Justus Winter on Apr 12 at 11:05 am:\r
+>> Quoting Austin Clements (2012-04-01 05:23:23)\r
+>> >Quoth Justus Winter on Mar 21 at 1:55 am:\r
+>> >> I propose to split the function notmuch_database_close into\r
+>> >> notmuch_database_close and notmuch_database_destroy so that long\r
+>> >> running processes like alot can close the database while still using\r
+>> >> data obtained from queries to that database.\r
+>> >\r
+>> >Is this actually safe? My understanding of Xapian::Database::close is\r
+>> >that, once you've closed the database, basically anything can throw a\r
+>> >Xapian exception. A lot of data is retrieved lazily, both by notmuch\r
+>> >and by Xapian, so simply having, say, a notmuch_message_t object isn't\r
+>> >enough to guarantee that you'll be able to get data out of it after\r
+>> >closing the database. Hence, I don't see how this interface could be\r
+>> >used correctly.\r
+>> \r
+>> I do not know how, but both alot and afew (and occasionally the\r
+>> notmuch binary) are somehow safely using this interface on my box for\r
+>> the last three weeks.\r
+>\r
+> I see. TL;DR: This isn't safe, but that's okay if we document it.\r
+>\r
+> The bug report [0] you pointed to was quite informative. At its core,\r
+> this is really a memory management issue. To sum up for the record\r
+> (and to check my own thinking): It sounds like alot is careful not to\r
+> use any notmuch objects after closing the database. The problem is\r
+> that, currently, closing the database also talloc_free's it, which\r
+> recursively free's everything derived from it. Python later GCs the\r
+> wrapper objects, which *also* try to free their underlying objects,\r
+> resulting in a double free.\r
+>\r
+> Before the change to expose notmuch_database_close, the Python\r
+> bindings would only talloc_free from destructors. Furthermore, they\r
+> prevented the library from recursively freeing things at other times\r
+> by internally maintaining a reverse reference for every library talloc\r
+> reference (e.g., message is a sub-allocation of query, so the bindings\r
+> keep a reference from each message to its query to ensure the query\r
+> doesn't get freed). The ability to explicitly talloc_free the\r
+> database subverts this mechanism.\r
+>\r
+>\r
+> So, I've come around to thinking that splitting notmuch_database_close\r
+> and _destroy is okay. It certainly parallels the rest of the API\r
+> better. However, notmuch_database_close needs a big warning similar\r
+> to Xapian::Database::close's warning that retrieving information from\r
+> objects derived from this database may not work after calling close.\r
+> notmuch_database_close is really a specialty interface, and about the\r
+> only thing you can guarantee after closing the database is that you\r
+> can destroy other objects. This is also going to require a SONAME\r
+> major version bump, as mentioned by others. Which, to be fair, would\r
+> be a good opportunity to fix some other issues, too, like how\r
+> notmuch_database_open can't return errors and how\r
+> notmuch_database_get_directory is broken on read-only databases. The\r
+> actual bump should be done at release time, but maybe we should drop a\r
+> note somewhere (NEWS?) so we don't forget.\r
+\r
+Can I just check that there is no way to reopen the Xapian database\r
+readonly? (I may be using the wrong term: I mean is there a way of\r
+switching an open read-write database to read-only without losing the\r
+attached structures/messages/threads etc) If I understand it this would\r
+be sufficient as it would free the lock, but could be more generally\r
+useful for long lived notmuch processes.\r
+\r
+Best wishes\r
+\r
+Mark\r
projects / notmuch-archives.git / commitdiff