Asar to UStar converter, used as follows:

$ < foo.asar python3 as2t.py | tar x

Does not include empty directories.

I’d like to dedicate 22% of this program to Asar’s Pickle nonsense.

 import json
 import tarfile
 from typing import NamedTuple
 
 
-_UINT32_SIZE = b'\x04\x00\x00\x00'
-
-
 class FileRecord(NamedTuple):
     path: str
     offset: int
     size: int
     executable: bool
 
 
 def _read_exact(f, count):
     result = f.read(count)
 
     if len(result) != count:
         raise ValueError('Unexpected end of Asar file')
 
     return result
 
 
 def _expect(expectation):
     if not expectation:
         raise ValueError('Unexpected data in Asar file')
 
 
 def _read_uint4_le(f):
     return int.from_bytes(_read_exact(f, 4), 'little')
 
 
-def _read_padding(f, size):
-    _expect(0 <= size < 4)
-
-    if size != 0:
-        _expect(f.read(size) == b'\0' * size)
-
-
 def _flatten(path, index):
     for key, value in index.items():
         _expect(key and '/' not in key and key not in {'.', '..'})
 
         if 'files' in value:
             yield from _flatten(path + (key,), value['files'])
         else:
             raw_offset = value['offset']
             raw_size = value['size']
             raw_executable = value.get('executable', False)
 
             _expect(isinstance(raw_offset, str))
             _expect(isinstance(raw_size, int))
             _expect(isinstance(raw_executable, bool))
 
             offset = int(raw_offset)
 
             _expect(offset >= 0)
             _expect(raw_size >= 0)
 
             yield FileRecord(path + (key,), offset, raw_size, raw_executable)
 
 
 def transform(f, out):
-    # header_size header
-    _expect(f.read(4) == _UINT32_SIZE)
-
     # header_size
     header_pickled_size = _read_uint4_le(f) - 4
 
-    # header header
-    _expect(_read_uint4_le(f) == header_pickled_size)
-
-    # header header 2: return of the length prefixes
-    header_unpadded_size = _read_uint4_le(f)
-    padding_size = header_pickled_size - 4 - header_unpadded_size
-
     header_bytes = _read_exact(f, header_unpadded_size)
     header = json.loads(header_bytes)
-
-    _read_padding(f, padding_size)
 
     offset = 0
 
     for r in sorted(_flatten((), header['files']), key=lambda r: r.offset):
         _expect(offset == r.offset)
         offset = r.offset + r.size
 
         info = tarfile.TarInfo(name='/'.join(r.path))
         info.size = r.size
         info.mode = 0o755 if r.executable else 0o644
         out.addfile(info, f)
 
 
 if __name__ == '__main__':
     import sys
 
     with tarfile.open(fileobj=sys.stdout.buffer, mode='w|') as out:
         transform(sys.stdin.buffer, out)