doc/go1.15: add note about crypto/tls error wrapping change

[rolandshoemaker]

After https://go-review.googlesource.com/c/go/+/227840 http.Client and tls.Client will now return an opaque tls.permanentError in place of what were previously public error types (i.e. net.OpErr). This means that in order to get the previously top level error users need to use errors.As (or errors.Unwrap) to get the useful error (best practice should presumably be to always use errors.As but directly casting to net.OpErr is definitely quite common).

cc @FiloSottile @katiehockman

[katiehockman]

Yeah that sounds good. I'll take care of that. Thanks @rolandshoemaker.

[gopherbot]

Change https://golang.org/cl/247698 mentions this issue: docs/go1.15: document crypto/tls permanentError

[UladBohdan]

Hi @rolandshoemaker @katiehockman

I've noticed the original description of this issue refers to direct casting of errors to net.OpError type, I can see how it was affected and I can see how error check was modified in tests to handle new behaviour https://go-review.googlesource.com/c/go/+/227840/4/src/crypto/tls/handshake_server_test.go

However, I can't really see why the final description of the change https://go-review.googlesource.com/c/go/+/247698/4/doc/go1.15.html refers to assertion of net.Error type. From what I see, net.Error interface type was not really affected: new permanentError type also implements net.Error, therefore casting will not be broken https://go-review.googlesource.com/c/go/+/227840/4/src/crypto/tls/conn.go

Could you please explain this to me? Does go1.15 really affect net.Error but not other public error types from net package i.e. net.OpError?

[FiloSottile]

Yeah the release note should probably say "the original error" rather than "the original net.Error". I'll send a CL later if no one beats me to it.