Decompress WeChat weapp binary file bundles (after zstd decompression)

decompress.py

# Decompress WeChat weapp binary file bundles (after zstd decompression)
import os
import sys
import struct
import errno

HEADER_LEN = 14

if (len(sys.argv) != 3):
  print(f"Usage: {sys.argv[0]} <file.bin> <outputdir>")
  sys.exit(1)

print(f"Opening file of size {os.path.getsize(sys.argv[1])}")
fin = open(sys.argv[1], "rb")
header = fin.read(HEADER_LEN)
start, pad, metalen, datalen, end = struct.unpack(">BLLLB", header)
if (start != 0xbe or pad != 0 or end != 0xed):
  print("Invalid header")
  sys.exit(1)

print(f"Metadata length {metalen}, data length {datalen} total {HEADER_LEN+metalen+datalen}")
item_count = struct.unpack(">L", fin.read(4))[0]
print(f"Metadata contains {item_count} items")
processed_bytes = 0
for i in range(item_count):
  (namelen,) = struct.unpack(">L", fin.read(4))
  filename = fin.read(namelen).decode()
  filepos, filelen = struct.unpack(">LL", fin.read(8))

  # Read the file data and return to same offset
  offset = fin.tell()
  fin.seek(filepos)
  filedata = fin.read(filelen)
  fin.seek(offset)
  processed_bytes += filelen

  # Write the file data to corresponding file
  filepath = f"{sys.argv[2]}{filename}"
  try:
    os.makedirs(os.path.dirname(filepath))
  except OSError as exc:
    if exc.errno != errno.EEXIST:
      raise
  with open(filepath, "wb") as fout:
    fout.write(filedata)
  print(f"Processed file: {filename}")
print(f"Processed metadata of {fin.tell()-HEADER_LEN} bytes, reading {processed_bytes} bytes of file data")
fin.close()