1 files changed, 91 insertions, 0 deletions
diff --git a/internal/serve/serve.go b/internal/serve/serve.go
new file mode 100644
index 0000000..0b3b340
--- /dev/null
+++ b/internal/serve/serve.go
@@ -0,0 +1,91 @@
+// Package serve hosts the audio directory and its feed over HTTP.
+package serve
+
+import (
+	"crypto/rand"
+	"crypto/subtle"
+	"encoding/hex"
+	"fmt"
+	"net/http"
+	"path"
+	"strings"
+)
+
+// Options configure the server.
+type Options struct {
+	Dir  string
+	Addr string
+	// Token, if set, must appear in every request as ?k=<token> or an
+	// Authorization: Bearer header.
+	//
+	// "Private" here means unlisted-and-unguessable, not authenticated in any
+	// serious sense: podcast clients do not do interactive login, so a shared
+	// secret in the URL is the mechanism that actually works with them. Anyone
+	// holding the URL holds the access.
+	Token string
+}
+
+// NewToken returns a random token suitable for Options.Token.
+func NewToken() (string, error) {
+	b := make([]byte, 16)
+	if _, err := rand.Read(b); err != nil {
+		return "", fmt.Errorf("generating token: %w", err)
+	}
+	return hex.EncodeToString(b), nil
+}
+
+// Handler builds the HTTP handler: the feed at /feed.xml, audio under /audio/.
+func Handler(opts Options) http.Handler {
+	mux := http.NewServeMux()
+
+	files := http.FileServer(http.Dir(opts.Dir))
+	mux.Handle("/audio/", http.StripPrefix("/audio/", files))
+
+	mux.HandleFunc("/feed.xml", func(w http.ResponseWriter, r *http.Request) {
+		http.ServeFile(w, r, path.Join(opts.Dir, "feed.xml"))
+	})
+
+	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/" {
+			http.NotFound(w, r)
+			return
+		}
+		w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+		fmt.Fprintln(w, "podkit — subscribe your podcast app to /feed.xml")
+	})
+
+	return withToken(opts.Token, mux)
+}
+
+// withToken gates every request on a shared secret.
+func withToken(token string, next http.Handler) http.Handler {
+	if token == "" {
+		return next
+	}
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if !tokenOK(r, token) {
+			// 404 rather than 401: a 401 confirms something is here worth
+			// guessing at, and no podcast client is going to act on the
+			// challenge anyway.
+			http.NotFound(w, r)
+			return
+		}
+		next.ServeHTTP(w, r)
+	})
+}
+
+func tokenOK(r *http.Request, want string) bool {
+	if got := r.URL.Query().Get("k"); got != "" && constantTimeEqual(got, want) {
+		return true
+	}
+	auth := r.Header.Get("Authorization")
+	if strings.HasPrefix(auth, "Bearer ") {
+		return constantTimeEqual(strings.TrimPrefix(auth, "Bearer "), want)
+	}
+	return false
+}
+
+// constantTimeEqual avoids leaking the token through response timing.
+func constantTimeEqual(a, b string) bool {
+	return subtle.ConstantTimeCompare([]byte(a), []byte(b)) == 1
+}