Closing the receiver tab will not throw an error in tabs.sendMessage()

[fregante]

Related:

• Chrome: "The message port closed before a response was received." #130

• webextension-polyfill/src/browser-polyfill.js

  Lines 488 to 489 in f818ecb

  	if (extensionAPIs.runtime.lastError.message === CHROME_SEND_MESSAGE_CALLBACK_NO_RESPONSE_MESSAGE) {
  	resolve();

From my understanding, the code above was added to silence the error Chrome throws when a message is received by the listener but not "handled" (i.e. the listener did not return a promise).

For Chrome, this is a user error: The user must call sendResponse, even without arguments.

Unfortunately the polyfill’s workaround means that I can't distinguish between:

• the tab never handled my request (if there's any listener registered)
• the tab closed before my request was answered

… because in both cases, the promise will resolve with undefined.

Since the solution to this problem is simple, I suggest surfacing this error instead of ignoring it. The solution for users is:

// use an async listener
browser.runtime.onMessage.addListener(async (message) => {
	// Handled, done
});

// return anything (or better, a resolved promise)
browser.runtime.onMessage.addListener((message) => {
	return Promise.resolve();
});

I understand however that this would be a breaking change, but the current situation means I can't use this polyfill to send long-running messages (e.g. send message, wait for HTTP response(2))

[fregante]

While looking into this, I also attempted to document some Chrome responses, which lead me to open an issue on the polyfill:

• Documentation: How sendMessage responds pixiebrix/webext-messenger#66