Communicating with external program over HTTP is just one approach. Let's try to run it directly instead.
To talk to it we'll be sending JSON messages, one JSON per line. It will reply with JSON messages, one JSON per line as well.
This "JSON streaming" approach is a fairly popular solution for communication between programs, mostly because how easy it is to code.
src/App.js
All our code will live in the preload, so all we need to change in the frontend is how we run code:
let runCode = async (code) => {
let {error, output} = await window.api.runCode(sessionId, code)
if (error) {
return output + "\n" + error
} else {
return output
}
}
We can also remove axios
and proxy
settings from package.json
.
Node IO
We already figured out how to create a new process, so now it's just await stdin.writeLine(...)
and await stdout.readLine()
, right?
Well, wrong! Node somehow never added any functionality for something as sophisticated like "reading a line". It's the first programming langugae I've ever see that just doesn't. Even C had gets
(which didn't check for memory overflows, but that's just C being C).
This shouldn't really be that surprising as JavaScript is a browser-first language, and it lacks a lot of the same functionality that we take for granted coding in Unix-first language like Ruby or Python.
Fortunately [promise-readline](https://www.npmjs.com/package/promise-readline)
can rescue us here, so let's install that. Not to be confused with readline-promise
and a lot of other similarly named packages which don't do anything like that. Searching that was pretty difficult.
src/preload.js
We have just one choice to make:
- create new process for each sessions
- create one process, and send it session id with every request
This time we'll implement the "one process, multiple sessions" solution, but separating them would lead to better isolation and wouldn't be that much more complex.
Other than lineReader(process.stdout)
and await stdin.write
from promise-readline
, everything else should be already familiar.
let child_process = require("child_process")
let lineReader = require("promise-readline")
let { contextBridge } = require("electron")
let languageServer = null
async function startLanguageServer() {
let process = child_process.spawn(
"./ruby_language_server",
[],
{
stdio: ["pipe", "pipe", "inherit"],
},
)
languageServer = {
process,
stdin: process.stdin,
stdout: lineReader(process.stdout),
}
}
async function runCode(session_id, code) {
if (!languageServer) {
await startLanguageServer()
}
let { stdin, stdout } = languageServer
await stdin.write(JSON.stringify({ code, session_id }) + "\n")
let line = await stdout.readLine()
return JSON.parse(line)
}
contextBridge.exposeInMainWorld(
"api", { runCode }
)
ruby_language_server
And now we can simply take the Sinatra server we had before, and replace Sinatra logic by a simple STDIN.each_line
loop. In Ruby there's zero trouble doing line-oriented I/O.
#!/usr/bin/env ruby
require "stringio"
require "json"
def capture_output(new_stdout, new_stderr)
begin
$stdout, save_stdout = new_stdout, $stdout
$stderr, save_stderr = new_stderr, $stderr
yield
ensure
$stdout = save_stdout
$stderr = save_stderr
end
end
def eval_and_capture_output(code, context)
response = {}
output = StringIO.new
capture_output(output, output) do
begin
eval(code, context)
rescue Exception => e
response["error"] = "#{e.class}: #{e.message}"
end
end
response["output"] = output.string
response
end
def new_binding
Object.new.instance_eval{ binding }
end
bindings = Hash.new{|h,k| h[k] = new_binding}
STDOUT.sync = true
STDIN.each_line do |line|
data = JSON.parse(line)
code = data["code"]
session_id = data["session_id"]
response = eval_and_capture_output(code, bindings[session_id])
puts response.to_json
end
What did we gain?
The biggest gain compared to the HTTP solution is that we also no longer have extremely insecure server literally executing any code we send to it.
We also have full control over starting and shutting down the language server, and it no longer needs to be started and stopped separately. We could even isolate different contexts in different processes, and we'll take advantage of that in the future.
Result
Here's the result if we press "Run All" button:
In the next episode we'll try to do the same thing for Python as we did for Ruby.
As usual, all the code for the episode is here.
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