with-cache

#!/bin/bash
#
# Downloads the latest images, extracts their caching keys, and provides them
# into the environment so local images may be built using downloaded cached
# layers.
#
# This is necessary because usually local builds only use other locally built
# layers, so you must pay the cost of a full build at the beginning.
#
set -euo pipefail

: "${TAG:=latest}"

main() {
    pushd "$(dirname "$0")/.." >/dev/null
        pull-nextstrain-images
        pull-base-image
        export-build-args
    popd >/dev/null

    log "Running your command now…"
    exec "$@"
}

pull-nextstrain-images() {
    log "Pulling latest Nextstrain images (tagged $TAG)"
    ./devel/pull "$TAG"
}

pull-base-image() {
    local image="$(base-image)"

    log "Pulling latest $image base image (tagged $TAG)"
    docker image pull "$image"

    if ! has-layer nextstrain/base:"$TAG" "$(final-layer "$image")"; then
        die "nextstrain/base:$TAG is built with a version of $image that I can't find."
    fi
}

export-build-args() {
    build_args="$(build-args)"
    log "Exporting build args $build_args"
    export $build_args
}

build-args() {
    list-values nextstrain/base-builder:"$TAG" .ContainerConfig.Cmd \
        | awk '/^CACHE_DATE=(.*)/ { print }'
}

base-image() {
    awk '/^FROM/ { print $2; exit }' Dockerfile
}

final-layer() {
    local image="$1"
    local layer_count=$(inspect "$image" "{{len .RootFS.Layers}}")
    inspect "$image" "{{index .RootFS.Layers $(($layer_count - 1))}}"
}

has-layer() {
    local image="$1"
    local layer="$2"
    list-layers "$image" | grep -qF "$layer" 2>/dev/null
}

list-layers() {
    list-values "$1" .RootFS.Layers
}

list-values() {
    local image="$1"
    local field="$2"
    inspect "$image" "{{range $field}}{{.}}"$'\n'"{{end}}"
}

inspect() {
    local image="$1"
    local format="$2"
    docker image inspect "$image" --format "$format"
}

log() {
    echo "==> $*" >&2
}

die() {
    echo "==> $*" >&2
    exit 1
}

main "$@"