Add support for indetifying whole directory trees using tar

This patch uses a clever invocation of GNU tar to produce a
deterministic bytestream from a directory tree. This stream is fed to a
hash function in 64KiB chunks (== Linux default pipe capacity) to
produce a fingerprint which can be displayed as an identicon.
Why would we do this instead of using the tarfile stdlib package or just
using os.walk plus some code?

The tarfile package is not capable of producing the output as a stream
of N byte chunks. The "most granual" mode of operation it can do is
producing all of the chunks belonging to a given file all at once.
This is problematic, because we could run out of memory or be forced to
write the tar archive to a temporary file - which would be painfully slow,
we could run out of disk space, wear out SSDs or outright refuse to run in a
container with a read-only rootfs and no tmpfs mounted.

An os.walk solution is doable, but will require some problem solving
which I am too lazy to do right now:
  - Forcing os.walk to walk in a deterministic order (should be easy)
  - Walks on different directory structures could theoretically produce
    the same bytestream (doable but requires some thinking)
The GNU tar solution is far from ideal (it forces an external dependency
and requires a subprocess call and some pipe juggling) but is very easy
to implement and should be fine performance wise:
  - The bottleneck on reasonable hardware configurations should
    be hashing or disk IO
  - The cost of doing a fork/exec is negligible compared to either

TL;DR os.walk: maybe in a future patch

main.py

@@ -1,37 +1,87 @@
-from sys import argv, stdin
+#!/usr/bin/env python3
+from sys import stdin
 from sys import exit as sysexit
 from hashlib import blake2b
 from io import BytesIO
+from subprocess import Popen, PIPE
+
+import click
 
 from identicon import Identicon
 
 
 DIGEST_SIZE = 20
-BUF_SIZE = 65536
+BUF_SIZE = 65536  # Linux default pipe capacity is 64KiB (64 * 2^10)
 
 
-def main():
-    if not (stream := get_input_stream()):
+@click.command(
+    help=(
+        'Print OpenSSH style randomart identicon for arbitrary data.\n\n'
+        'If TEXT or --file is not supplied, data is read from STDIN.'
+    )
+)
+@click.argument('text', default=None, type=str, required=False)
+@click.option(
+    '--file', '-f', default=None, type=click.Path(exists=True),
+    help='Calculate from file or directory (recursive).'
+)
+def main(**kwargs):
+    if not (stream := get_input_stream(kwargs)):
         print_usage_and_exit()
 
-    i = Identicon(get_digest(stream))
+    digest = get_digest(stream.stream)
+    stream.close()
+
+    i = Identicon(digest)
     i.calculate()
     print(i)
 
 
-def get_input_stream():
-    io = None
-    if len(argv) == 2:
-        io = BytesIO(argv[1].encode())
+def get_input_stream(kwargs):
+    stream = None
+    if (text := kwargs["text"]) is not None:
+        stream = ClosableStream(BytesIO(text.encode()))
+    elif file := kwargs["file"]:
+        stream = get_deterministic_tar_stream(file)
     elif not stdin.isatty():
-        io = stdin.buffer
-    return io
+        stream = ClosableStream(stdin.buffer)
+    return stream
+
+
+class ClosableStream:
+    def __init__(self, stream, close_func=None):
+        self.stream = stream
+        self._close_func = close_func or (lambda: None)
+
+    def close(self):
+        return self._close_func()
+
+
+def get_deterministic_tar_stream(file):
+    # pylint: disable=consider-using-with
+    cmd = (
+        'tar',
+        f'--blocking-factor={BUF_SIZE//512}',
+        '--sort=name',
+        '--mtime=UTC 1970-01-01',
+        '--owner=root:0', '--group=root:0', '--numeric-owner',
+        '--mode=a=rwX', '--no-acls', '--no-xattrs', '--no-selinux',
+        '-C', file, '-cf', '-', '.'
+    )
+    p = Popen(cmd, stdout=PIPE, stderr=PIPE)
+
+    def wait_and_check_exitcode():
+        exit_code = p.wait()
+        if exit_code != 0:
+            raise RuntimeError(f"Tar failed: {p.stderr.read().decode()}")
+
+    return ClosableStream(p.stdout, wait_and_check_exitcode)
 
 
 def print_usage_and_exit():
-    print('Usage: identicon [TEXT]')
-    print('Print OpenSSH style randomart identicon for arbitrary data.\n')
-    print('If TEXT is not supplied, data is read from STDIN.')
+    command = main
+    with click.Context(command) as ctx:
+        click.echo(command.get_help(ctx))
     sysexit(1)