Allow other extensions to connect

[jakubsta]

As discussed in #940 this PR allows to connect to MetaMask from other extensions.

Resolves #940

[danfinlay]

This looks good at a cursory glance, but please also understand we need to treat this carefully, and QA/security review it thoroughly before merging. I will try to prioritize completing this in the next sprint or the one after. Sorry for the wait, but I'm sure you don't want us neglecting security either ;)

[june07]

In the meantime here's a workaround...

https://june07.com/crypto/metamaskpay.html?amount=1&payee=0x69F3C9210091A0E5cb7D01459683447173D4BDDA

It works for my extension NiM https://june07.com/nim
http://recordit.co/OLMqQXl6kR

[kumavis]

adding label to make sure this doesn't get in without extra review

[kumavis]

This does look good! Need to read up on the APIs and make sure they match our assumptions.

[danfinlay]

I've reviewed the API, it does seem to do what we expect here:
https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/runtime/onConnectExternal

[kumavis]

Can you connect externally to *? I think I remember this being a problem when I looked at it a while back
maybe the "ids": ["*"] is the trick

Edit:
https://developer.chrome.com/extensions/manifest/externally_connectable#reference

ids checks apps/extensions, matches checks web pages

ids (array of string) - optional

The IDs of extensions or apps that are allowed to connect. If left empty or unspecified, no extensions or apps can connect.

The wildcard "*" will allow all extensions and apps to connect.

matches (array of string) - optional

The URL patterns for web pages that are allowed to connect. This does not affect content scripts. If left empty or unspecified, no web pages can connect.

Patterns cannot include wildcard domains nor subdomains of (effective) top level domains; ://google.com/ and http://.chromium.org/ are valid, while <all_urls>, http:///, ://.com/, and even http://.appspot.com/* are not.

[jakubsta]

Yes I can connect from external extension like here: https://github.com/jakubsta/example-extension/blob/master/background.js#L7 (it connects to extension built from this PR)