ExternalWebSession: avoid extra slashes when the path is ""

[zenhack]

Currently, if the caller makes a request to an ExternalWebSession with
path = "", An extra trailing slash is appended to the URL. This makes
capabilities for non-"directory" URLs useless; e.g. if the
ExternalWebSession points to the URL:

http://example.com/rss.xml

Then when trying to fetch that exact URL (rather than something under
it), the requested URL will most likely be:

http://example.com/rss.xml/

...which will probably result in 404.

This patch fixes the behavior, such that no additional slash is added if
path = "".

[kentonv]

Hmm, unfortunately this means there's now no way to specify that you want the trailing slash.

Maybe we could trigger this when the path is null, rather than the empty string?

[zenhack]

One problem with that is that afaik there's no way for an app using the bridge's proxy functionality for this to actually set the path to null, so it would remain unusable except for apps speaking capnp directly.

I think you could still send the trailing slash if you're willing to put the trailing slash in canonicalUrl when making the powerbox request in the first place? That means you'd need a separate cap if you needed to do both though...

[kentonv]

I know this is ugly but I think we need to be explicit somehow. As it stands this change could change behavior of existing apps, and I'm not completely confident saying no one depends on this.

It sucks that HTTP treats /foo and /foo/ differently, ugh...

[zenhack]

Looking back at this, I'm not sure it makes a ton of sense to try to twist ApiSession into handling this case; the docs already suggest you're supposed to be using this to access oauth endpoints and such, and this use case probably won't come up in that context.

In the original context of this PR, I was trying to request individual URLs as part of the bridge I wrote for ttrss -- but I ended up just switching to requesting the whole domain anyway, since otherwise you got way too many pb requests. So this has lost its immediacy (which is why it's been sitting here for months...)

Perhaps we should just close this, and if we find we want to be able to request individual URLs, we add a separate interface that users can request for that purpose; that way we don't have to worry about backwards compatibility concerns, or shoehorn this exact interface into that use case.

I'm going to go ahead and close, though if anyone has opinions I'm willing to discuss further.