Slurp a file into stdout. Disk space is freed as soon as contents are read

slurp.py

#!/usr/bin/env python3

import argparse
import os
import sys

from ctypes import *

FALLOC_FL_KEEP_SIZE = 0x01
FALLOC_FL_PUNCH_HOLE = 0x02

def main():
  parser = argparse.ArgumentParser()
  parser.add_argument('fn', type=str, help='file name to slurp')
  args = parser.parse_args()

  libc = CDLL("libc.so.6", use_errno=True)
  offset = 0
  with open(args.fn, 'rb+') as f:
    while True:
      chunk = f.read(100 * 1024**2)  # 100MB block
      if not chunk:
        break
      sys.stdout.buffer.write(chunk)
      if libc.fallocate64(f.fileno(), FALLOC_FL_PUNCH_HOLE+FALLOC_FL_KEEP_SIZE, c_longlong(offset), c_longlong(len(chunk))) != 0:
        print(f"fallocate error: {os.strerror(ctypes.get_errno())} @ offset={offset}, chunk={len(chunk)}", file=sys.stderr)
      offset += len(chunk)

if __name__ == "__main__":
    main()