feat: add BroadcastChannel JS bridge for browser-based link cable multiplayer

[bangnokia]

BroadcastChannel JS Bridge for Browser-Based Link Cable Multiplayer

Add a lightweight JavaScript bridge that enables GBA link cable multiplayer between browser tabs using the BroadcastChannel API — no server required for same-origin local multiplayer.

What this does

Adds an Emscripten JS bridge (globalThis.__gpspLinkBridge) that lets a web frontend configure and drive gpSP's netplay serial protocols via send(dst, payload) / poll() calls, using BroadcastChannel for cross-tab communication.

Changes

libretro/libretro.c

• Add gpsp_bridge_set_client_id() / gpsp_bridge_set_client_count() EMSCRIPTEN_KEEPALIVE exports for configuring netplay_client_id and netplay_num_clients from JavaScript
• Add js_gpsp_bridge_has/send/poll EM_JS functions for BroadcastChannel communication via globalThis.__gpspLinkBridge
• Add gpsp_bridge_poll_receive() that polls the JS bridge and dispatches packets to serial protocol handlers
• Guard netpacket_start() to not clobber bridge-configured values when the JS bridge is already active
• Bugfix: Add netpacket_poll_receive() call for SERIAL_MODE_SERIAL_POKE in retro_run() — was completely missing, causing Pokemon link cable packets to never be delivered

serial_proto.c

• Add netpacket_poll_receive() per-tick in serialpoke_update() for low-latency packet delivery (matching the pattern already used by AW mode)
• Bugfix: Fix CONNECTED → HANDSHAKE re-transition in both master and slave paths — reset offset/checksum, notify peer via network packet, fire IRQ on slave side (fixes "Communication error" when exiting Pokemon trade room)

Testing

• Pokemon Fire Red: Trading works end-to-end — enter trade room, select Pokemon, trade, exit trade room cleanly without "Communication error"
• Transport: BroadcastChannel between two browser tabs (same origin)
• Serial mode: mul_poke (Pokemon link cable protocol)

[Copilot]

Pull request overview

This pull request adds BroadcastChannel JavaScript bridge support for browser-based GBA link cable multiplayer and includes important bug fixes for the Pokemon serial protocol.

Changes:

• Adds Emscripten JS bridge (globalThis.__gpspLinkBridge) for cross-tab multiplayer using BroadcastChannel API
• Fixes Pokemon link cable handshake re-transition bug (CONNECTED → HANDSHAKE) by resetting state variables and notifying peers
• Fixes missing netpacket_poll_receive() call for SERIAL_MODE_SERIAL_POKE that was preventing Pokemon packets from being delivered
• Adds always-on AW_TRACE diagnostic logging for AW protocol debugging
• Adds heartbeat command handling for AW protocol to improve state synchronization

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
libretro/libretro.c	Adds Emscripten JS bridge functions (send/poll/has), KEEPALIVE exports for client ID/count configuration, bridge polling in netpacket_poll_receive(), guard logic to preserve bridge-configured values, and missing netpacket_poll_receive() calls in retro_run()
serial_proto.c	Adds AW_TRACE macro for diagnostic logging, heartbeat_cmd field for AW protocol state sync, fixes handshake re-transition in both master/slave paths, adds netpacket_poll_receive() calls for low-latency packet delivery, clears SIOCNT busy bit, and adds diagnostic counters
bios_data.S	Adds ELF .type and .size directives for BIOS symbols

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[Copilot]

Missing bounds check on client_id parameter. The client_id is used directly as an array index (e.g., lines 800, 807, 808, 810, 820) without validation, but the peer array only has 4 elements (indices 0-3). When using the JavaScript bridge, a malicious or buggy bridge implementation could provide a client_id >= 4, causing out-of-bounds memory access. Add a bounds check at the start of the function after validating the header: if (client_id >= 4) return;

[Copilot]

Inconsistent indentation in switch case statement. The cases for SERIAL_MODE_SERIAL_AW1 and SERIAL_MODE_SERIAL_AW2 use one space for indentation while other cases use two spaces. The body also uses three spaces instead of the expected indentation pattern. This should match the indentation pattern used for SERIAL_MODE_SERIAL_POKE case above.

Suggested change

	case SERIAL_MODE_SERIAL_AW1:
	case SERIAL_MODE_SERIAL_AW2:
	netpacket_poll_receive();
	break;
	case SERIAL_MODE_SERIAL_AW1:
	case SERIAL_MODE_SERIAL_AW2:
	netpacket_poll_receive();
	break;

[Copilot]

Redundant condition check. The condition "!len || len <= 0" is redundant because "len <= 0" already covers the case when len is 0. The "!len" check is unnecessary and should be removed. The condition should be simplified to just "if (len <= 0) return;"

Suggested change

	if (!len || len <= 0) return;
	if (len <= 0) return;

[Copilot]

AW_TRACE macro is always enabled and will print diagnostic messages to console in production builds. While the comment indicates this is intentional for "AW protocol debugging", several trace calls are unlimited (lines 557, 589, 733, 735-736, 852) and could print frequently during gameplay, potentially impacting performance and cluttering console output. Consider either: (1) guarding AW_TRACE with a compile-time flag similar to SRPT_DEBUG_LOG, or (2) adding rate limiting to all trace calls, or (3) documenting that these will be removed/disabled before merging to main.