add stuff for translating x-forwarded-for to player id lol

Fixes #5.

serve/proxy.go

@@ -0,0 +1,51 @@
+package serve
+
+import (
+	"context"
+	"net/http"
+	"net/netip"
+	"strings"
+
+	"git.sunturtle.xyz/studio/shotgun/player"
+)
+
+// WithPlayerID is a middleware that adds a player ID to the request context
+// based on the X-Forwarded-For header. If there is no such header, or the
+// originator addr otherwise cannot be parsed from it, the request fails with
+// a 500 error.
+func WithPlayerID(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		ff := r.Header.Get("X-Forwarded-For")
+		addr, err := originator(ff)
+		if err != nil {
+			http.Error(w, "missing or invalid X-Forwarded-For header; check server configuration", http.StatusInternalServerError)
+			return
+		}
+		id := player.ID(addr.As16())
+		ctx := ctxWith(r.Context(), id)
+		next.ServeHTTP(w, r.WithContext(ctx))
+	})
+}
+
+// Player returns the player ID set by WithPlayerID in the request context.
+func PlayerID(ctx context.Context) player.ID {
+	return ctxValue[player.ID](ctx)
+}
+
+// originator parses the IP of the client that originated a request from the
+// content of its X-Forwarded-For header.
+func originator(ff string) (netip.Addr, error) {
+	ff, _, _ = strings.Cut(ff, ",")
+	return netip.ParseAddr(ff)
+}
+
+type ctxKey[T any] struct{}
+
+func ctxValue[T any](ctx context.Context) T {
+	r, _ := ctx.Value(ctxKey[T]{}).(T)
+	return r
+}
+
+func ctxWith[T any](ctx context.Context, v T) context.Context {
+	return context.WithValue(ctx, ctxKey[T]{}, v)
+}

serve/proxy_test.go

@@ -0,0 +1,12 @@
+package serve
+
+import "testing"
+
+func TestOriginator(t *testing.T) {
+	// We could do plenty of tests here, but the one I really care about is
+	// that we get an error on an empty string.
+	_, err := originator("")
+	if err == nil {
+		t.Error("originator should have returned an error on an empty string")
+	}
+}