Python script for downloading NAR (Nix ARchive) files

download-nar.py

#!/usr/bin/env nix-shell
#! nix-shell --pure -i python3 -p nix python3 python3Packages.requests
import argparse
import requests
import subprocess
from pathlib import Path

# https://nix.dev/manual/nix/2.24/store/store-path
DIGEST_SIZE_BYTES = 32
STORE_PATH = "/nix/store"


def digest_from_store_path(store_path: str) -> str:
    start_pos = len(STORE_PATH) + 1
    return store_path[start_pos : start_pos + DIGEST_SIZE_BYTES]


def parse_narinfo(narinfo: str) -> dict:
    res = dict()
    for line in narinfo.splitlines():
        key, value = line.split(": ")
        res[key] = value
    return res


parser = argparse.ArgumentParser()
parser.add_argument("-b", "--binary-cache", default="cache.nixos.org")
parser.add_argument("-f", "--file")
parser.add_argument("-o", "--output")
parser.add_argument("installable")
args = parser.parse_args()

eval_command = ["nix", "eval", "--raw"]
if args.file is not None:
    eval_command += ["--file", args.file]
eval_command += [f"{args.installable}.outPath"]

store_path = subprocess.run(
    eval_command,
    capture_output=True,
    check=True,
    text=True,
).stdout
path_digest = digest_from_store_path(store_path)

s = requests.Session()

narinfo_resp = s.get(f"https://{args.binary_cache}/{path_digest}.narinfo")
narinfo_resp.raise_for_status()

nar_url = parse_narinfo(narinfo_resp.text)["URL"]

nar_resp = s.get(f"https://{args.binary_cache}/{nar_url}")
nar_resp.raise_for_status()

out_path = Path()
if args.output is not None:
    out_path = Path(args.output)
else:
    out_path = Path(nar_url).name

with open(out_path, "xb") as file:
    for chunk in nar_resp.iter_content(chunk_size=1024):
        file.write(chunk)