handle thundering herds #3
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This changes the startup logic pretty significantly, to provide comprehensive protection against many clients all attempting to start the daemon at the same time. A lockfile is used to ensure that only a single contender is responsible for either starting the daemon script, or usurping a wedged or misbehaving server. The lockfile contains the pid of the server attempting to take over, and is checked to ensure that it STILL contains the correct pid before confirming, to handle the race when multiple contenders all try to delete a stale lockfile at the same time. That is, the lockfile used as a mutex against startup races is also subject to races, since it can be disregarded and deleted if it's stale, so that has to be protected against. This also makes it so that it attempts to listen *first*, and only usurps a wedged process if the listen fails, which is more efficient in the common happy-path case, and only involves minimal waiting in the edge cases.
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This changes the startup logic pretty significantly, to provide comprehensive protection against many clients all attempting to start the daemon at the same time.
A lockfile is used to ensure that only a single contender is responsible for either starting the daemon script, or usurping a wedged or misbehaving server.
The lockfile contains the pid of the server attempting to take over, and is checked to ensure that it STILL contains the correct pid before confirming, to handle the race when multiple contenders all try to delete a stale lockfile at the same time. That is, the lockfile used as a mutex against startup races is also subject to races, since it can be disregarded and deleted if it's stale, so that has to be protected against.
This also makes it so that it attempts to listen first, and only usurps a wedged process if the listen fails, which is more efficient in the common happy-path case, and only involves minimal waiting in the edge cases.