mime: wrong extensions list for content type "image/jpeg" when using built-in types #36524
Description
What version of Go are you using (go version)?
$ go version go version go1.13.6 linux/amd64
Does this issue reproduce with the latest release?
yes
What operating system and processor architecture are you using (go env)?
go env Output
$ go env GO111MODULE="" GOARCH="amd64" GOBIN="" GOCACHE="xyz/.cache/go-build" GOENV="xyz/.config/go/env" GOEXE="" GOFLAGS="" GOHOSTARCH="amd64" GOHOSTOS="linux" GONOPROXY="xyz" GONOSUMDB="xyz" GOOS="linux" GOPATH="xyz/go" GOPRIVATE="xyz" GOPROXY="https://proxy.golang.org,direct" GOROOT="/usr/local/go" GOSUMDB="sum.golang.org" GOTMPDIR="" GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64" GCCGO="gccgo" AR="ar" CC="gcc" CXX="g++" 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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build284189108=/tmp/go-build -gno-record-gcc-switches"(Some values sanitized with "xyz".)
What did you do?
I was trying to retrieve file extensions for the content type "image/jpeg" with mime.ExtensionsByType("image/jpeg").
https://play.golang.org/p/gfVzW9oAdiq
What did you expect to see?
On systems where none of the following files are present, the built-in types list is used according to the documentation.
/etc/mime.types /etc/apache2/mime.types /etc/apache/mime.types
The built-in list can be viewed here: https://golang.org/src/mime/type.go#L59
It contains two entries for "image/jpeg". So I would expect to get these two entries as result of mime.ExtensionsByType("image/jpeg"): []string{".jpg", ".jpeg"}
There is also a code comment in that mentions this exact case: https://golang.org/src/mime/type.go#L19
What did you see instead?
On my machine either []string{".jpg"} or []string{".jpeg"} at random. On the playground always []string{".jpeg"}.
Here is a test case for the issue that can be run in "mime/type_test.go":
func TestExtensionsByType2(t *testing.T) {
cleanup := setMimeInit(func() {
clearMimeTypes()
// Initialize built-in types like it is done in "type.go"
// https://golang.org/src/mime/type.go#L84
setMimeTypes(builtinTypesLower, builtinTypesLower)
})
defer cleanup()
tests := []struct {
typ string
want []string
}{
{typ: "image/jpeg", want: []string{".jpg", ".jpeg"}},
}
for _, tt := range tests {
got, err := ExtensionsByType(tt.typ)
if err != nil {
t.Error("error")
}
if !reflect.DeepEqual(got, tt.want) {
t.Errorf("ExtensionsByType(%q) = %q; want %q", tt.typ, got, tt.want)
}
}
}