The slot was kept as soon as search.list was about to be called, so a search refused before it went out (no connection, no token) cost the user a slot for nothing. It is kept now only once the client recorded the call's units, or the request went out and its answer never came (YouTube may have counted it): YouTubeError says whether a transport failure was sent, and the client counts what it spent.
The process lock around the budget check and the call is waited for at most ten seconds; past that the search is refused as busy (search_busy, a retry can help) and gets its slot back, instead of parking a request worker behind every slow call before it. The lock test now pins one hold around both steps, and the busy test fails on a wait without a bound.
peter
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