test: make subr mocks variadic for native-comp, add arity meta-test

Re-enabling native-comp surfaced a suite-wide fragility. When a test redefines a C primitive (or a native-compiled function), native-comp routes native callers through a trampoline that calls the mock with the primitive's maximum arity. A fixed-arity mock narrower than the primitive then throws wrong-number-of-arguments, intermittently, as the eln-cache fills.

I swept every arity-narrow subr mock to append &rest _ (188 sites, preserving any named args the body uses), and added tests/test-meta-subr-mock-arity.el, which fails make test on any subr mock too narrow for the primitive's arity. The rule isn't "never mock a subr". The suite mocks message and completing-read freely. It's "a subr mock must accept the primitive's arity."

Background, the three failure modes, and the research are in docs/native-comp-subr-mocking.org.

1 files changed, 8 insertions, 8 deletions

diff --git a/tests/test-media-utils.el b/tests/test-media-utils.el
index 9384d568f..841b6faf9 100644
--- a/tests/test-media-utils.el
+++ b/tests/test-media-utils.el
@@ -24,7 +24,7 @@
 (ert-deftest test-media-get-available-players-filters-by-executable ()
   "Normal: only players whose :command is on PATH are reported."
   (cl-letf (((symbol-function 'executable-find)
-             (lambda (cmd) (and (member cmd '("mpv" "vlc")) cmd))))
+             (lambda (cmd &rest _) (and (member cmd '("mpv" "vlc")) cmd))))
     (let ((result (cj/get-available-media-players)))
       (should (memq 'mpv result))
       (should (memq 'vlc result))
@@ -32,7 +32,7 @@
 
 (ert-deftest test-media-get-available-players-none-installed ()
   "Boundary: with nothing on PATH, the list is empty."
-  (cl-letf (((symbol-function 'executable-find) (lambda (_) nil)))
+  (cl-letf (((symbol-function 'executable-find) (lambda (_ &rest _) nil)))
     (should-not (cj/get-available-media-players))))
 
 ;; ----------------------------- cj/media-play-it ------------------------------
@@ -41,7 +41,7 @@
   "Normal: a player that needs no stream URL gets a plain command, no yt-dlp."
   (let (captured cj/default-media-player)
     (setq cj/default-media-player 'mpv)
-    (cl-letf (((symbol-function 'executable-find) (lambda (_) "/usr/bin/mpv"))
+    (cl-letf (((symbol-function 'executable-find) (lambda (_ &rest _) "/usr/bin/mpv"))
               ((symbol-function 'start-process-shell-command)
                (lambda (_n _b cmd) (setq captured cmd) 'proc))
               ((symbol-function 'set-process-sentinel) #'ignore)
@@ -56,7 +56,7 @@
   "Normal: a player needing a stream URL wraps the URL in a yt-dlp -g call."
   (let (captured cj/default-media-player)
     (setq cj/default-media-player 'vlc)
-    (cl-letf (((symbol-function 'executable-find) (lambda (_) "/usr/bin/vlc"))
+    (cl-letf (((symbol-function 'executable-find) (lambda (_ &rest _) "/usr/bin/vlc"))
               ((symbol-function 'start-process-shell-command)
                (lambda (_n _b cmd) (setq captured cmd) 'proc))
               ((symbol-function 'set-process-sentinel) #'ignore)
@@ -71,7 +71,7 @@
   "Error: an unavailable player command signals an error before launching."
   (let (cj/default-media-player)
     (setq cj/default-media-player 'mpv)
-    (cl-letf (((symbol-function 'executable-find) (lambda (_) nil)))
+    (cl-letf (((symbol-function 'executable-find) (lambda (_ &rest _) nil)))
       (should-error (cj/media-play-it "https://example.com/v")))))
 
 ;; ------------------------------- cj/yt-dl-it ---------------------------------
@@ -79,19 +79,19 @@
 (ert-deftest test-media-yt-dl-it-errors-without-yt-dlp ()
   "Error: a missing yt-dlp aborts the download."
   (cl-letf (((symbol-function 'executable-find)
-             (lambda (cmd) (unless (equal cmd "yt-dlp") "/usr/bin/x"))))
+             (lambda (cmd &rest _) (unless (equal cmd "yt-dlp") "/usr/bin/x"))))
     (should-error (cj/yt-dl-it "https://example.com/v"))))
 
 (ert-deftest test-media-yt-dl-it-errors-without-tsp ()
   "Error: yt-dlp present but tsp missing aborts the download."
   (cl-letf (((symbol-function 'executable-find)
-             (lambda (cmd) (unless (equal cmd "tsp") "/usr/bin/x"))))
+             (lambda (cmd &rest _) (unless (equal cmd "tsp") "/usr/bin/x"))))
     (should-error (cj/yt-dl-it "https://example.com/v"))))
 
 (ert-deftest test-media-yt-dl-it-builds-tsp-yt-dlp-process ()
   "Normal: with both tools present, the URL is queued via tsp + yt-dlp."
   (let (captured (videos-dir "/tmp/videos"))
-    (cl-letf (((symbol-function 'executable-find) (lambda (_) "/usr/bin/x"))
+    (cl-letf (((symbol-function 'executable-find) (lambda (_ &rest _) "/usr/bin/x"))
               ((symbol-function 'start-process)
                (lambda (&rest args) (setq captured args) 'proc))
               ((symbol-function 'set-process-sentinel) #'ignore)