For accurate app descriptor release, it is required to count the number of
use counts. Use count increased when:
- app linked to configuration app queue;
- socket conf stores pointer to app;
- request for start app process posted to router service thread;
Application port has pointer to app, but it does not increase use count
to avoid use count loop.
Timer needs a pointer to nxt_timer_t which is stored in engine timers tree.
nxt_timer_t now resides in nxt_app_joint_t and does not lock the application.
Start process port RPC handlers is also linked to nxt_app_joint_t.
App joint (nxt_app_joint_t) is a 'weak pointer':
- single threaded;
- use countable;
- store pointer to nxt_app_t (which can be NULL);
nxt_app_t has pointer to nxt_app_joint_t and update its pointer to app.
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