#!/bin/sh
# atgc-change-id-hook v1
#
# Stamp a `Change-Id:` trailer on a commit that has none. The id is what a
# stacked pull request is matched to its commits by, and it is the one thing
# about a commit that survives an amend or a rebase — so it has to be written
# when the commit is, not bolted on afterwards by rewriting the branch.
#
# Installed by `atgc repo configure`. Deleting this file is a supported way
# to turn it off; atgc reinstalls it only when the slot is empty.
#
# Nothing here prompts, and nothing here fails a commit: every path that
# cannot produce an id leaves the message exactly as it found it and exits 0.

msg="$1"
[ -n "$msg" ] || exit 0
[ -f "$msg" ] || exit 0

# Already carries one. Also covers a commit being amended, whose trailer is
# already in the message being re-edited.
if grep -qi '^Change-Id:[[:space:]]*I' "$msg"; then
    exit 0
fi

# A fixup or squash belongs to the commit it names, and will be folded into
# it: an id of its own would be one more id than there are changes.
if head -n 1 "$msg" | grep -qE '^(fixup|squash|amend)! '; then
    exit 0
fi

# An empty message aborts the commit. Stamping one would turn that abort into
# a commit whose entire message is a trailer.
if [ -z "$(sed -e 's/#.*//' -e '/^[[:space:]]*$/d' "$msg")" ]; then
    exit 0
fi

# Unique, not reproducible: author, time, pid and the message itself. Cut to
# the 40 hex digits a change-id is written with, since a repository using
# sha256 hashes would otherwise hand back 64.
id=$(
    {
        git var GIT_AUTHOR_IDENT 2>/dev/null
        echo "$$"
        date +%s 2>/dev/null
        cat "$msg"
    } | git hash-object --stdin 2>/dev/null | cut -c1-40
)
[ -n "$id" ] || exit 0

# `interpret-trailers` places it in the trailer block, after a Signed-off-by
# and before nothing, and knows about comments and the scissors line — all
# the reasons not to append a line by hand here.
git interpret-trailers --if-exists doNothing --trailer "Change-Id: I$id" --in-place "$msg" 2>/dev/null

exit 0