57 lines
1.8 KiB
Go
57 lines
1.8 KiB
Go
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package credits
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import "sync"
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// A minimal single-flight, because four signals can name the same episode.
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//
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// Live playback, a predicted next episode, another viewer's prediction and a refresh cycle
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// all legitimately want the same media version at the same moment, and each of them arriving
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// as its own scan is the one way this subsystem could become expensive. The queue already
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// deduplicates by item; this closes the gap for anything that reaches Process directly — a
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// forced scan from the console, the benchmark, or a live push racing the worker.
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//
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// Written rather than taken from golang.org/x/sync so the gateway keeps its two direct
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// dependencies. It is thirty lines and the semantics are not subtle: the first caller does
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// the work, everybody else waits and receives the same answer.
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type flightGroup struct {
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mu sync.Mutex
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calls map[string]*flightCall
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}
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type flightCall struct {
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done chan struct{}
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value any
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err error
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}
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func newFlightGroup() *flightGroup {
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return &flightGroup{calls: map[string]*flightCall{}}
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}
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// Do runs fn unless an identical key is already in flight, in which case it waits for that
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// one and returns its result.
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func (g *flightGroup) Do(key string, fn func() (any, error)) (any, error) {
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g.mu.Lock()
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if existing, found := g.calls[key]; found {
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g.mu.Unlock()
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<-existing.done
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return existing.value, existing.err
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}
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call := &flightCall{done: make(chan struct{})}
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g.calls[key] = call
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g.mu.Unlock()
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// The deferred cleanup matters more than it looks: a detector that panics must not leave
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// a key permanently in flight, which would make that episode unscannable for the life of
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// the process and give every later caller a channel that never closes.
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defer func() {
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g.mu.Lock()
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delete(g.calls, key)
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g.mu.Unlock()
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close(call.done)
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}()
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call.value, call.err = fn()
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return call.value, call.err
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}
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