[pulseaudio-discuss] [patch] ops: catch repeated finalisation

[jnqnfe at gmail.com]

The internal operation_set_state function already returns early if the
new state is the same as the existing state. The attached patch extends
this to return early if already in a finalised (done/cancelled) state,
i.e. blocks attempts to re-finalise into a different state.

This helps avoid unlinking more than once (or crashing on ref count
assertion).

I was not certain whether an assertion would be a better alternative -
with such a crash helping highlight usage problems...

The situation that lead to this was the thought of someone stupidly
trying to pa_operation_cancel() a callback within the callback
execution itself, while designing a solution for a memory leak related
to cancellation within my Rust binding. While no-one should do such a
thing, if they did, they'd either trip up a ref count assertion, or the
operation would be unlinked twice, which would be bad. It's a simple
thing to catch and mitigate, and could prove to be a useful
bulletproofing measure for this function in general.
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