Golang net/http: "http: invalid Read on closed Body" on "io.Copy(w, r.Body)"

Go version

go version go1.24.3 darwin/arm64

Output of go env in your module/workspace:

AR='ar'
CC='clang'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='clang++'
GCCGO='gccgo'
GO111MODULE=''
GOARCH='arm64'
GOARM64='v8.0'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/Users/REDACTED/Library/Caches/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/Users/REDACTED/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/3m/m4kn6p6510zbvxpx5nyzpbfr0000gn/T/go-build1423855236=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD='REDACTED'
GOMODCACHE='/Users/REDACTED/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/REDACTED/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/REDACTED/Library/Application Support/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.24.3'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

func Test_HttpEcho(t *testing.T) {
    httpServer := &http.Server{
        Addr: ":5555",
        Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            _, err := io.Copy(w, r.Body)
            if err != nil {
                t.Fatal(err)
            }
            r.Body.Close()
        }),
    }
    go httpServer.ListenAndServe()
    defer httpServer.Close()

    http.Post("http://127.0.0.1:5555/", "", strings.NewReader(strings.Repeat("X", 1<<10)))
}

Results in http: invalid Read on closed Body

What did you see happen?

io.Copy returned an error http: invalid Read on closed Body

w.w.Flush() in /usr/local/go/src/net/http/server.go line 610 is closing the r.Body after sniffing the first 512 bytes. The closing itself happens at /usr/local/go/src/net/http/server.go line 1471 err = w.reqBody.Close().

What did you expect to see?

I did not expect an error from io.Copy.

The following alternative code does NOT error out and works as expected.

func Test_HttpEcho(t *testing.T) {
    httpServer := &http.Server{
        Addr: ":5555",
        Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            body, err := io.ReadAll(r.Body)
            if err != nil {
                t.Fatal(err)
            }
            _, err = w.Write(body)
            if err != nil {
                t.Fatal(err)
            }
            r.Body.Close()
        }),
    }
    go httpServer.ListenAndServe()
    defer httpServer.Close()

    http.Post("http://127.0.0.1:5555/", "", strings.NewReader(strings.Repeat("X", 1<<10)))
}

Comment From: seankhliao

From http.ResponseWriter.Write:

// Depending on the HTTP protocol version and the client, calling // Write or WriteHeader may prevent future reads on the // Request.Body. For HTTP/1.x requests, handlers should read any // needed request body data before writing the response.

See https://pkg.go.dev/net/http#ResponseController.EnableFullDuplex

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