Golang net/http/httputil: export ReverseProxy.defaultErrorHandler

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

$ go version
go version go1.11.1 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/harald/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/harald/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/Cellar/go/1.11.1/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.11.1/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/w3/11rv94110qnc2l400nlz7gmc0000gn/T/go-build136867624=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you expect to see?

It seems to me that when using ReverseProxy.ErrorHandler (from package httputil) a nice pattern would be to introspect the error, do special processing on or ignore certain types of errors, but then fall back to the default error handling.

Therefore, I would like to export this function so I don't have to reimplement it (and reimplement logf):

func (p *ReverseProxy) defaultErrorHandler(rw http.ResponseWriter, req *http.Request, err error) {
    p.logf("http: proxy error: %v", err)
    rw.WriteHeader(http.StatusBadGateway)
}

An added bonus is also that you then receive updates to this logic as Go versions progress, whereas the copied-over code would get stale.

Comment From: HaraldNordgren

My current workaround is to turn the method into a function with the ReverseProxy as first argument to be able to define it locally:

// Adapted from https://github.com/golang/go/blob/go1.11.4/src/net/http/httputil/reverseproxy.go
func logf(p *httputil.ReverseProxy, format string, args ...interface{}) {
    if p.ErrorLog != nil {
        p.ErrorLog.Printf(format, args...)
    } else {
        log.Printf(format, args...)
    }
}

// Adapted from https://github.com/golang/go/blob/go1.11.4/src/net/http/httputil/reverseproxy.go
func defaultErrorHandler(p *httputil.ReverseProxy) func(rw http.ResponseWriter, req *http.Request, err error) {
    return func(rw http.ResponseWriter, req *http.Request, err error) {
        logf(p, "http: proxy error: %v", err)
        rw.WriteHeader(http.StatusBadGateway)
    }
}