Bounded zlib decompression: enforce the cumulative output budget while decoding

What it is

Python's zlib decompression object accepts max_length to limit output from one decompress call. Unprocessed input can remain in unconsumed_tail; eof distinguishes a complete stream, while unused_data describes bytes beyond its end. The limit is per call, so a caller must separately enforce its cumulative output budget. Python zlib

Why it matters

An agent may add a fixed chunk size and assume total memory is bounded while appending every chunk to a growing list. Define limits on compressed input, produced output and processing effort before decoding. Also decide whether the protocol permits concatenated streams or trailing bytes.

How to apply

• Choose a task-specific maximum expanded size and track the remaining budget after every produced chunk. Reject excess output instead of retaining it for later validation.
• Pass an explicit positive output allowance to each bounded decompress call. Treat zero carefully because the documented default means unbounded output, not permission for zero bytes.
• Handle unconsumed_tail before advancing past input that the decoder has not processed. At the output boundary, use a deliberate completion or excess-data detection policy rather than an unbounded flush.
• After input ends, inspect eof and apply the protocol's policy for unused_data. Distinguish a truncated stream from an otherwise valid stream with forbidden trailing content.
• Propose fixtures for an exact-budget result, an over-budget result, truncation, trailing data and chunk boundaries inside the compressed representation.

Pitfalls

An output-size ceiling does not by itself bound CPU time, compressed-input storage or the memory of downstream parsing. The flush length parameter describes an initial buffer size rather than a cumulative safety ceiling. Consider an external execution budget where the threat model requires it. These are proposed defensive checks; no decompression benchmark or resource-exhaustion test is claimed.