wasm_sapi_send_headers produces broken JSON due to incorrect escaping

[adamziel]

When using the new Font Library in WordPress 6.5, I ran into the following error:

register-service-worker.ts:64 Uncaught (in promise) SyntaxError: Bad escaped character in JSON at position 601 (line 9 column 122) (at comlink.js?v=01749073:42:27)
    at JSON.parse (<anonymous>)
    at #getResponseHeaders (base-php.ts:349:28)
    at #handleRequest (base-php.ts:631:44)
    at async WebPHP.run (base-php.ts:267:21)
    at async #dispatchToPHP (php-request-handler.ts:243:11)
    at async PHPRequestHandler.request (php-request-handler.ts:119:11)
    at async PHPBrowser.request (php-browser.ts:61:20)

Turns out, it was in this line in base-php.ts:

const headersData = JSON.parse(this.readFileAsText(headersFilePath));

It tries to parse the JSON-encoded headers emited by PHP.wasm. Here's the output that tripped it up:

{ "status": 201, "headers": ["X-Powered-By: PHP/8.0.30-dev",
"Content-Type: application/json; charset=UTF-8",
"X-Robots-Tag: noindex",
"Link: <http://localhost:5400/scope:0.4235061639379647/index.php?rest_route=/>; rel=\"https://api.w.org/\"",
"X-Content-Type-Options: nosniff",
"Access-Control-Expose-Headers: X-WP-Total, X-WP-TotalPages, Link",
"Access-Control-Allow-Headers: Authorization, X-WP-Nonce, Content-Disposition, Content-MD5, Content-Type",
"X-WP-Nonce: bfcdf46c4b",
"Location: http://localhost:5400/scope:0.4235061639379647/index.php?rest_route=/wp/v2/font-families/(?P<font_family_id>[\d]+)/font-faces/13",
"Allow: GET, POST",
"Expires: Wed, 11 Jan 1984 05:00:00 GMT",
"Cache-Control: no-cache, must-revalidate, max-age=0, no-store, private"  ]}

Turns out, it's about \d. We should probably escape backslashes like we escape double quotes: in the wasm_sapi_send_headers C function:

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

Lines 1423 to 1431 in 1467766

	for (int i = 0, max = sapi_header->header_len; i < max; i++)
	{
	if (sapi_header->header[i] == '"')
	{
	fwrite(&"\\", sizeof(char), 1, headers_file);
	}
	fwrite(&sapi_header->header[i], sizeof(char), 1, headers_file);
	}

cc @bgrgicak – would you like to tackle this one?