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

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 {

packages/php-wasm/node/public/php_7_0.js

@@ -1,6 +1,6 @@
 const dependencyFilename = __dirname + '/7_0_33/php_7_0.wasm'; 
 export { dependencyFilename }; 
-export const dependenciesTotalSize = 11391146; 
+export const dependenciesTotalSize = 11391167; 
 export function init(RuntimeName, PHPLoader) {
     /**
      * Overrides Emscripten's default ExitStatus object which gets
@@ -1475,6 +1475,216 @@ var NODEFS = {
  }
 };
 
+var PROXYFS = {
+ mount(mount) {
+  return PROXYFS.createNode(null, "/", mount.opts.fs.lstat(mount.opts.root).mode, 0);
+ },
+ createNode(parent, name, mode, dev) {
+  if (!FS.isDir(mode) && !FS.isFile(mode) && !FS.isLink(mode)) {
+   throw new FS.ErrnoError(ERRNO_CODES.EINVAL);
+  }
+  var node = FS.createNode(parent, name, mode);
+  node.node_ops = PROXYFS.node_ops;
+  node.stream_ops = PROXYFS.stream_ops;
+  return node;
+ },
+ realPath(node) {
+  var parts = [];
+  while (node.parent !== node) {
+   parts.push(node.name);
+   node = node.parent;
+  }
+  parts.push(node.mount.opts.root);
+  parts.reverse();
+  return PATH.join.apply(null, parts);
+ },
+ node_ops: {
+  getattr(node) {
+   var path = PROXYFS.realPath(node);
+   var stat;
+   try {
+    stat = node.mount.opts.fs.lstat(path);
+   } catch (e) {
+    if (!e.code) throw e;
+    throw new FS.ErrnoError(ERRNO_CODES[e.code]);
+   }
+   return {
+    dev: stat.dev,
+    ino: stat.ino,
+    mode: stat.mode,
+    nlink: stat.nlink,
+    uid: stat.uid,
+    gid: stat.gid,
+    rdev: stat.rdev,
+    size: stat.size,
+    atime: stat.atime,
+    mtime: stat.mtime,
+    ctime: stat.ctime,
+    blksize: stat.blksize,
+    blocks: stat.blocks
+   };
+  },
+  setattr(node, attr) {
+   var path = PROXYFS.realPath(node);
+   try {
+    if (attr.mode !== undefined) {
+     node.mount.opts.fs.chmod(path, attr.mode);
+     node.mode = attr.mode;
+    }
+    if (attr.timestamp !== undefined) {
+     var date = new Date(attr.timestamp);
+     node.mount.opts.fs.utime(path, date, date);
+    }
+    if (attr.size !== undefined) {
+     node.mount.opts.fs.truncate(path, attr.size);
+    }
+   } catch (e) {
+    if (!e.code) throw e;
+    throw new FS.ErrnoError(ERRNO_CODES[e.code]);
+   }
+  },
+  lookup(parent, name) {
+   try {
+    var path = PATH.join2(PROXYFS.realPath(parent), name);
+    var mode = parent.mount.opts.fs.lstat(path).mode;
+    var node = PROXYFS.createNode(parent, name, mode);
+    return node;
+   } catch (e) {
+    if (!e.code) throw e;
+    throw new FS.ErrnoError(ERRNO_CODES[e.code]);
+   }
+  },
+  mknod(parent, name, mode, dev) {
+   var node = PROXYFS.createNode(parent, name, mode, dev);
+   var path = PROXYFS.realPath(node);
+   try {
+    if (FS.isDir(node.mode)) {
+     node.mount.opts.fs.mkdir(path, node.mode);
+    } else {
+     node.mount.opts.fs.writeFile(path, "", {
+      mode: node.mode
+     });
+    }
+   } catch (e) {
+    if (!e.code) throw e;
+    throw new FS.ErrnoError(ERRNO_CODES[e.code]);
+   }
+   return node;
+  },
+  rename(oldNode, newDir, newName) {
+   var oldPath = PROXYFS.realPath(oldNode);
+   var newPath = PATH.join2(PROXYFS.realPath(newDir), newName);
+   try {
+    oldNode.mount.opts.fs.rename(oldPath, newPath);
+    oldNode.name = newName;
+    oldNode.parent = newDir;
+   } catch (e) {
+    if (!e.code) throw e;
+    throw new FS.ErrnoError(ERRNO_CODES[e.code]);
+   }
+  },
+  unlink(parent, name) {
+   var path = PATH.join2(PROXYFS.realPath(parent), name);
+   try {
+    parent.mount.opts.fs.unlink(path);
+   } catch (e) {
+    if (!e.code) throw e;
+    throw new FS.ErrnoError(ERRNO_CODES[e.code]);
+   }
+  },
+  rmdir(parent, name) {
+   var path = PATH.join2(PROXYFS.realPath(parent), name);
+   try {
+    parent.mount.opts.fs.rmdir(path);
+   } catch (e) {
+    if (!e.code) throw e;
+    throw new FS.ErrnoError(ERRNO_CODES[e.code]);
+   }
+  },
+  readdir(node) {
+   var path = PROXYFS.realPath(node);
+   try {
+    return node.mount.opts.fs.readdir(path);
+   } catch (e) {
+    if (!e.code) throw e;
+    throw new FS.ErrnoError(ERRNO_CODES[e.code]);
+   }
+  },
+  symlink(parent, newName, oldPath) {
+   var newPath = PATH.join2(PROXYFS.realPath(parent), newName);
+   try {
+    parent.mount.opts.fs.symlink(oldPath, newPath);
+   } catch (e) {
+    if (!e.code) throw e;
+    throw new FS.ErrnoError(ERRNO_CODES[e.code]);
+   }
+  },
+  readlink(node) {
+   var path = PROXYFS.realPath(node);
+   try {
+    return node.mount.opts.fs.readlink(path);
+   } catch (e) {
+    if (!e.code) throw e;
+    throw new FS.ErrnoError(ERRNO_CODES[e.code]);
+   }
+  }
+ },
+ stream_ops: {
+  open(stream) {
+   var path = PROXYFS.realPath(stream.node);
+   try {
+    stream.nfd = stream.node.mount.opts.fs.open(path, stream.flags);
+   } catch (e) {
+    if (!e.code) throw e;
+    throw new FS.ErrnoError(ERRNO_CODES[e.code]);
+   }
+  },
+  close(stream) {
+   try {
+    stream.node.mount.opts.fs.close(stream.nfd);
+   } catch (e) {
+    if (!e.code) throw e;
+    throw new FS.ErrnoError(ERRNO_CODES[e.code]);
+   }
+  },
+  read(stream, buffer, offset, length, position) {
+   try {
+    return stream.node.mount.opts.fs.read(stream.nfd, buffer, offset, length, position);
+   } catch (e) {
+    if (!e.code) throw e;
+    throw new FS.ErrnoError(ERRNO_CODES[e.code]);
+   }
+  },
+  write(stream, buffer, offset, length, position) {
+   try {
+    return stream.node.mount.opts.fs.write(stream.nfd, buffer, offset, length, position);
+   } catch (e) {
+    if (!e.code) throw e;
+    throw new FS.ErrnoError(ERRNO_CODES[e.code]);
+   }
+  },
+  llseek(stream, offset, whence) {
+   var position = offset;
+   if (whence === 1) {
+    position += stream.position;
+   } else if (whence === 2) {
+    if (FS.isFile(stream.node.mode)) {
+     try {
+      var stat = stream.node.node_ops.getattr(stream.node);
+      position += stat.size;
+     } catch (e) {
+      throw new FS.ErrnoError(ERRNO_CODES[e.code]);
+     }
+    }
+   }
+   if (position < 0) {
+    throw new FS.ErrnoError(ERRNO_CODES.EINVAL);
+   }
+   return position;
+  }
+ }
+};
+
 var FS = {
  root: null,
  mounts: [],
@@ -2550,7 +2760,8 @@ var FS = {
   FS.createSpecialDirectories();
   FS.filesystems = {
    "MEMFS": MEMFS,
-   "NODEFS": NODEFS
+   "NODEFS": NODEFS,
+   "PROXYFS": PROXYFS
   };
  },
  init: (input, output, error) => {
@@ -5280,6 +5491,7 @@ var allocateUTF8OnStack = stringToUTF8OnStack;
 
 var PHPWASM = {
  init: function() {
+  FS.mkdir("/internal");
   PHPWASM.EventEmitter = ENVIRONMENT_IS_NODE ? require("events").EventEmitter : class EventEmitter {
    constructor() {
     this.listeners = {};
@@ -7334,6 +7546,8 @@ Module["ccall"] = ccall;
 
 Module["FS_createPreloadedFile"] = FS.createPreloadedFile;
 
+Module["PROXYFS"] = PROXYFS;
+
 var calledRun;
 
 dependenciesFulfilled = function runCaller() {