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p--b macOS: Pass JackMachSemaphore send right via mach_msg IPC (#788)
1ab3445 Aug 14, 2021
macOS: Pass JackMachSemaphore send right via mach_msg IPC (#788) 
* macOS: Pass JackMachSemaphore send right via mach_msg IPC

Previously, JackMachSemaphore would communicate the send right for the
semaphore object from the server to a client via a named service
registered via `bootstrap_register`. However, to do this, it would
register the semaphore's port as the service port directly.

In theory this ought to be fine, however in practice, macOS `launchd`,
which provides the `bootstrap_register` interface, does not correctly
detect when such a port becomes dead, and incorrectly believes that the
service that it provides is forever alive, even past the end of the
`jackd` process' (and therefore the semaphore's) existence. This seems
to be *specific* to semaphore ports, as `launchd` is expecting a
standard IPC port, owned by the task, not the kernel. This prevents
`jackd` from later registering another service with the same name, as
`launchd` rejects the registration as conflicting with an active service.

To get around this, `jackd` previously added a counter to the end of the
named service registrations, allowing old services to remain in the
system until the end of the session. To prevent things getting out of
hand, this was capped at 98 service registrations for a given semaphore
name. This led to #784, in which running a client for the 99th time
resulted in the semaphore creation failing and the client failing to
connect.

As `launchd` outlives multiple runs of `jackd`, this situation persisted
across restarts of `jackd`, requiring a restart of the user's session
(i.e. a reboot) to fix.

An initial attempt at fixing this (see #785) tried passing the port
rights directly via shared memory, however mach is too clever for us and
foils that plan by having port names be looked up in a per-task table
(sensible when you think about it).

In this commit, we use mach IPC messages to transfer the send right for
the semaphore from the server to the client. By registering a standard
IPC port with the bootstrap server, the service registrations are
correctly torn down when the ports are destroyed.

It works something like this:

* Server creates IPC port and registers it globally via `bootstrap_register`
* Server listens on IPC port for messages
* Client looks up IPC port via `bootstrap_look_up`
* Client sends it a message
* Server replies with a message containing a send right to the
semaphore's port
* Client is then free to use the semaphore port as before.

This resolves #784.

* Improve error handling

* Add myself to Authors
1ab3445

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README.rst

JACK2

https://travis-ci.org/jackaudio/jack2.svg?branch=master

JACK2 aka jackdmp is a C++ version of the JACK low-latency audio server for multi-processor machines. It is a new implementation of the JACK server core features that aims at removing some limitations of the JACK1 design. The activation system has been changed for a data flow model and lock-free programming techniques for graph access have been used to have a more dynamic and robust system.

  • uses a new client activation model, that allows simultaneous client execution (on a SMP machine) when parallel clients exist in the graph (client that have the same inputs). This activation model allows to better use available CPU on a smp machine, but also works on mono-processor machine.
  • uses a lock-free way to access (read/write) the client graph, thus allowing connections/disconnection to be done without interrupting the audio stream. The result is that connections/disconnections are glitch-free.
  • can work in two different modes at the server level:
    • synchronous activation: in a given cycle, the server waits for all clients to be finished (similar to normal jackd)
    • asynchronous activation: in a given cycle, the server does not wait for all clients to be finished and use output buffer computed the previous cycle. The audible result of this mode is that if a client is not activated during one cycle, other clients may still run and the resulting audio stream will still be produced (even if its partial in some way). This mode usually result in fewer (less audible) audio glitches in a loaded system.

For further information, see the JACK homepage and wiki. There are also the #jack and #lad chat channels on freenode IRC.

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GPL-2.0 License

Releases 41

1.9.19 Latest
Jul 15, 2021
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