Special case /register for application services

[matrixbot]

This issue was originally created by @babolivier at https://github.com/matrix-org/dendrite/issues/630.

• [ ] Allow an application service to register usernames starting with underscores if its configuration file allows it.
• [ ] Support non-exclusive namespaces.

The latter means we shouldn't respond with Only 1 match allowed-like errors if a user ID provided during registration from an appservice matches more than one namespace and none of these namespaces is defined as exclusive. This requires checking whether a namespace is exclusive during registration.

[matrixbot]

This comment was originally posted by @MohitKS5 at https://github.com/matrix-org/dendrite/issues/630#issuecomment-565467829.

For the first part, should I assume that permission to use underscore will be implied by application service namespace 's regex or add a new parameter to config?

[matrixbot]

This comment was originally posted by @rhymefororange at https://github.com/matrix-org/dendrite/issues/630#issuecomment-991217039.

Is this still an open issue? #1785 fixed the first part and #828 fixed the second, right? And #1599 mentions that #1784 is being used instead of this issue (which is closed). Can someone clear up the confusion?

[matrixbot]

This comment was originally posted by @rhymefororange at https://github.com/matrix-org/dendrite/issues/630#issuecomment-994919441.

@babolivier @kegsay any thoughts?

[matrixbot]

This comment was originally posted by @kegsay at https://github.com/matrix-org/dendrite/issues/630#issuecomment-1008774271.

Unsure, I haven't been keeping track of appservice support. If the tests pass, then it should be ok to close, though I don't know which sytests relate to this issue.