hgeditor

rust: pure Rust lazyancestors iterator

This is the first of a patch series aiming to provide an
alternative implementation in the Rust programming language
of the _lazyancestorsiter from the ancestor module.

This iterator has been brought to our attention by the people at
Octobus, as a potential good candidate for incremental "oxydation"
(rewriting in Rust), because it has shown performance issues lately
and it merely deals with ints (revision numbers) obtained by calling
the index, whih should be directly callable from Rust code,
being itself implemented as a C extension.

The idea behind this series is to provide a minimal example of Rust code
collaborating with existing C and Python code. To open the way to gradually
rewriting more of Mercurial's Python code in Rust, without being forced to pay
a large initial cost of rewriting the existing fast core into Rust.

This patch does not introduce any bindings to other Mercurial code
yet. Instead, it introduces the necessary abstractions to address the problem
independently, and unit-test it.

Since this is the first use of Rust as a Python module within Mercurial,
the hg-core crate gets created within this patch. See its Cargo.toml for more
details.

Someone with a rustc/cargo installation may chdir into rust/hg-core and
run the tests by issuing:

   cargo test --lib

The algorithm is a bit simplified (see details in docstrings),
and at its simplest becomes rather trivial, showcasing that Rust has
batteries included too: BinaryHeap, the Rust analog of Python's heapq
does actually all the work.

The implementation can be further optimized and probably be made more
idiomatic Rust.

#!/bin/sh
#
# This is an example of using HGEDITOR to create of diff to review the
# changes while committing.

# If you want to pass your favourite editor some other parameters
# only for Mercurial, modify this:
case "${EDITOR}" in
    "")
        EDITOR="vi"
        ;;
    emacs)
        EDITOR="$EDITOR -nw"
        ;;
    gvim|vim)
        EDITOR="$EDITOR -f -o"
        ;;
esac


HGTMP=""
cleanup_exit() {
    rm -rf "$HGTMP"
}

# Remove temporary files even if we get interrupted
trap "cleanup_exit" 0 # normal exit
trap "exit 255" HUP INT QUIT ABRT TERM

HGTMP=$(mktemp -d ${TMPDIR-/tmp}/hgeditor.XXXXXX)
[ x$HGTMP != x -a -d $HGTMP ] || {
  echo "Could not create temporary directory! Exiting." 1>&2
  exit 1
}

(
    grep '^HG: changed' "$1" | cut -b 13- | while read changed; do
        "$HG" diff "$changed" >> "$HGTMP/diff"
    done
)

cat "$1" > "$HGTMP/msg"

MD5=$(which md5sum 2>/dev/null) || \
    MD5=$(which md5 2>/dev/null)
[ -x "${MD5}" ] && CHECKSUM=`${MD5} "$HGTMP/msg"`
if [ -s "$HGTMP/diff" ]; then
    $EDITOR "$HGTMP/msg" "$HGTMP/diff" || exit $?
else
    $EDITOR "$HGTMP/msg" || exit $?
fi
[ -x "${MD5}" ] && (echo "$CHECKSUM" | ${MD5} -c >/dev/null 2>&1 && exit 13)

mv "$HGTMP/msg" "$1"

exit $?