Recover corrupted xz files with missing end-of-stream markers

read-partial-xz.py

#! /usr/bin/env python3
# Work around unxz throwing "Unexpected end of input" errors and refusing to decompress anything
# This can't recover everything, but it'll try, byte by byte if necessary

import lzma
import pathlib
import sys
# Adjust this to your workload: Seeks are more expensive than small buffer copies, 
# setting this too high does not help performance
INITIAL_BLOCK_SIZE=20

for infile in sys.argv[1:]:
	outfile = pathlib.Path(infile[:-3] if infile.endswith(".xz") else infile+".raw")
	print(f"Decompressing as much as possible of {infile} to {outfile}")

	if outfile.exists():
		print(f"Skipping {infile}, decompress file {outfile} already exists")
		continue

	out_fd = outfile.open("wb")
	# Recourd the last successful read position, so we can retry from it with small block sizes when errors occur
	last_seek = 0
	# Optimistically start at 16 MiB reads
	block_size = 1 << INITIAL_BLOCK_SIZE
	errors = 0
	with lzma.open(infile) as in_fd:
		while True:
			try:
				out_fd.write(in_fd.read(block_size))
				last_seek += block_size
				errors = 0
				print("\tRead", block_size, "bytes, reached position", last_seek)
			except EOFError as e:
				errors += 1
				block_size = block_size // 2
				in_fd.seek(last_seek)
				if errors > INITIAL_BLOCK_SIZE or block_size < 1:
					print ("\tError count exceeded or minimum block size reached, giving up")
					break
				print(f"\tIgnoring exception {type(e)} ({e}) and re-trying from {last_seek} with block size {block_size}")
	print(f"{infile}: {last_seek} bytes written to {outfile}")