hgmerge

fix race in localrepo.addchangegroup.

localrepo.addchangegroup writes to changelog, then manifest, then normal
files.  this breaks access ordering.  if reader reads changelog while
manifest is being written, can find pointers into places in manifest
that are not yet written.  same can happen for manifest and normal files.

fix is to make almost no change to localrepo.addchangegroup.  it must
to write changelog and manifest data early because it has to read them
while writing other files.  instead, write changelog and manifest data
to temp file that reader cannot see, then append temp data to manifest
after all normal files written, finally append temp data to changelog.

temp file code is in new appendfile module.  can be used in other places
with small changes.

much smaller race still left.  we write all new data in one write call,
but reader can maybe see partial update because python or os or filesystem
cannot always make write really atomic. file locking no help: slow, not
portable, not reliable over nfs.  only real safe other plan is write to
temp file every time and rename, but performance bad when manifest or
changelog is big.

#!/bin/sh
#
# hgmerge - default merge helper for Mercurial
#
# This tries to find a way to do three-way merge on the current system.
# The result ought to end up in $1.  Script is run in root directory of
# repository.
#
# Environment variables set by Mercurial:
# HG_FILE            name of file within repo
# HG_MY_NODE         revision being merged
# HG_OTHER_NODE      revision being merged

set -e # bail out quickly on failure

LOCAL="$1"
BASE="$2"
OTHER="$3"

if [ -z "$EDITOR" ]; then
    EDITOR="vi"
fi

# find decent versions of our utilities, insisting on the GNU versions where we
# need to
MERGE="merge"
DIFF3="gdiff3"
DIFF="gdiff"
PATCH="gpatch"

type "$MERGE" >/dev/null 2>&1 || MERGE=
type "$DIFF3" >/dev/null 2>&1 || DIFF3="diff3"
$DIFF3 --version >/dev/null 2>&1 || DIFF3=
type "$DIFF"  >/dev/null 2>&1 || DIFF="diff"
type "$DIFF"  >/dev/null 2>&1 || DIFF=
type "$PATCH" >/dev/null 2>&1 || PATCH="patch"
type "$PATCH" >/dev/null 2>&1 || PATCH=

# find optional visual utilities
FILEMERGE="/Developer/Applications/Utilities/FileMerge.app/Contents/MacOS/FileMerge"
KDIFF3="kdiff3"
TKDIFF="tkdiff"
MELD="meld"

type "$FILEMERGE" >/dev/null 2>&1 || FILEMERGE=
type "$KDIFF3"    >/dev/null 2>&1 || KDIFF3=
type "$TKDIFF"    >/dev/null 2>&1 || TKDIFF=
type "$MELD"      >/dev/null 2>&1 || MELD=

# random part of names
RAND="$RANDOM$RANDOM"

# temporary directory for diff+patch merge
HGTMP="${TMPDIR-/tmp}/hgmerge.$RAND"

# backup file
BACKUP="$LOCAL.orig.$RAND"

# file used to test for file change
CHGTEST="$LOCAL.chg.$RAND"

# put all your required cleanup here
cleanup() {
    rm -f "$BACKUP" "$CHGTEST"
    rm -rf "$HGTMP"
}

# functions concerning program exit
success() {
    cleanup
    exit 0
}

failure() {
    echo "merge failed" 1>&2
    mv "$BACKUP" "$LOCAL"
    cleanup
    exit 1
}

# Ask if the merge was successful
ask_if_merged() {
    while true; do
        echo "$LOCAL seems unchanged."
        echo "Was the merge successful? [y/n]"
        read answer
        case "$answer" in
            y*|Y*) success;;
            n*|N*) failure;;
        esac
    done
}

# Clean up when interrupted
trap "failure" 1 2 3 6 15 # HUP INT QUIT ABRT TERM

# Back up our file (and try hard to keep the mtime unchanged)
mv "$LOCAL" "$BACKUP"
cp "$BACKUP" "$LOCAL"

# Attempt to do a non-interactive merge
if [ -n "$MERGE" -o -n "$DIFF3" ]; then
    if [ -n "$MERGE" ]; then
        $MERGE "$LOCAL" "$BASE" "$OTHER" 2> /dev/null && success
    elif [ -n "$DIFF3" ]; then
        $DIFF3 -m "$BACKUP" "$BASE" "$OTHER" > "$LOCAL" && success
    fi
    if [ $? -gt 1 ]; then
        echo "automatic merge failed! Exiting." 1>&2
        failure
    fi
fi

# on MacOS X try FileMerge.app, shipped with Apple's developer tools
if [ -n "$FILEMERGE" ]; then
    cp "$BACKUP" "$LOCAL"
    cp "$BACKUP" "$CHGTEST"
    # filemerge prefers the right by default
    $FILEMERGE -left "$OTHER" -right "$LOCAL" -ancestor "$BASE" -merge "$LOCAL"
    [ $? -ne 0 ] && echo "FileMerge failed to launch" && failure
    test "$LOCAL" -nt "$CHGTEST" && success || ask_if_merged
fi

if [ -n "$DISPLAY" ]; then
    # try using kdiff3, which is fairly nice
    if [ -n "$KDIFF3" ]; then
        $KDIFF3 --auto "$BASE" "$BACKUP" "$OTHER" -o "$LOCAL" || failure
        success
    fi

    # try using tkdiff, which is a bit less sophisticated
    if [ -n "$TKDIFF" ]; then
        $TKDIFF "$BACKUP" "$OTHER" -a "$BASE" -o "$LOCAL" || failure
        success
    fi

    if [ -n "$MELD" ]; then
        cp "$BACKUP" "$CHGTEST"
        # protect our feet - meld allows us to save to the left file
        cp "$BACKUP" "$LOCAL.tmp.$RAND"
        # Meld doesn't have automatic merging, so to reduce intervention
        # use the file with conflicts
        $MELD "$LOCAL.tmp.$RAND" "$LOCAL" "$OTHER" || failure
        # Also it doesn't return good error code
        test "$LOCAL" -nt "$CHGTEST" && success || ask_if_merged
    fi
fi

# Attempt to do a merge with $EDITOR
if [ -n "$MERGE" -o -n "$DIFF3" ]; then
    echo "conflicts detected in $LOCAL"
    cp "$BACKUP" "$CHGTEST"
    $EDITOR "$LOCAL" || failure
    # Some editors do not return meaningful error codes
    # Do not take any chances
    test "$LOCAL" -nt "$CHGTEST" && success || ask_if_merged
fi

# attempt to manually merge with diff and patch
if [ -n "$DIFF" -a -n "$PATCH" ]; then

    (umask 077 && mkdir "$HGTMP") || {
        echo "Could not create temporary directory $HGTMP" 1>&2
        failure
    }

    $DIFF -u "$BASE" "$OTHER" > "$HGTMP/diff" || :
    if $PATCH "$LOCAL" < "$HGTMP/diff"; then
        success
    else
        # If rejects are empty after using the editor, merge was ok
        $EDITOR "$LOCAL" "$LOCAL.rej" || failure
        test -s "$LOCAL.rej" || success
    fi
    failure
fi

echo
echo "hgmerge: unable to find any merge utility!"
echo "supported programs:"
echo "merge, FileMerge, tkdiff, kdiff3, meld, diff+patch"
echo
failure