client: use error wrapping for WithReturnConnectionError

[anitgandhi]

Use case(s) - what problem will this feature solve?

This is essentially a re-open of #4155, because I don't believe it was actually fixed for Dial.

We use grpc.WithBlock on Dial by default in our organization, and we've love to switch to grpc.WithReturnConnectionError instead, but being able to Unwrap to get one of the underlying errors would be nice.

Proposed Solution

Solution #1 as described in #4155.
Basically, use %w instead of %v.

As things stand on the latest release, %v is still used: https://github.com/grpc/grpc-go/blob/v1.41.0/clientconn.go#L227

As of v1.41.0, support for Go <=1.13 has been dropped anyway (#4700 ), so in my opinion it's safe to start using %w going forward.

Alternatives Considered

%w: %w would have been nice, but go vet doesn't allow that, and it also causes errors.Is to return false for both of them, negating the benefits completely.

I'm not sure which one would necessarily be better to use %w on , ctx.Err() or the transport err. But either way, I think it would still be nice to have one of them.

Thanks in advance for considering it!

[menghanl]

Thanks for the FR.
We can wrap the connection error (%v: %w). But wrapping both errors doesn't work I think (not sure what the behavior is with two %ws).

Another thing is that we might add support for older go versions back, for reasons outside of our control. So we may need to hold this a bit longer.

[anitgandhi]

Right yeah, i tested %w %w and it completely breaks all wrapping (see end section of issue description).

As a possible alternative - we could just change the behavior to return either context.DeadlineExceeded OR the last transport error (when the opt is used, of course).

could make it a bit less confusing