net/http: Client panics if RoundTripper returns neither response nor error

[Teelevision]

What did you do?

Use a http.RoundTripper that returns neither response nor error.

https://play.golang.org/p/EGEjS2P2xQI

What did you expect to see?

Documentation that the RoundTripper MUST return either response or error.

What did you see instead?

go/src/net/http/client.go

Lines 119 to 125 in 1dc0110

	// RoundTrip should not attempt to interpret the response. In
	// particular, RoundTrip must return err == nil if it obtained
	// a response, regardless of the response's HTTP status code.
	// A non-nil err should be reserved for failure to obtain a
	// response. Similarly, RoundTrip should not attempt to
	// handle higher-level protocol details such as redirects,
	// authentication, or cookies.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0xffffffff addr=0x0 pc=0x24bc42]

goroutine 1 [running]:
net/http.redirectBehavior(0x311a6b, 0x3, 0x0, 0x84c510, 0x0, 0x0, 0x0, 0x0)
	/usr/local/go/src/net/http/client.go:422 +0x22
net/http.(*Client).do(0x840940, 0x84c510, 0x0, 0x0, 0x0, 0x0)
	/usr/local/go/src/net/http/client.go:655 +0x3a0
net/http.(*Client).Do(...)
	/usr/local/go/src/net/http/client.go:509
net/http.(*Client).Get(0x840940, 0x3122b2, 0x7, 0x5503, 0x0, 0x0, 0x0, 0x5503)
	/usr/local/go/src/net/http/client.go:398 +0xa0
main.main()
	/tmp/sandbox656600978/prog.go:15 +0xc0

Program exited: status 2.

[odeke-em]

Hello @Teelevision, thank you for filing this issue and welcome to the Go project!

So my apologies if am preaching to the choir; in Go, the idiomatic way of communicating
between functions is by returning values as well as error as per (values..., error). You'll commonly see in Go code handling for which:

1. A non-nil error signals a non-normal flow/state of the program and requires a form of handling
2. If no error was encountered, the value is then assumed to be okay and can be dereferenced/used

if we have the case of (*http.Response)(nil), (error)(nil) both being nil, we are in an invalid state where the expectation of a valid *http.Response is no longer fulfilled. It would break from the norm to expect that every function handle both error == nil and values == nil

Right above the lines that you quoted, we've got

go/src/net/http/client.go

Lines 115 to 117 in 9c1f14f

	type RoundTripper interface {
	// RoundTrip executes a single HTTP transaction, returning
	// a Response for the provided Request.

which says that RoundTrip returns a Response.

Also I believe this doc already says what you'd like it to report

go/src/net/http/client.go

Lines 120 to 123 in 9c1f14f

	// particular, RoundTrip must return err == nil if it obtained
	// a response, regardless of the response's HTTP status code.
	// A non-nil err should be reserved for failure to obtain a
	// response. Similarly, RoundTrip should not attempt to

and implicitly a nil Response can't have a status code.

What do you think?

[Teelevision]

Ok, thank you, this clarifies it for me.