podkit: playlist URL to private podcast feed

One Go binary with fetch / tag / feed / serve / all. The stages run separately
on purpose: the download is the slow part and should not be repeated while
adjusting titles or the base URL.

Built from a shell + Python + Go sketch. The pieces that needed fixing on the
way, all of which produce a feed that loads wrong or not at all:

- Enclosure URLs were unescaped. Track titles routinely contain spaces and
  ampersands, which make both the URL and the XML invalid — the likeliest way
  for a feed to fail to load entirely. Only the final path segment is escaped;
  escaping the base would mangle scheme and host.
- Ordering came from filename sort, which is correct only while zero-padding is
  applied consistently. One unpadded file puts lecture 10 after lecture 1.
  Order now comes from the parsed track number.
- No pubDate and no itunes:type. Podcast clients default to newest-first, which
  starts a lecture course at the final lecture. Items now carry distinct
  ascending dates and the feed is marked serial.
- Missing channel description and link, which several clients reject outright.
- A struct field with no xml tag emitted a stray <EnclosureURL> element.
- guid had no isPermaLink="false", so a reader may treat it as a fetchable URL.
- Errors from ReadDir, Info and Marshal were discarded, so a missing directory
  produced an empty feed that looked like a working setup with nothing in it.
- os.Args indexed without a length check.

Beyond the sketch: a download archive so an interrupted series resumes, an
up-front check for yt-dlp and ffmpeg before the long download rather than a
post-processing failure after the bytes are on disk, non-audio files excluded
by extension so cover art and .part files cannot become episodes, and a serve
command with token gating that 404s rather than 401s.

Found by the first live run against a LibriVox collection: sources whose own
titles begin with a track number came out as "01 - 01 - April.mp3", because the
output template prepends the playlist index regardless. Collapsed, but only
when the two numbers agree — "01 - 1964 - Lecture" is a year and "05 - 12 Angry
Men" is a title, and neither is a repeat.

47 tests. Verified end to end against public-domain audio: downloaded,
converted, tagged, served, and the delivered file is a playable 128kbps MP3
with correct ID3 title/artist/album/track frames.

1 files changed, 145 insertions, 0 deletions

diff --git a/internal/serve/serve_test.go b/internal/serve/serve_test.go
new file mode 100644
index 0000000..04b25ee
--- /dev/null
+++ b/internal/serve/serve_test.go
@@ -0,0 +1,145 @@
+package serve
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func setup(t *testing.T, token string) (http.Handler, string) {
+	t.Helper()
+	dir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(dir, "feed.xml"), []byte("<rss/>"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(dir, "01 - One.mp3"), []byte("audio"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	return Handler(Options{Dir: dir, Token: token}), dir
+}
+
+func get(h http.Handler, target string, bearer string) *httptest.ResponseRecorder {
+	req := httptest.NewRequest(http.MethodGet, target, nil)
+	if bearer != "" {
+		req.Header.Set("Authorization", "Bearer "+bearer)
+	}
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+	return rec
+}
+
+func TestServesFeedAndAudioWithoutToken(t *testing.T) {
+	h, _ := setup(t, "")
+	if rec := get(h, "/feed.xml", ""); rec.Code != http.StatusOK {
+		t.Errorf("/feed.xml = %d, want 200", rec.Code)
+	}
+	if rec := get(h, "/audio/01%20-%20One.mp3", ""); rec.Code != http.StatusOK {
+		t.Errorf("audio = %d, want 200", rec.Code)
+	}
+}
+
+// The escaped URL the feed generator emits must actually resolve against the
+// server that serves those files — the two halves have to agree.
+func TestEscapedFilenameResolves(t *testing.T) {
+	h, dir := setup(t, "")
+	if err := os.WriteFile(filepath.Join(dir, "10 - Probability & Uncertainty.mp3"), []byte("x"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	rec := get(h, "/audio/10%20-%20Probability%20&%20Uncertainty.mp3", "")
+	if rec.Code != http.StatusOK {
+		t.Errorf("escaped filename = %d, want 200", rec.Code)
+	}
+}
+
+func TestTokenRequiredWhenSet(t *testing.T) {
+	h, _ := setup(t, "s3cret")
+	for _, target := range []string{"/feed.xml", "/audio/01%20-%20One.mp3", "/"} {
+		if rec := get(h, target, ""); rec.Code != http.StatusNotFound {
+			t.Errorf("%s without a token = %d, want 404", target, rec.Code)
+		}
+	}
+}
+
+func TestTokenAcceptedInQuery(t *testing.T) {
+	h, _ := setup(t, "s3cret")
+	if rec := get(h, "/feed.xml?k=s3cret", ""); rec.Code != http.StatusOK {
+		t.Errorf("query token = %d, want 200", rec.Code)
+	}
+}
+
+// Podcast clients cannot do interactive auth, so the query form is the one
+// that matters; the bearer header is for curl and for testing.
+func TestTokenAcceptedAsBearer(t *testing.T) {
+	h, _ := setup(t, "s3cret")
+	if rec := get(h, "/feed.xml", "s3cret"); rec.Code != http.StatusOK {
+		t.Errorf("bearer token = %d, want 200", rec.Code)
+	}
+}
+
+func TestWrongTokenRejected(t *testing.T) {
+	h, _ := setup(t, "s3cret")
+	if rec := get(h, "/feed.xml?k=wrong", ""); rec.Code != http.StatusNotFound {
+		t.Errorf("wrong token = %d, want 404", rec.Code)
+	}
+	if rec := get(h, "/feed.xml", "wrong"); rec.Code != http.StatusNotFound {
+		t.Errorf("wrong bearer = %d, want 404", rec.Code)
+	}
+}
+
+// A gated server must not confirm that anything is there.
+func TestRejectionLooksLikeAbsence(t *testing.T) {
+	h, _ := setup(t, "s3cret")
+	rec := get(h, "/feed.xml", "")
+	if rec.Code == http.StatusUnauthorized || rec.Code == http.StatusForbidden {
+		t.Errorf("status %d advertises that something exists here", rec.Code)
+	}
+	if strings.Contains(strings.ToLower(rec.Body.String()), "token") {
+		t.Error("the rejection body mentions a token, which tells a prober what to look for")
+	}
+}
+
+// http.FileServer resolves .. before dispatch, but this is the kind of thing
+// worth a standing assertion rather than a memory of having checked once.
+func TestNoPathTraversal(t *testing.T) {
+	h, dir := setup(t, "")
+	secret := filepath.Join(filepath.Dir(dir), "secret.txt")
+	if err := os.WriteFile(secret, []byte("do not serve"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	defer func() { _ = os.Remove(secret) }()
+
+	for _, target := range []string{
+		"/audio/../secret.txt",
+		"/audio/..%2fsecret.txt",
+		"/audio/%2e%2e/secret.txt",
+	} {
+		rec := get(h, target, "")
+		if strings.Contains(rec.Body.String(), "do not serve") {
+			t.Errorf("%s escaped the served directory", target)
+		}
+	}
+}
+
+func TestNewTokenIsRandomAndLongEnough(t *testing.T) {
+	a, err := NewToken()
+	if err != nil {
+		t.Fatalf("NewToken: %v", err)
+	}
+	b, _ := NewToken()
+	if a == b {
+		t.Error("two generated tokens were identical")
+	}
+	if len(a) < 32 {
+		t.Errorf("token is only %d chars; it is the only thing protecting the feed", len(a))
+	}
+}
+
+func TestUnknownPathIs404(t *testing.T) {
+	h, _ := setup(t, "")
+	if rec := get(h, "/nope", ""); rec.Code != http.StatusNotFound {
+		t.Errorf("unknown path = %d, want 404", rec.Code)
+	}
+}