Elixir/Ports and external process wiring: Difference between revisions

This exploration began as I wrote a simple library to run rsync from Elixir.<ref>https://hexdocs.pm/rsync/Rsync.html</ref>  I was hoping to learn how to interface with long-lived external processes, in this case to transfer files and monitor progress.  Starting and reading from rsync went very well, thanks to the <code>--info=progress2</code> option which reports progress in a fairly machine-readable format.  I was able to start the file transfer, capture status, and report it back to the Elixir caller in various ways.

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Since I was calling my rsync library from an application under development, I would often kill the program abruptly by crashing or by typing <control>-C in the terminal.  What I found is that the rsync transfer would continue to run in the background even after Elixir had completely shut down.

That would have to change—leaving overlapping file transfers running unmonitored is exactly what I wanted to avoid by having Elixir control the process in the first place.

A common use case is to use external processes for something like compression and decompression.  A program like <code>gzip</code> or <code>cat</code> will stop once it detects that its input has ended, using a C system call like this:<syntaxhighlight lang="c">

ssize_t n_read = read (input_desc, buf, bufsize);

BEAM assumes the connected process behaves like this, so nothing needs to be done to clean up a dangling external process because it will end itself as soon as the Port is closed or the BEAM exits.  If the external process is known to not behave this way, the recommendation is to wrap it in a shell script which converts a closed stdin into a kill signal.<ref>https://hexdocs.pm/elixir/main/Port.html#module-orphan-operating-system-processes</ref>

[[W:BEAM (Erlang virtual machine)|BEAM]] applications are built out of supervision trees and excel at managing huge numbers of parallel actor processes, all scheduled internally.  Although the communities' mostly share a philosophy of running as much as possible inside of the VM because it builds on this strength, and simplifies away much interface glue and context switching, on many occasions it will still start an external OS process.  There are some straightforward ways to simply run a command line, which might be familiar to programmers coming from another language: <code>[https://www.erlang.org/doc/apps/kernel/os.html#cmd/2 os:cmd]</code> takes a string and runs the thing.  At a lower level, external programs are managed through a [https://www.erlang.org/doc/system/ports.html Port] which is a flexible abstraction allowing a backend driver to communicate data in and out, and to send some control signals such as reporting an external process's exit and exit status.

When it comes to internal processes, BEAM is among the most mature and robust, achieved by good isolation and by its hierarchical [https://www.erlang.org/doc/system/sup_princ supervisors] liberally pruning entire subprocess trees at the first sign of going out of specification.  But for external processes, results are mixed.  Some programs are twitchy and crash easily, for example <code>cat</code>, but others like the BEAM itself or a long-running server are built to survive any ordinary I/O glitch or accidental mashing of the keyboard.  Furthermore, this will usually be a fundamental assumption of that program and there will be no configuration to make the program behave differently depending on stimulus.

What I discovered is that the BEAM external process library assumes that its spawned processes will respond to standard input and output shutting down or so called end of file, for example what happens when <control>-d is typed into the shell.  This works very well for a subprocess like <code>bash</code> but has no effect on a program like <code>sleep</code> or <code>rsync</code>.

Which signal to use is still an open issue, there's a softer version <code>HUP</code> which says "Goodbye!" and the program is free to interpret as it will, the mid-level <code>TERM</code> that I prefer because it makes the intention explicit but can still be blocked or handled gracefully if needed, and <code>KILL</code> which is bursting with destructive potential.  The world of unix signals is a wild and scary place, on which there's a refreshing diversity of opinion around the Internet.

Despite its retro-futuristic appearance of being one of the most time-tested yet forward-facing programming environments, I was brought back to Earth by digging around inside the VM to find that it's just a C program like any other.  There's nothing holy about the BEAM emulator, there are some good and some great ideas about functional languages and they're buried in a mass of ancient procedural ifdefs, with unnerving memory management and typedefs wrapping the size of an integer on various platforms, just like you might find in other relics from the dark ages of computing, next to the Firefox or linux kernel source code.

Tantalizingly, message-passing is at the core of the VM, but is not a first-class concept when reaching out to external processes.  There's some fancy footwork with [[W:Anonymous pipe|pipes]] and [[W:Dup (system call)|dup]], but communication is done with enums, unions, and bit-rattling stdlib.  I love it, but... it might something to look at on another rainy day.