Async callback in browser.webRequest.onBeforeSendHeaders not working

[JacobVarghese1992]

Sorry if this part of the known limitations mentioned but I can't get this to work for the life of me. Synchronously everything works.

Editted: This is an example now straight from the MDN webdocs.

var targetPage = "https://httpbin.org/*";

/*
Set UA string to Opera 12
*/
var ua = "Opera/9.80 (X11; Linux i686; Ubuntu/14.10) Presto/2.12.388 Version/12.16";

/*
Rewrite the User-Agent header to "ua".
*/
function rewriteUserAgentHeaderAsync(e) {
var asyncRewrite = new Promise((resolve, reject) => {
window.setTimeout(() => {
for (var header of e.requestHeaders) {
if (header.name.toLowerCase() === "user-agent") {
header.value = ua;
}
}
resolve({requestHeaders: e.requestHeaders});
}, 2000);
});

return asyncRewrite;
}

/*
Add rewriteUserAgentHeader as a listener to onBeforeSendHeaders,
only for the target page.

Make it "blocking" so we can modify the headers.
*/
browser.webRequest.onBeforeSendHeaders.addListener(
rewriteUserAgentHeaderAsync,
{urls: [targetPage]},
["blocking", "requestHeaders"]
);

manifest.json:
{
"name": "Getting Started Example",
"version": "1.0",
"description": "Build an Extension!",
"permissions": ["tabs", "activeTab", "declarativeContent", "storage", "webRequest", "webRequestBlocking", "://.httpbin.org/"],
"options_page": "options.html",
"background": {
"scripts": ["browser-polyfill.js", "background.js"],
"persistent": true
},
.
.
.
.
"manifest_version": 2
}

[mixedpuppy]

use blocking, not asyncblocking. You'll also need webRequestBlocking in the manifest permissions.

[JacobVarghese1992]

@mixedpuppy : Updated to a standard exmple from MDN docs. (My old code was me experimenting which obviously didn't even Load, sorry) Like I mentioned if I have a vanilla non-promise callback it all works fine which should mean I have the basic setup right.

[Rob--W]

Chrome's implementation does not support promises as a return value. Return values other than objects in the "BlockingResponse" format are ignored in Chrome.

[JacobVarghese1992]

@Rob--W Thanks, Yea I was naively hoping this polyfill could do some magic to get this to work in chrome. I basically need to sign a header with crypo.subtle.sign (which returns a promise) and replace the header before being sent. I'm assuming this can't be done and I can't sign it before hand.

Should this be in the list of known limitations ?

[Rob--W]

Good point, let's document it.

For your use case you would have to either generate the signatures for possible return values beforehand, or use a JS library that offers a synchronous interface, such as CryptoJS.