feat(music): write an m3u path absolute when it leaves the playlist's directory

A track under the playlist's own directory still writes relative. Anything
outside it now writes the absolute path instead of a ../ chain. Playlists in
the mpd directory point into ~/music, so the relative form there was four
levels of .. and broke the moment anything moved.

I took the trade knowingly. ambience.m3u is the only playlist anywhere using
../ lines, and it reaches its audio through a sibling directory. So a newly
appended track there writes absolute while its hand-written lines stay
relative, and its header comment needs a matching edit. The alternative was
an exception for tracks inside the deployed tree, which would be a rule
serving exactly one file.

The comment I wrote for the earlier half of this argued one direction only.
Absolute survives the playlist moving, which I said. Relative survives the
library moving, which I didn't, and for a track in the same tree the ../ form
survives strictly more moves rather than fewer. Rewritten to name the trade
instead of implying there isn't one.

The boundary case is a directory whose name begins with two dots. The check
looks for a leading "../", so a real subdirectory called ..hidden sits under
the playlist and must stay relative. It has a test now, because loosening
that check to ".." would break that case and nothing else.

2 files changed, 80 insertions, 26 deletions

diff --git a/modules/music-config.el b/modules/music-config.el
index 075ed49b..0834aae8 100644
--- a/modules/music-config.el
+++ b/modules/music-config.el
@@ -677,16 +677,25 @@ M3U-FILE should be an existing, writable M3U file path."
   (unless (file-writable-p m3u-file)
     (error "M3U file is not writable: %s" m3u-file))
 
-  ;; Write the path relative to the playlist's own directory, which is the base
-  ;; `cj/music--m3u-file-tracks' and EMMS both resolve against.  This used to be
-  ;; `cj/music-root'.  For a playlist inside the music root the two are the same
-  ;; directory, so the disagreement stayed invisible until a playlist lived
-  ;; somewhere else -- then the appended line resolved against the wrong base and
-  ;; pointed at a file that was never there.
-  (let ((relative-path (if (file-name-absolute-p track-path)
-                           (file-relative-name track-path
-                                               (file-name-directory m3u-file))
-                         track-path)))
+  ;; Relative when the track sits under the playlist's own directory, absolute
+  ;; otherwise.
+  ;;
+  ;; The base is the playlist rather than `cj/music-root' because that is what
+  ;; both readers resolve against -- `cj/music--m3u-file-tracks' and EMMS's
+  ;; `emms-source-playlist-parse-m3u'.  Inside the music root the two are the
+  ;; same directory, which is why basing on the root went unnoticed: it only
+  ;; wrote an unresolvable line once a playlist lived somewhere else.
+  ;;
+  ;; Falling back to absolute keeps a cross-tree reference readable, and it
+  ;; survives the playlist being moved again.  A playlist in the mpd directory
+  ;; pointing into ~/music would otherwise carry a four-level ../ chain that
+  ;; breaks the moment anything moves.
+  (let* ((dir (file-name-directory m3u-file))
+         (relative-path
+          (if (not (file-name-absolute-p track-path))
+              track-path
+            (let ((rel (file-relative-name track-path dir)))
+              (if (string-prefix-p "../" rel) track-path rel)))))
     ;; Determine if we need a leading newline
     (let ((needs-prefix-newline nil)
           (file-size (file-attribute-size (file-attributes m3u-file))))
diff --git a/tests/test-music-config--append-track-to-m3u-file.el b/tests/test-music-config--append-track-to-m3u-file.el
index 06327b9d..9e5d0578 100644
--- a/tests/test-music-config--append-track-to-m3u-file.el
+++ b/tests/test-music-config--append-track-to-m3u-file.el
@@ -149,15 +149,35 @@ why the defect stayed invisible until a playlist moved out of it."
                        (list track-path))))
     (test-music-config--append-track-to-m3u-file-teardown)))
 
-(ert-deftest test-music-config--append-track-to-m3u-file-normal-base-is-the-playlist-not-the-music-root ()
-  "Normal: the written line is relative to the playlist's own directory.
-Stated directly as well as via the round-trip, so a reader change could not
-quietly make both sides agree on the wrong base."
+(ert-deftest test-music-config--append-track-to-m3u-file-normal-under-playlist-dir-is-relative ()
+  "Normal: a track under the playlist's directory is written relative to it.
+The music root sits at a different depth on purpose.  Put it alongside the
+playlist directory instead and both candidate bases produce the same string,
+so the assertion would hold against a writer using either one."
+  (test-music-config--append-track-to-m3u-file-setup)
+  (unwind-protect
+      (let* ((base (cj/create-test-base-dir))
+             (playlists (expand-file-name "mpd/playlists/" base))
+             (cj/music-root (expand-file-name "music/" base))
+             (m3u-file (expand-file-name "album.m3u" playlists))
+             (track-path (expand-file-name "sub/song.mp3" playlists)))
+        (make-directory (expand-file-name "sub/" playlists) t)
+        (make-directory cj/music-root t)
+        (with-temp-buffer (write-file m3u-file))
+        (cj/music--append-track-to-m3u-file track-path m3u-file)
+        (with-temp-buffer
+          (insert-file-contents m3u-file)
+          (should (string= (buffer-string) "sub/song.mp3\n")))
+        (should (equal (cj/music--m3u-file-tracks m3u-file) (list track-path))))
+    (test-music-config--append-track-to-m3u-file-teardown)))
+
+(ert-deftest test-music-config--append-track-to-m3u-file-normal-sibling-dir-is-absolute ()
+  "Normal: a track outside the playlist's directory is written absolute.
+A sibling would otherwise come out as \"../audio/x.mp3\".  Absolute is the
+convention for cross-tree references here, and it survives the playlist being
+moved again later, which a ../ chain does not."
   (test-music-config--append-track-to-m3u-file-setup)
   (unwind-protect
-      ;; Same depth asymmetry as the round-trip test above, and for the same
-      ;; reason: with the music root alongside playlists/ both bases agree and
-      ;; the assertion holds against the broken writer.
       (let* ((base (cj/create-test-base-dir))
              (playlists (expand-file-name "mpd/playlists/" base))
              (audio (expand-file-name "mpd/audio/" base))
@@ -171,15 +191,15 @@ quietly make both sides agree on the wrong base."
         (cj/music--append-track-to-m3u-file track-path m3u-file)
         (with-temp-buffer
           (insert-file-contents m3u-file)
-          (should (string= (buffer-string) "../audio/rain-loop.mp3\n"))))
+          (should (string= (buffer-string) (concat track-path "\n"))))
+        (should (equal (cj/music--m3u-file-tracks m3u-file) (list track-path))))
     (test-music-config--append-track-to-m3u-file-teardown)))
 
-(ert-deftest test-music-config--append-track-to-m3u-file-normal-deep-parent-chain-round-trips ()
-  "Normal: a track several levels above the playlist round-trips.
-The single \"../\" the tests above exercise is the shallow case.  Once
-playlists and audio can live anywhere, a multi-level chain is the shape most
-likely to be broken by a later \"let's normalize these paths\" edit, and
-nothing else here would catch it."
+(ert-deftest test-music-config--append-track-to-m3u-file-normal-deep-parent-chain-goes-absolute ()
+  "Normal: a track several levels away is written absolute, not as a ../ chain.
+This is the case the absolute fallback exists for.  A four-level chain is
+unreadable and breaks the moment the playlist moves, so distance from the
+playlist is exactly when an absolute path earns its keep."
   (test-music-config--append-track-to-m3u-file-setup)
   (unwind-protect
       (let* ((base (cj/create-test-base-dir))
@@ -194,14 +214,39 @@ nothing else here would catch it."
         (cj/music--append-track-to-m3u-file track-path m3u-file)
         (with-temp-buffer
           (insert-file-contents m3u-file)
-          ;; playlists/ -> c -> b -> a -> base is four hops up.
-          (should (string= (buffer-string) "../../../../faraway/song.mp3\n")))
+          ;; Four hops up (playlists -> c -> b -> a -> base) would be the
+          ;; relative form; the writer declines it and emits the absolute path.
+          (should (string= (buffer-string) (concat track-path "\n"))))
         (should (equal (cj/music--m3u-file-tracks m3u-file)
                        (list track-path))))
     (test-music-config--append-track-to-m3u-file-teardown)))
 
 ;;; Boundary Cases
 
+(ert-deftest test-music-config--append-track-to-m3u-file-boundary-dotdot-named-dir-stays-relative ()
+  "Boundary: a directory whose name merely begins with two dots stays relative.
+This is the input the relative-vs-absolute test actually turns on.  The check
+looks for a leading \"../\", so a real subdirectory named \"..hidden\" is under
+the playlist and must not be mistaken for an escape.  Loosening the check to
+\"..\" would break exactly this case and nothing else in the suite would catch
+it."
+  (test-music-config--append-track-to-m3u-file-setup)
+  (unwind-protect
+      (let* ((base (cj/create-test-base-dir))
+             (playlists (expand-file-name "mpd/playlists/" base))
+             (cj/music-root (expand-file-name "music/" base))
+             (m3u-file (expand-file-name "p.m3u" playlists))
+             (track-path (expand-file-name "..hidden/song.mp3" playlists)))
+        (make-directory (expand-file-name "..hidden/" playlists) t)
+        (make-directory cj/music-root t)
+        (with-temp-buffer (write-file m3u-file))
+        (cj/music--append-track-to-m3u-file track-path m3u-file)
+        (with-temp-buffer
+          (insert-file-contents m3u-file)
+          (should (string= (buffer-string) "..hidden/song.mp3\n")))
+        (should (equal (cj/music--m3u-file-tracks m3u-file) (list track-path))))
+    (test-music-config--append-track-to-m3u-file-teardown)))
+
 (ert-deftest test-music-config--append-track-to-m3u-file-boundary-very-long-path-appends-successfully ()
   "Append very long track path without truncation."
   (test-music-config--append-track-to-m3u-file-setup)