Fetch file

[lastmjs]

This PR implements file:// for fetch from a canister, not from the frontend. It also has example code and tests. This is a very simple version of file://

[bdemann]

What is the feature?

Does it have an issue?

No. Looks like its a bullet point of #1691

What is it doing?

Explanation

Implements fetch file presumably for the frontend since
#1706 is going to take care of the
backend

Questions

• What do I need to know about the fetch file protocol before I can competently
  preform this review?

Answers?

What is the File protocol?

The file protocol is used to get files on the local file system¹. So I would
imagine that on the client side the file protocol would get files on the user's
or developer's computer and on the canister side it would be used to get files
in the canister's file system. So I would expect that on the client side there
isn't much to do besides just pass off to the default implementation of fetch
right?
So presumably fetch with the file protocol on the backend would be used to get
files in the canister's file system.

Considerations

Security restrictions

According to Gemini "Most browsers and environments restrict file protocol
fetching for security reasons. Fetching from arbitrary locations on the user's
device could be a privacy or security risk. Your implementation might need to
consider restrictions or permissions." So in the event that we can't just pass
it off we will have to consider that.

Edge Cases

Non-existent Files

We might have to handle non-existent files gracefully if we can't just use the
default fetch

Permissions

We might have to worry about permissions if we can't just use the default
fetch

In short I hope we can just use the default fetch.

Expectations

Tests

I guess I would expect a front end that is able to get a local file and... do
something with it? Send it to the canister?

Code

Hopefully just hand off to the default implementation of fetch.

Footnotes

1. Citation Needed ↩

[bdemann]

Okay this is definitely a canister side fetch, that's a horse of a different color. Let's start over

[bdemann]

Should this be implemented?

I've started reading some stuff about how node fetch doesn't actually implement the file protocol, so we might not should do it either. I'll do some more research including working more with tensor flow to see what exactly it needs the file protocol for and make sure we are understanding its needs correctly.