Spawning PHP sub-processes in Web Workers (#1069)

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.](#1027)

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

```ts
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

<img width="500"
src="https://github.com/WordPress/wordpress-playground/assets/205419/470d79b2-2f10-4f1a-806c-5f26463766da"
/>

#### /wordpress/spawn.php

```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
<?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

Author: adamziel

packages/php-wasm/compile/php/Dockerfile

@@ -812,6 +812,7 @@ RUN set -euxo pipefail; \
 	-I TSRM/ \
 	-I /root/lib/include \
 	-L/root/lib -L/root/lib/lib/ \
+	-lproxyfs.js \
 	$(cat /root/.emcc-php-wasm-flags) \
 	-o /build/output/php.js \
 	-s EXPORTED_FUNCTIONS="$EXPORTED_FUNCTIONS" \

packages/php-wasm/compile/php/php_wasm.c

@@ -954,7 +954,7 @@ void wasm_set_request_port(int port)
  *
  *   stream: The stream to redirect, e.g. stdout or stderr.
  *
- *   path: The path to the file to redirect to, e.g. "/tmp/stdout".
+ *   path: The path to the file to redirect to, e.g. "/internal/stdout".
  *
  *   returns: The exit code: 0 on success, -1 on failure.
  */
@@ -1175,8 +1175,13 @@ int wasm_sapi_request_init()
 	// Write to files instead of stdout and stderr because Emscripten truncates null
 	// bytes from stdout and stderr, and null bytes are a valid output when streaming
 	// binary data.
-	stdout_replacement = redirect_stream_to_file(stdout, "/tmp/stdout");
-	stderr_replacement = redirect_stream_to_file(stderr, "/tmp/stderr");
+	// We'll use the /internal directory instead of /tmp, because a child process sharing
+	// the same filesystem and /tmp mount would write to the same stdout and stderr files
+	// and produce unreadable output intertwined with the parent process output. The /internal
+	// directory should always stay in per-process MEMFS space and never be shared with
+	// any other process.
+	stdout_replacement = redirect_stream_to_file(stdout, "/internal/stdout");
+	stderr_replacement = redirect_stream_to_file(stderr, "/internal/stderr");
 	if (stdout_replacement == -1 || stderr_replacement == -1)
 	{
 		return -1;
@@ -1434,7 +1439,7 @@ FILE *headers_file;
  */
 static int wasm_sapi_send_headers(sapi_headers_struct *sapi_headers TSRMLS_DC)
 {
-	headers_file = fopen("/tmp/headers.json", "w");
+	headers_file = fopen("/internal/headers.json", "w");
 	if (headers_file == NULL)
 	{
 		return FAILURE;

packages/php-wasm/compile/php/phpwasm-emscripten-library.js

@@ -16,6 +16,11 @@ const LibraryExample = {
 	// JavaScript library under the PHPWASM object:
 	$PHPWASM: {
 		init: function () {
+			// The /internal directory is required by the C module. It's where the
+			// stdout, stderr, and headers information are written for the JavaScript
+			// code to read later on.
+			FS.mkdir("/internal");
+			
 			PHPWASM.EventEmitter = ENVIRONMENT_IS_NODE
 				? require('events').EventEmitter
 				: class EventEmitter {