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Add dispatchers for running actors on separate carrier pools - #12
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An actor stuck in a blocking foreign call pins its carrier, and every actor placed on that carrier waits with it. A dispatcher is a named jolt fiber pool, as Akka's dispatchers are thread pools: spawn, gen_server, gen_statem and gen_event take :dispatcher, and process-info reports it. :default is jolt's pool and :blocking has 16 carriers; define! adds more, and their carriers start on first use. The actor keeps its pool so a hibernating actor wakes on it, or on :default if the dispatcher was shut down in the meantime. Needs jolt with jolt.fibers/pool (jolt-lang/jolt#1307).
The nightly pre-release hasn't been rebuilt since before fiber pools landed, so CI was getting a jolt without jolt.fibers/pool. 0.8.21 is the first release with both fiber kills and pools.
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An actor that makes a blocking foreign call (a C database driver, say) pins its carrier, and every actor placed on that carrier waits until the call returns. Fibers can't migrate, so the only fix is to keep that work on carriers of its own. This adds Akka-style dispatchers on top of jolt's new fiber pools:
:defaultis jolt's pool, and:blocking(16 carriers, like Akka's default-blocking-io-dispatcher) is predefined. A pool starts on first use and can be redefined until then. Children don't inherit the dispatcher. An unknown dispatcher is a badarg and spawns nothing.shutdown!refuses new actors and lets running ones finish, and a hibernating actor whose dispatcher went away wakes on:default.Depends on jolt-lang/jolt#1307. CI runs against jolt's nightly build, so it will fail until that PR is merged and the nightly is rebuilt. Locally the full suite passes against that branch (370 tests), including a test where a usleep FFI call pins one dispatcher while another keeps running.