net/http header not being set as expected

[evanlurvey]

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

tested with go 1.17.5 and 1.17.6

Does this issue reproduce with the latest release?

yes

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

GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/elurvey/Library/Caches/go-build"
GOENV="/Users/elurvey/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/elurvey/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/elurvey/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.17.6"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/pq/pmt4ss412gz7_4r9dl_ny35c0000gn/T/go-build3784969989=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

package main

import "net/http"

func transactionsEndpoint(w http.ResponseWriter, r *http.Request) {
	w.WriteHeader(200)
	w.Header().Set("Content-Type", "application/json")
	w.Write([]byte(`{"why": "you no work"}`))
}

func main() {
	handler := http.HandlerFunc(transactionsEndpoint)
	http.Handle("/", handler)
	http.ListenAndServe(":8080", nil)
}

What did you expect to see?

I expect to see the header set Content-Type header set to application/json

What did you see instead?

header is set to text/plain instead

$ curl localhost:8080 -v
*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 8080 (#0)
> GET / HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.64.1
> Accept: */*
> 
< HTTP/1.1 200 OK
< Date: Wed, 12 Jan 2022 21:51:12 GMT
< Content-Length: 22
< Content-Type: text/plain; charset=utf-8
< 
* Connection #0 to host localhost left intact
{"why": "you no work"}* Closing connection 0

[seankhliao]

Changing the header map after a call to WriteHeader (or Write) has no effect unless the modified headers are trailers

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