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# This script lets you update a hierarchy of files in an atomic way by
# first creating a new hierarchy using rsync's --link-dest option, and
# then swapping the hierarchy into place.  **See the usage message for
# more details and some important caveats!**

import os, sys, re, subprocess, shutil

ALT_DEST_ARG_RE = re.compile('^--[a-z][^ =]+-dest(=|$)')

RSYNC_PROG = '/usr/bin/rsync'

def main():
    cmd_args = sys.argv[1:]
    if '--help' in cmd_args:
        usage_and_exit()

    if len(cmd_args) < 2:
        usage_and_exit(True)

    dest_dir = cmd_args[-1].rstrip('/')
    if dest_dir == '' or dest_dir.startswith('-'):
        usage_and_exit(True)

    if not os.path.isdir(dest_dir):
        die(dest_dir, "is not a directory or a symlink to a dir.\nUse --help for help.")

    bad_args = [ arg for arg in cmd_args if ALT_DEST_ARG_RE.match(arg) ]
    if bad_args:
        die("You cannot use the", ' or '.join(bad_args), "option with atomic-rsync.\nUse --help for help.")

    # We ignore exit-code 24 (file vanished) by default.
    allowed_exit_codes = '0 ' + os.environ.get('ATOMIC_RSYNC_OK_CODES', '24')
    try:
        allowed_exit_codes = set(int(num) for num in re.split(r'[, ]+', allowed_exit_codes) if num != '')
    except ValueError:
        die('Invalid integer in ATOMIC_RSYNC_OK_CODES:', allowed_exit_codes[2:])

    symlink_content = os.readlink(dest_dir) if os.path.islink(dest_dir) else None

    dest_arg = dest_dir
    dest_dir = os.path.realpath(dest_dir) # The real destination dir with all symlinks dereferenced
    if dest_dir == '/':
        die('You must not use "/" as the destination directory.\nUse --help for help.')

    old_dir = new_dir = None
    if symlink_content is not None and dest_dir.endswith(('-1','-2')):
        if not symlink_content.endswith(dest_dir[-2:]):
            die("Symlink suffix out of sync with dest_dir name:", symlink_content, 'vs', dest_dir)
        num = 3 - int(dest_dir[-1]);
        old_dir = None
        new_dir = dest_dir[:-1] + str(num)
        symlink_content = symlink_content[:-1] + str(num)
    else:
        old_dir = dest_dir + '~old~'
        new_dir = dest_dir + '~new~'

    cmd_args[-1] = new_dir + '/'

    if old_dir is not None and os.path.isdir(old_dir):
        shutil.rmtree(old_dir)
    if os.path.isdir(new_dir):
        shutil.rmtree(new_dir)

    child = subprocess.run([RSYNC_PROG, '--link-dest=' + dest_dir, *cmd_args])
    if child.returncode not in allowed_exit_codes:
        die('The rsync copy failed with code', child.returncode, exitcode=child.returncode)

    if not os.path.isdir(new_dir):
        die('The rsync copy failed to create:', new_dir)

    if old_dir is None:
        atomic_symlink(symlink_content, dest_arg)
    else:
        os.rename(dest_dir, old_dir)
        os.rename(new_dir, dest_dir)


def atomic_symlink(target, link):
    newlink = link + "~new~"
    try:
        os.unlink(newlink); # Just in case
    except OSError:
        pass
    os.symlink(target, newlink)
    os.rename(newlink, link)


def usage_and_exit(use_stderr=False):
    usage_msg = """\
Usage: atomic-rsync [RSYNC-OPTIONS] [HOST:]/SOURCE/DIR/ /DEST/DIR/
       atomic-rsync [RSYNC-OPTIONS] HOST::MOD/DIR/ /DEST/DIR/

This script lets you update a hierarchy of files in an atomic way by first
creating a new hierarchy (using hard-links to leverage the existing files),
and then swapping the new hierarchy into place.  You must be pulling files
to a local directory, and that directory must already exist.  For example:

    mkdir /local/files-1
    ln -s files-1 /local/files
    atomic-rsync -aiv host:/remote/files/ /local/files/

If /local/files is a symlink to a directory that ends in -1 or -2, the copy
will go to the alternate suffix and the symlink will be changed to point to
the new dir.  This is a fully atomic update.  If the destination is not a
symlink (or not a symlink to a *-1 or a *-2 directory), this will instead
create a directory with "~new~" suffixed, move the current directory to a
na