[code] Textadept: preliminary question to the better-experienced about best-practice(s) for: event-handlers waiting for external command response

From: Phil S. <accountz.att.mailbox.org>
Date: Fri, 9 Nov 2018 16:28:32 +0100

This is a thing that will come up repeatedly for other scenarios as
well, but for now just consider this simple scenario: format-on-save.

Now if your formatter logic is inside your local Lua scripts, this is
straightforward with FILE_BEFORE_SAVE. Even if invoking a command from
scratch, this would be equally straightforward with io.popen which isn't
callback-based or on a separate thread.

BUT if your script is "perma-connected" (eg. via stdout/stdin IPC pipes)
to your long-running backend program (kinda like a 'language server'
perhaps, started via os.spawn), then you're going to post your request
to its stdin and some time later your `stdout_cb` callback handler
receives a message containing some indication that it is the response to
your earlier fmt request via some request-ID. Other messages might occur
prior while waiting for the fmt-response.

(Nevermind that this takes mere milliseconds in practice, it's still
cross-process IO.)

In other environments like Node, you wouldn't
noop-loop-until-response-received in your FILE_BEFORE_SAVE handler
because that would be blocking the whole single-threaded process and
prevent your `stdout_cb` from ever being called. (In the very old
VisualBasic days, you could put a DoEvents call inside your waiting/noop
loop and done ;)

Now normally (ignoring Windows-terminal-version special case) Textadept
manages its `os.spawn`ed processes in (a) separate thread(s) so I guess
those pipes aren't blocked from my wait-loop, if I were to go try that
route. But in Textadept Lua, are your `os.spawn` callbacks in that
separate thread too, or in the same one as other userland script code?
Docs say "Stdout is read asynchronously" but that doesn't tell me about
where my callback is invoked.

How would a more seasoned than me Lua scripter approach this scenario
when scripting within the Textadept environment and API context?
Wait-loops are a bit ugly in general and won't occur often, usually
there's no point for them but in a few select scenarios such as
fmt-on-save it'd be nice if one could stick to the existing
FILE_BEFORE_SAVE event-handling infra (rather than say re-route
all-user-interactions-for-Save to some handcrafted custom replacement
scaffolding --- but maybe that would be the recommended approach in the
end, I don't know, hence my asking =)

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Received on Fri 09 Nov 2018 - 10:28:32 EST

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