net/http: improve useFakeNetwork check to allow browsers to run tests in wasm

[agnivade]

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

$ go version
go version devel +1531192272 Mon May 27 17:58:39 2019 +0000 linux/amd64

net/http/rountrip_js.go has this code:

// useFakeNetwork is used to determine whether the request is made
// by a test and should be made to use the fake in-memory network.
func useFakeNetwork() bool {
	return len(os.Args) > 0 && strings.HasSuffix(os.Args[0], ".test")
}

This code has gone through several iterations. First it was:

host, _, err := net.SplitHostPort(req.Host)
if err != nil {
	host = req.Host
}
if ip := net.ParseIP(host); ip != nil {
	return ip.IsLoopback(ip)
}
return host == "localhost"

Then, it was changed to:

return len(os.Args) > 0 && path.Base(os.Args[0]) == "node"

And finally,

return len(os.Args) > 0 && strings.HasSuffix(os.Args[0], ".test")

The latest version will always return true if any code is invoked with go test. That prevents wasm codepath from being taken if the test is being run using a binary (by replacing the -exec flag) which spins up a browser, and loads the wasm file in it and captures the logs. Essentially a test environment for wasm code inside the browser.

I stumbled onto this while writing a tool like that. The current workaround from my end is to rename the binary to .wasm if it ends with .test. But ideally, this should be fixed in the code.

@johanbrandhorst @neelance @bradfitz

[bradfitz]

What do you propose?

[agnivade]

Actually, I can't recall why the 2nd version was changed. The only difference between 2nd and 3rd is that from catching all node-spawned processes, now it catches all tests under js,wasm flag.

Perhaps -

len(os.Args) > 0 && path.Base(os.Args[0]) == "node" && strings.HasSuffix(os.Args[0], ".test") 

sounds better ? Since it filters both node processes and tests, allowing browser tests to run.

[johanbrandhorst]

The reason it was introduced was to allow the gerrit tests to pass, so if such a change would still allow the gerrit tests to pass while allowing tests to be run in the browser then I think that sounds good.

[agnivade]

I meant what exactly changed in the logic from v2 to v3 to allow trybots to pass. AFAIU, v3 generalizes the logic to follow a non-wasm path for all tests. But v2 already allowed node. So how did it fail ? (assuming wasm trybots failed)

Anyways, I am afk at the moment, but I will check if my suggestion passes both node and browser tests.

[gopherbot]

Change https://golang.org/cl/179118 mentions this issue: net/http: allow WASM fetch RoundTripper in non-node envs