feat(receive): add maxsize argument to bound memory usage

[Tieske]

client:receive("*l") and receive("*a") are unbounded: a peer that never sends a newline, or never closes, makes LuaSocket buffer until the process runs out of memory (e.g. src/http.lua reading response headers in a loop). Add an optional maxsize argument that caps the payload a single call may accumulate, including prefix.

• Hoist all argument validation ahead of timeout_markstart() so bad calls (maxsize < 1, #prefix >= maxsize, numeric pattern > maxsize) raise before any I/O and leave the socket untouched.
• recvline/recvall take a budget and return a new internal BUF_OVERSIZED code, surfaced to Lua as the "oversized" error alongside "timeout"/"closed", with the partial held in the third return value.
• recvraw is left untouched: argument checks make the cap unreachable for numeric patterns.
• Preserve three invariants: a timeout partial is always shorter than maxsize (safe to retry as prefix), completion beats the cap for *a, and no bytes are lost or skipped on overflow.
• tcp.c, unixstream.c and serial.c all share this code path unchanged.

Adds test coverage (argument errors, *l/*a boundaries, timeout/close at the cap, the drain idiom, numeric patterns, unix-stream mirror) and documents the new argument, error, and recovery idioms in docs/tcp.html.

[Copilot]

Pull request overview

Adds an optional maxsize argument to client:receive() to cap how much data a single call may accumulate for *l and *a, preventing unbounded buffering when peers never send a newline or never close. This extends the existing receive API with a bounded-read mode and a new "oversized" error while preserving socket state and retry/drain invariants.

Changes:

• Add maxsize argument handling and early argument validation in buffer_meth_receive, plus a new internal BUF_OVERSIZED completion path.
• Implement budget-aware recvline/recvall behavior to stop at the cap and surface "oversized" with an exact-cap partial.
• Add tests covering boundary conditions and recovery idioms, and document the new argument/error semantics.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/utestclnt.lua	Adds unit tests for receive(..., maxsize) on key boundaries and error behavior.
test/testclnt.lua	Adds broader integration-style coverage for argument errors, *l/*a boundaries, drain idiom, and numeric-pattern behavior.
src/buffer.c	Implements maxsize validation, budgets for recvline/recvall, and maps cap hits to "oversized".
docs/tcp.html	Documents the new maxsize argument, the "oversized" error, and recommended drain/retry patterns.
CHANGELOG.md	Notes the new receive maxsize capability and "oversized" behavior in Unreleased.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.