net/http: Expose "http: server gave HTTP response to HTTPS client" error

[AkihiroSuda]

What version of Go are you using (go version)?

$ go version
go version go1.16 linux/amd64

Does this issue reproduce with the latest release?

Yes

What did you expect to see?

I'd like to see the http: server gave HTTP response to HTTPS client error in the net/http package to be exposed as a constant that can be compared with errors.Is

go/src/net/http/client.go

Line 262 in f21be2f

err = errors.New("http: server gave HTTP response to HTTPS client")

Context

I'm working on a clone of Docker called nerdctl (contaiNERD ctl).
I want to compare http: server gave HTTP response to HTTPS client with errors.Is, for ease of reimplementing dockerd --insecure-registries, which attempts HTTPS first and then falls back to HTTP.

What did you see instead?

The http: server gave HTTP response to HTTPS client error is not exposed.

[toothrot]

/cc @bradfitz @rsc @empijei @neild

[neild]

This seems reasonable to me, especially given that this error is replacing a more-informative (to programs) tls.RecordHeaderError.

Perhaps the error returned here should wrap the tls.RecordHeaderError rather than replacing it.

[neild]

Three concrete possibilities that I can think of:

1. Expose an ErrHTTPResponse var:

   ErrHTTPResponse = errors.New("http: server gave HTTP response to HTTPS client")

   [+] Simple.
   [-] Inconsistently returns ErrHTTPResponse or tls.RecordHeaderError depending on the response.

2. Don't expose a var, but make the error wrap the underlying tls.RecordHeaderError. In the case where someone wants to detect this condition, they can use errors.As to recover the underlying error and examine it:

   var tlsErr tls.RecordHeaderError
   if errors.As(err, &tlsErr) && string(tlsErr.RecordHeader[:]) == "HTTP/" {
       // ...
   }

   [+] Consistently return the underlying tls.RecordHeaderError.
   [-] Detecting the error condition is more work for the caller.
   [+] Contract with the caller is clear: We provide the tls.RecordHeaderError and they can decide how to interpret it.

3. Both: Return an error which errors.Is equivalent to an ErrHTTPResponse sentinel and which also wraps tls.RecordHeaderError.
   [-] Lot of mechanism for a fairly obscure case.

Option (2) requires the least change to the http API surface, but (1) is the simplest. Either of those seems reasonable to me.

[AkihiroSuda]

Can we wrap (2) in a function like func IsNotHTTPS(err error) bool?

[bradfitz]

I'm fine with (2). Or (1) if we name it, say, ErrSchemeMismatch ("HTTP" in "ErrHTTPResponse" sounds too generic, as "HTTP" often also means "HTTPS").

I wouldn't want to see a new function. That's too heavy in godoc for how fringe this is.