view CONTRIBUTORS @ 42963:8502f76dbfd7

transaction: detect an attempt to truncate-to-extend on playback, raise error

On some networked filesystems, writes can have delayed finalization/confirmation
and write races can occur such that a remote modification will "win" and
modifications will be lost. There is no functionality for providing this
feedback to userspace programs (in fact, there's not even functionality for
providing this information to the Linux kernel...), so these programs may see
the files suddenly change.

We've noticed that there have been cases where Mercurial has detected something
has gone wrong and attempts to abort (rolling back the transaction), which is
good. However, when rolling back the transaction, for the append-only files,
we attempt to "truncate" the file back to the size it was in before the hg
transaction started, but end up *extending* it. This may be harmless, but if
this happens to the 00changelog.i file, we get a bunch of nulls on the end of
the file and this causes hg to become *really* confused. :)

If we detect that some modification of the file outside of this Mercurial
process has caused the file to be smaller than the size we are attempting to
truncate to, let's just exit and stop trying to clean up the repository -
continuing will likely just cause more damage.

Differential Revision: https://phab.mercurial-scm.org/D6867

[This file is here for historical purposes, all recent contributors
should appear in the changelog directly]

Andrea Arcangeli <andrea at suse.de>
Thomas Arendsen Hein <thomas at intevation.de>
Goffredo Baroncelli <kreijack at libero.it>
Muli Ben-Yehuda <mulix at mulix.org>
Mikael Berthe <mikael at lilotux.net>
Benoit Boissinot <bboissin at gmail.com>
Brendan Cully <brendan at kublai.com>
Vincent Danjean <vdanjean.ml at free.fr>
Jake Edge <jake at edge2.net>
Michael Fetterman <michael.fetterman at intel.com>
Edouard Gomez <ed.gomez at free.fr>
Eric Hopper <hopper at omnifarious.org>
Alecs King <alecsk at gmail.com>
Volker Kleinfeld <Volker.Kleinfeld at gmx.de>
Vadim Lebedev <vadim at mbdsys.com>
Christopher Li <hg at chrisli.org>
Chris Mason <mason at suse.com>
Colin McMillen <mcmillen at cs.cmu.edu>
Wojciech Milkowski <wmilkowski at interia.pl>
Chad Netzer <chad.netzer at gmail.com>
Bryan O'Sullivan <bos at serpentine.com>
Vicent Seguí Pascual <vseguip at gmail.com>
Sean Perry <shaleh at speakeasy.net>
Nguyen Anh Quynh <aquynh at gmail.com>
Ollivier Robert <roberto at keltia.freenix.fr>
Alexander Schremmer <alex at alexanderweb.de>
Arun Sharma <arun at sharma-home.net>
Josef "Jeff" Sipek <jeffpc at optonline.net>
Kevin Smith <yarcs at qualitycode.com>
TK Soh <teekaysoh at yahoo.com>
Radoslaw Szkodzinski <astralstorm at gorzow.mm.pl>
Samuel Tardieu <sam at rfc1149.net>
K Thananchayan <thananck at yahoo.com>
Andrew Thompson <andrewkt at aktzero.com>
Michael S. Tsirkin <mst at mellanox.co.il>
Rafael Villar Burke <pachi at mmn-arquitectos.com>
Tristan Wibberley <tristan at wibberley.org>
Mark Williamson <mark.williamson at cl.cam.ac.uk>