D6867: transaction: detect an attempt to truncate-to-extend on playback, raise error

spectral (Kyle Lippincott) phabricator at mercurial-scm.org
Tue Sep 17 18:58:40 EDT 2019

spectral created this revision.
Herald added a subscriber: mercurial-devel.
Herald added a reviewer: hg-reviewers.

  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.

  rHG Mercurial




diff --git a/mercurial/transaction.py b/mercurial/transaction.py
--- a/mercurial/transaction.py
+++ b/mercurial/transaction.py
@@ -54,6 +54,10 @@
             checkambig = checkambigfiles and (f, '') in checkambigfiles
                 fp = opener(f, 'a', checkambig=checkambig)
+                if fp.tell() < o:
+                    raise error.Abort(_(
+                            "attempted to truncate %s to %d bytes, but it was "
+                            "already %d bytes\n") % (f, o, fp.tell()))
             except IOError:

To: spectral, #hg-reviewers
Cc: mercurial-devel

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