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unix.listen

Synopsis

local unix = require "unix"

unix.stream.listen()
unix.seqpacket.listen()

function(ep: string[, mode: integer]) -> acceptor

Description

  1. Creates a socket.

  2. Set common options.

  3. If mode is given, changes the process file mode creation mask (umask) such that the next call to bind() will create an UNIX socket whose permissions will be bit.band(mode, filesystem.mode(7,7,7)). Old umask is saved.

  4. Binds the socket to ep. On errors, old umask (if umask was modified on the previous step) is restored before the error is propagated up.

  5. If mode is given, restores the old umask now.

  6. Put the socket in the listening state.

  7. Returns the socket.

If ep starts with @ then it’s assumed to represent an abstract UNIX socket.
If mode is given, only the master VM is allowed to use this function.

Rationale

mode as an extra parameter

To understand why mode is not part of the address string, we must understand why port is part of the address string in ip.tcp.listen(). ip.tcp.listen() accepts the port number as part of the address string because this info is usually stored in config files where there’s a single string to identify the endpoint to bind to. Having this logic embedded in ip.tcp.listen() makes it easier to parse these config files.

However the permission access mode is not part of the endpoint address. mode is not an address. It doesn’t identify an endpoint. It’s a separate value in the config file (possibly fully omitted from the config altogether and hardcoded in the program logic). It’s not even required in many situations (hence why it’s an optional parameter here).

mode is only usable in the master VM

It’d be possible to make this function fallback to change the permissions of the socket afterwards when it’s not called from the master VM. However this approach would not be atomic and would be unsafe as an unwanted client could connect in the window of time where the file held the wrong permissions.

During code refactors, the call to this function could be moved to the wrong VM and the dangerous approach would be chosen w/o the user’s knowledge. This property is undesired and that’s a strong reason why we don’t do it.

If the lack of atomicity is not a problem, the user can explicitly call filesystem.chmod() after listen() returns.

(Not) Removing files by default

This function could simplify the user’s life even further if it also removed the file pointed to by ep before it binds the socket. However it’d make the function unusable in scenarios where the file must be removed by a different process (e.g. a supervised daemon, or many processes contending over the address with custom fallback code).

In other words, the presence/possibility of EADDRINUSE may be a desired property in this algorithm by some programs.

This function is a high-level API and it’s not intended to replace every usage of the lower-level API so the previous point may not be that strong of a reason. However an explicit call to filesystem.remove() in user’s code is not that big of a deal. It doesn’t add that much boilerplate.