#!/bin/sh
self=watch-git-tags

# List sorted local tags
local_tags() {
    for repo ; do
        git -C "$repo" tag --list | LC_COLLATE=C sort
    done
}

# List sorted remote tags
remote_tags() {
    for repo ; do
        git -C "$repo" ls-remote --quiet --refs --tags ||
            printf >&2 'Failed to retrieve tags for repository %s\n' "$PWD"
    done |
    while read -r _ tag ; do
        tag=${tag#refs/tags/}
        printf '%s\n' "$tag"
    done
}

# Create a temporary directory with name in $td, and handle POSIX-ish traps to
# remove it when the script exits; requires mktemp(1) (not POSIX)
td=$(mktemp -d) || exit
cleanup() {
    [ -n "$td" ] || return
    rm -fr -- "$td"
}
for sig in EXIT HUP INT TERM ; do
    # shellcheck disable=SC2064
    trap "cleanup $sig" "$sig"
done

# Use current directory if no other arguments
[ "$#" -gt 0 ] || set -- .

# Iterate through each repo in a subshell in parallel
for repo ; do (

    # Make a temporary directory with a hash in its name for uniqueness
    df=$(printf %s "$repo" | sed s:/:_:g)
    cs=$(printf %s "$repo" | cksum)
    sd=$td/$df.${cs%% *}
    mkdir -- "$sd" || exit

    # Step in and write repo path to file
    cd -- "$sd" || exit
    printf '%s\n' "$repo" > path || exit

    # Write local and remote tags to files
    local_tags "$repo" > "$sd"/a || exit
    remote_tags "$repo" > "$sd"/b || exit

    # Write new tags to file
    LC_COLLATE=C comm -13 -- [ab] > new

    # Attempt to quietly fetch new tags so that we don't notify about the same
    # ones next time
    [ -s new ] || continue
    git -C "$repo" fetch --quiet --tags

) & done

# Wait for all of those to finish
wait

# Iterate through the temp dirs in order
for dir in "$td"/* ; do (
    cd -- "$dir" || exit 0

    # Look for non-zero "new" files (at least one new tag)
    [ -s new ] || exit 0

    # Print repository path and new tags
    sed '1!s/^/\t/' -- path new
    exit 1

) ; done