net/http: http.nothingWrittenError incompatible with errors.Is/As

[zeisss]

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

$ go version
go1.15.2

Does this issue reproduce with the latest release?

I think so, yes.

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/zeisss/Library/Caches/go-build"
GOENV="/Users/zeisss/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/zeisss/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/zeisss/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.15.2/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.15.2/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
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/x0/0_rddyy16qn1sy9n_pclmxqh0000gn/T/go-build239938081=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

1. Performed a http request via net/http.Client
2. Received a net/http.nothingWrittenError (write tcp 172.31.16.237:54422->1.2.3.4:80: write: connection reset by peer)
3. Tried checking it with var opError *net.OpError; if errors.As(err, &opsError) { ... } to map to a user friendly errorcode
4. Error gets reported as an unknown error Rollbar, thus I know it is a http.nothingWrittenError.

What did you expect to see?

I expected errors.As() to return true for this error

What did you see instead?

I saw errors.As() returning false.

I believe the problem to occur, since the error is neither the right type nor does Unwrap() return the OpError error at any point (since it is directly in http.nothingWrittenError. The same problem can be observed for errors.Is(). See https://play.golang.org/p/0XhEPLe0bRy

Use Case

We are running a load test generator written in Go and we need to provide our users with an aggregated error report, thus we need to be able to unpack and understand the errors we receive.

[toothrot]

It seems like implementing Unwrap on net/http.nothingWrittenError makes sense to me. Alternatively, as it's not exported, it could be changed to work with errors.Is/As, which is essentially what it is doing before that existed.

/cc @neild

I'd be happy to do this change if others agree.

[neild]

I don't see a problem in principle, but I wonder if we should be returning nothingWrittenError at all. Perhaps it should be unwrapped before being returned to the user, as is done with transportReadFromServerError here:

https://go.googlesource.com/go/+/refs/heads/master/src/net/http/transport.go#605

I haven't traced all the code pathways, so I'm not sure if adding a case for nothingWrittenError at that location would be sufficient or not.

[zeisss]

If it makes a difference: nothing written is an information we would be happy to expose to our users. We are currently thinking about using "net/http/httptrace" for that though, so I guess nothing gets lost if you never return nothingWrittenError.