55 lines
2.1 KiB
Go
55 lines
2.1 KiB
Go
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package api
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import "sync"
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// consideredFollows bounds the memory below. A household has a handful of episodes in
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// flight; this is generous enough that a whole evening's viewing fits and small enough
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// that nothing a client can do grows it into a leak.
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const consideredFollows = 512
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// followChecks remembers which (viewer, episode) pairs have already been put through
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// the automatic My Shows check.
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//
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// The check is hung off playback reports, and a report arrives every ten seconds for the
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// length of an episode. Its own guard — half watched — is true for every one of those
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// after the halfway point, so on a forty-minute episode the second half of the viewing
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// ran the whole check a hundred times: two serial Emby lookups, the Sonarr catalogue and
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// a Postgres write, all of it on the response path of a request the television makes
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// while somebody is watching something. Every run after the first reached the same
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// `inserted == false` and discarded its own work.
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//
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// Deduplicating on the durable insert is correct and was never the problem; what it
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// could not do is prevent the work leading up to it. This is deliberately in memory and
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// deliberately lossy, the playbackTitles arrangement: a restarted gateway does the check
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// once more per episode, which is the cost of one lookup rather than a schema change.
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type followChecks struct {
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mu sync.Mutex
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seen map[string]struct{}
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order []string
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}
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// claim reports whether this is the first time the pair has been offered, and records it.
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// One call does both because two would be a race between the check and the record, and
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// the thing being protected is precisely a burst of concurrent reports.
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func (f *followChecks) claim(userID, itemID string) bool {
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if userID == "" || itemID == "" {
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return false
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}
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key := userID + "|" + itemID
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f.mu.Lock()
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defer f.mu.Unlock()
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if f.seen == nil {
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f.seen = make(map[string]struct{}, consideredFollows)
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}
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if _, known := f.seen[key]; known {
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return false
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}
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f.seen[key] = struct{}{}
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f.order = append(f.order, key)
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if len(f.order) > consideredFollows {
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delete(f.seen, f.order[0])
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f.order = f.order[1:]
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}
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return true
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}
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