net/http: ServeContent add Cache-Control header

[mhewedy]

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

$ go version
go version go1.16.5 darwin/amd64

Does this issue reproduce with the latest release?

I don't know

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/mhewedy/Library/Caches/go-build"
GOENV="/Users/mhewedy/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/mhewedy/.go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/mhewedy/.go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.16.5/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.16.5/libexec/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.16.5"
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/_m/6fmdhfh952d1kvxm2h85c2rm0000gn/T/go-build4269072314=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

calling http.ServeContent to serve file from the server, it doesn't send "Cache-Control" header at all, and even the source code fs.go doesn't mention it.

What did you expect to see?

According to retrofit (the famous Android HTTP client), it needs to see Cache-Control with value must-revalidate on the response header so it can send the HTTP request with "if-modified-since" HTTP request header. and hence http.ServeContent doesn't send such a header, then the resources are kept in Retrofit cache forever.

What did you see instead?

http.ServeContent doesn't send "Cache-Control" response header at all.

[seankhliao]

cc @neild

[neild]

Given that there isn't a universally correct Cache-Control value, not setting one by default and leaving it up to the user to it if desired seems like the best approach.