lib/user-methods.js's performFetch API is unstable across platforms

[kraenhansen]

Goals

We have to make a choice: Either the performFetch method of lib/user-methods.js should adhere to the fetch API, which means that we should be able to read header values from a response such as response.headers.get('Content-Type') or we should simplify the way we call the method making less assumptions on the shape of the response object (I don't recommend the latter).

In its current implementation performFetch performs very differently whether Realm JS is running on Node.js or in an environment with fetch available.

We currently have tests failing on master due to this discrepancy in behaviour across platforms.

Version of Realm and Tooling

• Realm JS SDK Version: 2.24.0
• Node or React Native: Both - but mostly React Native and Electron exhibits regressions.
• Client OS & Version: N/A
• Which debugger for React Native: N/A

[tgoyne]

That test failed because the network request to github failed, not because it couldn't read the content type header. The bit about not getting the correct content type is the expected result.

[kraenhansen]

Okay - I might have been a little too quick in my conclusion.

[bradjonesca]

FYI: whatwg/fetch#878

[kraenhansen]

I am sorry to report that we still have an issue with this implementation on Electron when called from the main process:

ReferenceError: XMLHttpRequest is not defined
     at fetchWithTimeout (/Users/kraenhansen/Repositories/realm-studio/node_modules/realm/lib/user-methods.js:82:17)
     at next (/Users/kraenhansen/Repositories/realm-studio/node_modules/realm/lib/user-methods.js:125:9)
     at Promise (/Users/kraenhansen/Repositories/realm-studio/node_modules/realm/lib/user-methods.js:140:13)
     at new Promise (<anonymous>)
     at performFetch (/Users/kraenhansen/Repositories/realm-studio/node_modules/realm/lib/user-methods.js:138:16)
     at _authenticate (/Users/kraenhansen/Repositories/realm-studio/node_modules/realm/lib/user-methods.js:337:19)
     at request.catch (/Users/kraenhansen/Repositories/realm-studio/node_modules/realm/lib/user-methods.js:341:20)
     at process._tickCallback (internal/process/next_tick.js:68:7)

[kneth]

Moreover, we should add support for exponential backoff and retrying for all platforms.

[kraenhansen]

This issue went away as we merged with the Stich SDK and introduced the realm-network-transport package (https://github.com/realm/realm-js/tree/master/packages/realm-network-transport).