lean-poker / boardgame.io

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fix(server): serialize sync and disconnect connection changes via per-match queue (#31)

Fixes #1100.

Master.onConnectionChange() does a read-modify-write on match metadata
without any locking. When multiple players sync simultaneously, one
player's isConnected=true could be overwritten by another player's
stale fetch, leaving the first player stuck on "Connecting..." forever.

The socket.io transport already had a perMatchQueue (PQueue with
concurrency:1) to serialize onUpdate calls. This fix wraps both the
sync handler (onSync + onConnectionChange) and the disconnect
handler (onConnectionChange) inside the same queue, forcing all
per-match operations to run serially.

Also adds CopyingInMemoryAsync test helper and two race-condition
tests that deterministically reproduce the bug by deep-copying fetch
results. Without the fix, the concurrent-sync test consistently fails
with one player's isConnected being undefined.

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