Xapian 1.4.5 "Db block overwritten - are there multiple writers?" with Glass

[Sylvain Taverne]

Thank's !!
I'll try during the week, and will let you know if all is fine ;)

Le mar. 10 juil. 2018 à 07:36, Olly Betts <olly at survex.com> a écrit :

> On Mon, Jul 09, 2018 at 10:29:18AM +0100, Olly Betts wrote:
> > The attached patch reset this cursor each time commit() is called, and
> > that fixes my C++ reproducer, though I think this ought to work as-is
> > and the real bug is at a lower level.
>
> I've dug deeper and that was indeed the case.  Here's a patch which
> addresses the root cause:
>
> https://oligarchy.co.uk/xapian/patches/glass-cursor-rebuild-fix.patch
>
> For the curious, the bug was in some code to rebuild the cursor when the
> underlying table changes in ways which require that.  That's a fairly
> rare occurrence (with my C++ reproducer it happens 99 times out of 5000
> commits).
>
> In chert the equivalent code just marks the cursor's blocks as not yet
> read, but in glass cursor blocks are reference counted and shared so we
> can't simply do that as it could affect other cursors sharing the same
> blocks.
>
> So instead the glass code was leaving them with the contents they
> previously had, except for copying the current root block from the
> table's "built-in cursor".  After the rebuild we seek the cursor to the
> same key it was on before, and that mostly works because we follow down
> each level in the Btree from the new root, except it can happen that the
> old cursor contained a block number which has since been released and
> reallocated, and in that case the block doesn't get reread and we try to
> use its old contents, which violates the rule that a parent can't be
> younger than its child and causes the exception.
>
> The simplest fix is to just reset the rebuilt cursor to match the
> current "built-in cursor" at all levels (not just the root), which is
> cheap because of the reference counting.  And that fixes my C++
> reproducer, which I converted from your Python reproducer.
>
> Please test and let me know if this fixes the original problem or not.
>
> Cheers,
>     Olly
>