Gaute Hope <[email protected]> wrote on Mon, 11 Aug 2014 14:17:54 +0200:
Hi,
I've been working on an application that keeps a read-only handle on
the notmuch database open for a long time. In some cases when a new
message is added along with some renames of other messages using
'notmuch new' while the application is running I get an Xapian
exception: DatabaseModifiedError:
A Xapian exception occurred performing query: The revision being
read has been discarded - you
should call Xapian::Database::reopen() and retry the operation.
Which seems to be printed from: notmuch_query_search_threads ->
notmuch_query_search_messages:294.
I have not been able to make a smaller test case at the moment (this
happens with offlineimap updating an maildir and notmuch new run
afterwards + some tagging).
I can work around this by checking for a NULL pointer returned from
notmuch_query_search_threads () and re-open the database
(notmuch_database_close () -> notmuch_database_open ()). But I have no
way of knowing programatically if this really is the error that has
happened. There should be some way of propagating the error
information or (even better for my case; for notmuch to reopen the
database), one option is the Gmime way of passing an pointer to an
error structure that is filled up by the notmuch interface function.
Hi again,
I have not yet been able to create a test for this with a fresh db (I
must be missing out on some of the parts triggering this).
I need to keep a query open for a while (because running it takes a
while), and this keeps coming back and biting me. I currently do a test
every time I need to access the query further (load more threads..). The
test is as follows, which is the easiest and quickest test I've found to
work reliably:
/* tries to provoke an Xapian::DatabaseModifiedError
*
* returns true if db is invalid. queries will be invalid
* afterwards, but might still respond true to notmuch_valid
* calls.
*
*/
time_t start = clock ();
bool invalid = false;
/* testing */
notmuch_query_t * q = notmuch_query_create (nm_db, "thread:fail");
/* this will produce an error, but no way to ensure failure */
notmuch_query_count_threads (q);
/* this returns NULL when error */
notmuch_threads_t * threads = notmuch_query_search_threads (q);
if (threads == NULL) invalid = true;
notmuch_query_destroy (q);
float diff = (clock () - start) * 1000.0 / CLOCKS_PER_SEC;
cout << "db: test query time: " << diff << " ms." << endl;
if (invalid) {
cout << "db: no longer valid, reopen required." << endl;
} else {
cout << "db: valid." << endl;
}
As mentioned before, I am basically guessing that the reason I get
threads == NULL is the DatabaseModifiedError. There is no way to know
for sure.
For portability I would suggest starting to move towards the GError
scheme provided by glib (also used by gmime). This is a somewhat major
effort though since ensuring that error propagation is done right [0] is
somewhat tricky. It provides more or less the same functionality as
exceptions do in C++.
There is also the problem of having to change the API - one way to avoid
that is to create wrappers that behave like the old API, but don't
handle errors that good. This requires the addition of addiontal _error
variations of the current set of functions. It will be a mess.
Cheers, Gaute
[0] https://developer.gnome.org/glib/stable/glib-Error-Reporting.html
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