Generate User-Agent when app starts, thus avoiding storing it on disk

[manavmehta]

Fixes: #921

What's this PR do?
Fetch and send UA from main process and use ipc call to catch it in renderer process: system-util.ts and use it elsewhere in the app as required, avoiding storing it on disk.

Any background context you want to provide?
Please refer #921

You have tested this PR on:

• Linux/Ubuntu
• macOS Catalina 10.15.2

[manavmehta]

@andersk @timabbott Can you please have a look? If this feels right, we can refine it :)

[timabbott]

This certainly looks plausible. It's probably a good idea to test this against a Zulip development server configured with a Python print() statement to print the User-Agent it received in both GET /server_settings and GET /events to confirm it's sending the right data in both contexts.

[akashnimare]

@manavmehta can you test this against a zulip test server and note down the UA in the following cases -
a) When user add a new server
b) API requests from the renderer process

[manavmehta]

The UA is correct in /server_settings and /events
But it is sent as null (or the default value) while adding a server, thereafter, it is correct in renderer API calls.

Reason: The new-server-form calls domain-util which calls system-utils afresh.

Workaround as suggested by @abhigyank ebb1238
Change the default value of UA
This would work because:

1. This is the only part we use to recognize Desktop app
2. A user adds server least often.

@timabbott @andersk any fixes in your mind or should we go with this?
(documenting this a TODO so that someone could come up with a fix)

[andersk]

A more robust approach would be for the renderer process to ask the main process for the user-agent with ipcRenderer.invoke (caching the resulting Promise) or ipcRenderer.sendSync (caching the resulting string).

[manavmehta]

@andersk That was a great suggestion.
It is ready for review and merge from my side.
cc: @akashnimare

P.S. If you want, I can send you the log where I printed a bunch of UAs received by the test server

[kanishk98]

Alright, this looks good to me, except for a few comments I've left that you may want to consider.
Great work, @manavmehta!

[andersk]

The goal is to avoid the race condition where we see ZulipElectron/5.0.0 for some time at startup instead of the full ZulipElectron/5.0.0 Mozilla/5.0 blah/blah blah string. Your code still doesn’t do that.

Either use ipcRenderer.invoke this way:

const userAgentPromise: Promise<str> = ipcRenderer.invoke('fetch-user-agent');

export async function getUserAgent(): string {
    return `ZulipElectron/${app.getVersion()} ${await userAgentPromise}`;
}

(which requires adjusting its callers to be async as well), or use ipcRenderer.sendSync.

[manavmehta]

@andersk I tested 37c420a against Zulip test server and logging both. Couldn't find a single instance of the race condition.

But since theoretically, the condition is possible, I think we may go with ipcRenderer.sendSync
Tested it and it works fine. Plus, since it fetches UA synchronously, I don't think there's a possibility for the race condition.

Though it adds a delay, which is probably lesser than a millisecond and better than the race condition anyway.
Thanks :)

[andersk]

I edited this to use ses.setUserAgent to set the User-Agent once for the entire session rather than individually for each webview. I also squashed all the commits together.