find max supported xattr size

find-max-xattr-size.py

#!/usr/bin/env python

import xattr

XATTR_NAME = 'user.xattr-test'
MAX_SIZE = 2**20 + 1  # if it's over 1 MiB, call it infinite


def works(fileno, size):
    try:
        xattr.setxattr(fileno, XATTR_NAME, 'a' * size)
        return True
    except IOError:
        return False


def find_max_xattr_size():
    lower = 1
    upper = None

    with open('xattr-testfile', 'w') as fh:
        fileno = fh.fileno()

        if not works(fileno, 1):
            return None

        # try to find an upper bound by repeated doubling
        size = 2
        while works(fileno, size) and size < MAX_SIZE:
            size *= 2
        if size >= MAX_SIZE:
            return float("inf")

        # binary search to find the maximum size that works
        upper = size
        while upper > (lower + 1):
            mid = (upper + lower) / 2
            if works(fileno, mid):
                lower = mid
            else:
                upper = mid
        return lower


if __name__ == '__main__':
    biggest = find_max_xattr_size()
    if biggest is None:
        print("FS doesn't seem to support extended attributes")
    elif biggest == float("inf"):
        print("FS supports really big extended attributes (over 1 MiB)")
    else:
        print("FS supports extended attributes up to %d bytes" % biggest)