view contrib/editmerge @ 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

#!/usr/bin/env bash
# A simple script for opening merge conflicts in the editor.
# Use the following Mercurial settings to enable it.
#
# [ui]
# merge = editmerge
#
# [merge-tools]
# editmerge.args=$output
# editmerge.check=changed
# editmerge.premerge=keep

FILE="$1"

getlines() {
  grep -n "^<<<<<<" "$FILE" | cut -f1 -d:
}

# editor preference loosely based on https://mercurial-scm.org/wiki/editor
# hg showconfig is at the bottom though, since it's slow to run (0.15 seconds)
ED="$HGEDITOR"
if [ "$ED" = "" ] ; then
  ED="$VISUAL"
fi
if [ "$ED" = "" ] ; then
  ED="$EDITOR"
fi
if [ "$ED" = "" ] ; then
  ED="$(hg showconfig ui.editor)"
fi
if [ "$ED" = "" ] ; then
  echo "merge failed - unable to find editor"
  exit 1
fi

if [ "$ED" = "emacs" ] || [ "$ED" = "nano" ] || [ "$ED" = "vim" ] ; then
  FIRSTLINE="$(getlines | head -n 1)"
  PREVIOUSLINE=""

  # open the editor to the first conflict until there are no more
  # or the user stops editing the file
  while [ ! "$FIRSTLINE" = "" ] && [ ! "$FIRSTLINE" = "$PREVIOUSLINE" ] ; do
    $ED "+$FIRSTLINE" "$FILE"
    PREVIOUSLINE="$FIRSTLINE"
    FIRSTLINE="$(getlines | head -n 1)"
  done
else
  $ED "$FILE"
fi

# get the line numbers of the remaining conflicts
CONFLICTS="$(getlines | sed ':a;N;$!ba;s/\n/, /g')"
if [ ! "$CONFLICTS" = "" ] ; then
  echo "merge failed - resolve the conflicts (line $CONFLICTS) then use 'hg resolve --mark'"
  exit 1
fi

exit 0