Fix IOWrapper indexing the path instead of the file

[ESultanik]

Stacked on #15.

IOWrapper advertises itself as a collections.abc.Sequence, but dispatched on isinstance(..., Sequence)/Sized at call time — and str is itself a Sequence. So for a file path, every sequence operation silently applied to the path text rather than the file's contents:

>>> open('/tmp/f.bin','wb').write(bytes(range(20)))
>>> len(IOWrapper('/tmp/f.bin'))
14                      # the length of the path string
>>> IOWrapper('/tmp/f.bin')[5]
'_'                     # a character of the path, not even an int

The source kind is now decided once in __init__ — path, stdin, stream, or bytes — so indexing always reads content. Four further defects fall out of that:

• The integer branch never seeked, so every index returned byte 0.
• The stepped-slice branch did ret.append(r) with the one-byte bytes from read(1); bytearray.append needs an int, so it raised TypeError.
• __enter__ recorded any handle that merely differed from self.wrapped, so IOWrapper('-') closed sys.stdin on exit. Ownership is now explicit: only handles we opened get closed.
• '-' returned the text sys.stdin, so reads yielded str and callers doing b[0] & 0xff hit TypeError. It now returns sys.stdin.buffer.

Negative indices and open-ended slices are handled, and the length is cached (via os.path.getsize for paths).

get_length seeks to the end instead of reading the whole stream in 1 KiB chunks, keeping a chunked fallback for non-seekable streams at 1 MiB.

Dead code replaced

AutoUnzippingStream and GzipIOWrapper — both referenced nowhere — are replaced by an auto_unzip context manager built on ExitStack. Two improvements over what they did: layers tear down outermost-first (the old __exit__ closed the underlying stream before the gzip wrapper), and buffering sits on top of the gzip layer rather than beneath it, which is where it matters since the varint decoder reads a byte at a time. Nothing consumes it yet; the CLI decrypt path adopts it next.

Verification

tests/test_iowrapper.py: 58 cases across path, str-path, bytes, bytearray, stream, and int-iterable sources, plus a subprocess check that stdin is read as binary and survives the context manager. Two xfail markers removed.

🤖 Generated with Claude Code

https://claude.ai/code/session_01367hFob9sd4xpDmVT4uoFy