Spawning PHP sub-processes in Web Workers

[adamziel]

Related to #1026, #1027, and #1051. Supersedes #1031

Adds support for spawning PHP subprocesses via <?php proc_open(['php', 'activate_theme.php']);. The spawned subprocess affects the filesystem used by the parent process.

Implementation details

This PR updates the default spawnHandler in worker-thread.ts that creates another WebPHP instance and mounts the parent filesystem using Emscripten's PROXYFS.

A shared filesystem didn't pan out. Synchronizing is the second best option.

This code snippet illustrates the idea – note the actual implementation is more nuanced:

php.setSpawnHandler(
	createSpawnHandler(async function (args, processApi) {
		const childPHP = new WebPHP();
		const { exitCode, stdout, stderr } = await childPHP.run({
			scriptPath: args[1]
		});
		processApi.stdout(stdout);
		processApi.stderr(stderr);
		processApi.exit(exitCode);
	})
);

Future work

• Stream stdout and stderr from childPHP to processApi instead of buffering the output and passing everything at once

Example of how it works

/wordpress/spawn.php

<?php
echo "<plaintext>";
echo "Spawning /wordpress/child.php\n";
$handle = proc_open('php /wordpress/child.php', [
	0 => ['pipe', 'r'],
	1 => ['pipe', 'w'],
	2 => ['pipe', 'w'],
], $pipes);

echo "stdout: " . stream_get_contents($pipes[1]) . "\n";
echo "stderr: " . stream_get_contents($pipes[2]) . "\n";
echo "Finished\n";
echo "Contents of the created file: " . file_get_contents("/wordpress/new.txt") . "\n";

/wordpress/child.php

<?php
echo "<plaintext>";
echo "Spawned, running";
error_log("Here's a message logged to stderr! " . rand());
file_put_contents("/wordpress/new.txt", "Hello, world!" . rand() . "\n");

Testing instructions

1. Update worker-thread.ts to create the two files listed above
2. In Playground, navigate to /spawn.php
3. Confirm the output is the same as on the screenshot above

cc @dmsnell @bgrgicak

[bgrgicak]

Nice! Glad you found a way without copying the files.

[bgrgicak]

Should this be /internal/stderr?

[adamziel]

Oh! Yeah if that remains a file path then yes. I wonder why this isn't php://stderr actually, I think I copied it over from #161 and, at that time, there was a bug preventing the use of php://stderr.

[adamziel]

I created an issue to track it here: #1071