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

--- a/mercurial/transaction.py	Tue Sep 17 15:09:25 2019 -0700
+++ b/mercurial/transaction.py	Tue Sep 17 14:01:26 2019 -0700
@@ -54,6 +54,10 @@
             checkambig = checkambigfiles and (f, '') in checkambigfiles
             try:
                 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()))
                 fp.truncate(o)
                 fp.close()
             except IOError: