DeComp(): make the grow step actually double + guard empty input

[IrPgFKS0]

Problem

The grow step in DeComp() (src/Common.cpp) doesn't do what its comment says:

// if decompression fails, we double the buffer size (up to the allowed limit) and try again
output_buffer.resize(std::max<size_t>(output_buffer.size() * 2, MAX_DECOMPRESSION_BUFFER_SIZE));

std::max(size * 2, 30MB) is almost always 30 MB, so every packet that needs one grow allocates the full 30 MB instead of doubling — on every such packet, on the hot receive path. The intended capped doubling is std::min.

Fix

• Change the grow to std::min<size_t>(output_buffer.size() * 2, MAX_DECOMPRESSION_BUFFER_SIZE) so it actually doubles up to the cap (the existing >= cap check above still terminates the loop).
• Add an empty-input guard (throws, same error path as other invalid inputs). This is required for the min-fix to be safe: with std::min, a zero-length body (e.g. a bare 4-byte ABG: frame from a buggy/malicious peer) would keep the buffer at 0 forever (0 * 2 stays 0) and spin the loop — today's std::max is accidentally the only thing preventing that. The guard also skips a pointless 30 MB allocation + two uncompress calls for input that can never decompress.

The two changes are intentionally in one commit because each alone changes the safety story: min without the guard introduces an infinite loop; the guard without min leaves the 30 MB-per-grow over-allocation.

How this was found

Found while auditing decompression robustness on a LAN fork of BeamMP (the launcher's DeComp has the unguarded-doubling variant of this bug, submitted separately as BeamMP-Launcher#257); the equivalent fix has been running in that fork's server for weeks of sessions.

Transparency

This fix comes from an AI-assisted fork: the bug was found and the patch written with the help of an AI coding tool (Claude), then tested by a human in real multiplayer sessions. Given this project's policy on AI-generated code, it's submitted as a draft for the maintainers to decide — happy to close it if that's not wanted, or for a maintainer to re-implement it independently.

🤖 Generated with Claude Code