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, 4 insertions, 4 deletions

diff --git a/tests/test-video-audio-recording--test-device.el b/tests/test-video-audio-recording--test-device.el
index e701b69fd..aa85b4388 100644
--- a/tests/test-video-audio-recording--test-device.el
+++ b/tests/test-video-audio-recording--test-device.el
@@ -20,7 +20,7 @@
   "Runs exactly 2 shell commands: ffmpeg to record, ffplay to playback."
   (let ((commands nil))
     (cl-letf (((symbol-function 'shell-command)
-               (lambda (cmd) (push cmd commands) 0)))
+               (lambda (cmd &rest _) (push cmd commands) 0)))
       (cj/recording--test-device "test-device" "test-" "GO!")
       (should (= 2 (length commands)))
       ;; ffmpeg runs first (pushed last due to stack order)
@@ -31,7 +31,7 @@
   "The provided device name appears in the ffmpeg command."
   (let ((commands nil))
     (cl-letf (((symbol-function 'shell-command)
-               (lambda (cmd) (push cmd commands) 0)))
+               (lambda (cmd &rest _) (push cmd commands) 0)))
       (cj/recording--test-device "alsa_input.usb-Jabra.mono" "mic-" "SPEAK!")
       (let ((ffmpeg-cmd (cadr commands)))
         (should (string-match-p "alsa_input.usb-Jabra.mono" ffmpeg-cmd))
@@ -43,7 +43,7 @@
   "Device names with special characters are shell-quoted."
   (let ((commands nil))
     (cl-letf (((symbol-function 'shell-command)
-               (lambda (cmd) (push cmd commands) 0)))
+               (lambda (cmd &rest _) (push cmd commands) 0)))
       (cj/recording--test-device "device with spaces" "test-" "GO!")
       (let ((ffmpeg-cmd (cadr commands)))
         ;; shell-quote-argument should have escaped the spaces
@@ -54,7 +54,7 @@
 (ert-deftest test-video-audio-recording--test-device-error-ffmpeg-failure-no-crash ()
   "Function completes without error even when ffmpeg returns non-zero."
   (cl-letf (((symbol-function 'shell-command)
-             (lambda (_cmd) 1)))
+             (lambda (_cmd &rest _) 1)))
     ;; Should not signal any error
     (cj/recording--test-device "dev" "test-" "GO!")
     (should t)))