drop_buffer_cache.py

# -*- coding: utf-8 -*-
import ctypes
import ctypes.util
import os
import sys


class DropBufferCacheException(Exception):
    pass


# Drop 'buffer' cache for the given range of the given file.

POSIX_FADV_DONTNEED = 4
OK = 0


# this function is used if we can't load the real libc function


def noop_libc_function(*args):
    return 0


# I have no idea what this code really does, but strace says it works.
# does this code work under Cygwin?


def load_libc_function(func_name):
    func = noop_libc_function
    try:
        libc = ctypes.CDLL(ctypes.util.find_library("c"))
        func = getattr(libc, func_name)
    except AttributeError:
        # print("Unable to locate %s in libc.  Leaving as a no-op."% func_name)
        pass
    except Exception:
        # libc not available
        pass
    return func


# do this at module load time

_posix_fadvise = load_libc_function("posix_fadvise64")


def drop_buffer_cache(fd, offset, length):
    ret = _posix_fadvise(
        fd, ctypes.c_uint64(offset), ctypes.c_uint64(length), POSIX_FADV_DONTNEED
    )
    if ret != OK:
        raise DropBufferCacheException(
            "posix_fadvise64(%s, %s, %s, 4) -> %s" % (fd, offset, length, ret)
        )


# unit test

if __name__ == "__main__":
    fd = os.open("/tmp/foo", os.O_WRONLY | os.O_CREAT)
    if sys.version.startswith("3"):
        ret = os.write(fd, bytes("hi there", "UTF-8"))
    elif sys.version.startswith("2"):
        ret = os.write(fd, "hi there")
    else:
        raise DropBufferCacheException("unrecognized python version %s" % sys.version)
    assert ret == 8
    drop_buffer_cache(fd, 0, 8)
    os.close(fd)