Subrouters implement http.Handler but don't do anything useful.

[hydrogen18]

Today I accidentally passed a gocraft/web subrouter to the net/http package. This was an accident on my part, but resulted in some strange behavior. It appears that for each request, it was performed twice. I was not even aware a subrouter could be pased to net/http.

This example demonstrates my mistake, but does not replicate the behavior.

// serve_subrouter.go
package main

import "net/http"
import "github.com/gocraft/web"
import "sync/atomic"
import "fmt"

var numcall uint32

type Context struct{}

func printNumCalls(rw web.ResponseWriter, req *web.Request, next web.NextMiddlewareFunc) {
    count := atomic.AddUint32(&numcall, 1)
    fmt.Printf("Call #%d\n", count)
    next(rw, req)
}

func helloHttp(rw web.ResponseWriter, req *web.Request) {
    rw.WriteHeader(http.StatusOK)
    fmt.Fprint(rw, "Hello HTTP\n")
}
func main() {
    router := web.New(Context{})
    subrouter := router.Subrouter(Context{}, "/foo")
    subrouter.Middleware(printNumCalls)
    subrouter.Get("/bar", helloHttp)

    http.ListenAndServe(":8080", subrouter) //use subrouter by accident
}

As you can see, subrouter can be used as it implements http.Handler. However, all I got was a panic when I used curl to get "http://localhost:8080/foo/bar".

ericu@ericu-desktop:~$ go run ./serve_subrouter.go 
2014/11/05 22:18:28 http: panic serving [::1]:51597: runtime error: makeslice: cap out of range
goroutine 20 [running]:
net/http.func·011()
    /home/ericu/builds/go/src/pkg/net/http/server.go:1100 +0xb7
runtime.panic(0x6cb260, 0x8bc0fc)
    /home/ericu/builds/go/src/pkg/runtime/panic.c:248 +0x18d
github.com/gocraft/web.(*Router).ServeHTTP(0xc208058180, 0x7fe673d358c8, 0xc20803cbe0, 0xc208029040)
    /home/ericu/gotmp/src/github.com/gocraft/web/router_serve.go:32 +0x82
net/http.serverHandler.ServeHTTP(0xc208004180, 0x7fe673d358c8, 0xc20803cbe0, 0xc208029040)
    /home/ericu/builds/go/src/pkg/net/http/server.go:1673 +0x19f
net/http.(*conn).serve(0xc20803e200)
    /home/ericu/builds/go/src/pkg/net/http/server.go:1174 +0xa7e
created by net/http.(*Server).Serve
    /home/ericu/builds/go/src/pkg/net/http/server.go:1721 +0x313
^Cexit status 2
ericu@ericu-desktop:~$ go run ./serve_subrouter.go 
2014/11/05 22:18:42 http: panic serving [::1]:51598: runtime error: makeslice: cap out of range
goroutine 20 [running]:
net/http.func·011()
    /home/ericu/builds/go/src/pkg/net/http/server.go:1100 +0xb7
runtime.panic(0x6cb260, 0x8bc0fc)
    /home/ericu/builds/go/src/pkg/runtime/panic.c:248 +0x18d
github.com/gocraft/web.(*Router).ServeHTTP(0xc208056180, 0x7f74ab7559c8, 0xc20803cbe0, 0xc208029450)
    /home/ericu/gotmp/src/github.com/gocraft/web/router_serve.go:32 +0x82
net/http.serverHandler.ServeHTTP(0xc208004180, 0x7f74ab7559c8, 0xc20803cbe0, 0xc208029450)
    /home/ericu/builds/go/src/pkg/net/http/server.go:1673 +0x19f
net/http.(*conn).serve(0xc20803e200)
    /home/ericu/builds/go/src/pkg/net/http/server.go:1174 +0xa7e
created by net/http.(*Server).Serve
    /home/ericu/builds/go/src/pkg/net/http/server.go:1721 +0x313

The obvious solution is not to pass subrouter to net/http. But it seems this mistake could be avoided by making the subrouter not implement the http.Handler type. This would generate a very obvious compile time error. If I come up with some patches to do this, would you be willing to accept them?

[cypriss]

@hydrogen18 Sorry for the delay in responding. I might be willing to accept them, but I worry that fixing it will introduce more complexity than it solves.

[elimisteve]

I've run into this a couple times. Thank you @hydrogen18 for spotting the issue -- I created a subrouter with the name of the original router, renamed the original router to something else, in which case I ended up passing the subrouter to http.ListenAndServe. Oops!

[hydrogen18]

I keep meaning to propose patches to work around this, but I've been too busy. Glad to know I'm not the only one @elimisteve