#!/usr/bin/python3

# This works around that `mount --bind /foo /bar` is non idempotent. Calling it
# 4 times in a row will cause it to be bound over itself 4 times.


import argparse
import os
import pathlib
import sys


def find_real_mount():
    """Look through $PATH to find the first 'mount' that isn't us"""
    myPath = pathlib.Path(sys.argv[0]).absolute()
    path = os.get_exec_path()
    for d in path:
        mountPath = pathlib.Path(d) / "mount"
        if not mountPath.exists() or not os.access(mountPath, os.X_OK):
            continue
        absPath = mountPath.absolute()
        if absPath == myPath:
            continue
        return absPath


def passthrough(args):
    """Find a system 'mount' command and execute it instead"""
    mntPath = find_real_mount()
    args = sys.argv[:]
    args[0] = str(mntPath)
    os.execv(args[0], args)


def parse_mount_line(line):
    mountID, parentID, major_minor, source, target, _ = line.split(' ', 5)
    # print(source, target)
    return source, target

def lookfor_mount_info_target(source, target):
    """Check if we are already bind mounted at this target"""
    source = os.path.abspath(source)
    target = os.path.abspath(target)
    with open('/proc/self/mountinfo', 'r') as f:
        for line in f:
            if target not in line:
                continue
            mntsource, mnttarget = parse_mount_line(line)
            if mnttarget != target:
                # print(f"not target: {line!r}")
                continue
            # this looks to be our target, does the source match?
            if source.endswith(mntsource):
                # found us, already there
                # print(f"target already found: {line!r}")
                return True
            # We found a mount target that was exactly us, but it didn't match, keep looking
    # We made it to the end, and didn't find anything
    return False

def main():
    args = sys.argv[1:]
    if len(args) != 3 or args[0] != '--bind':
        passthrough(args)
        return
    source, target = args[1:]
    if lookfor_mount_info_target(source, target):
        print(f"{source} is already bind mounted to {target}")
    else:
        passthrough(args)


if __name__ == '__main__':
    main()