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-system-commands-resolve-and-run.el b/tests/test-system-commands-resolve-and-run.el
index 2c9d98d0c..af2288fd9 100644
--- a/tests/test-system-commands-resolve-and-run.el
+++ b/tests/test-system-commands-resolve-and-run.el
@@ -118,19 +118,19 @@ does not run the command."
 
 (ert-deftest test-system-cmd-service-available-true-on-zero-exit ()
   "Normal: service is available when systemctl exists and `cat' exits 0."
-  (cl-letf (((symbol-function 'executable-find) (lambda (_p) "/usr/bin/systemctl"))
+  (cl-letf (((symbol-function 'executable-find) (lambda (_p &rest _) "/usr/bin/systemctl"))
             ((symbol-function 'call-process) (lambda (&rest _) 0)))
     (should (cj/system-cmd--emacs-service-available-p))))
 
 (ert-deftest test-system-cmd-service-available-false-on-nonzero-exit ()
   "Boundary: a nonzero exit (no such unit) means not available."
-  (cl-letf (((symbol-function 'executable-find) (lambda (_p) "/usr/bin/systemctl"))
+  (cl-letf (((symbol-function 'executable-find) (lambda (_p &rest _) "/usr/bin/systemctl"))
             ((symbol-function 'call-process) (lambda (&rest _) 1)))
     (should-not (cj/system-cmd--emacs-service-available-p))))
 
 (ert-deftest test-system-cmd-service-available-false-when-systemctl-absent ()
   "Error: with no systemctl on PATH the service can't be available."
-  (cl-letf (((symbol-function 'executable-find) (lambda (_p) nil))
+  (cl-letf (((symbol-function 'executable-find) (lambda (_p &rest _) nil))
             ((symbol-function 'call-process)
              (lambda (&rest _) (error "must not shell out without systemctl"))))
     (should-not (cj/system-cmd--emacs-service-available-p))))
@@ -220,7 +220,7 @@ kill-emacs directly (the service owns the daemon lifecycle)."
     (cl-letf (((symbol-function 'completing-read)
                (lambda (&rest _) "Lock Screen"))
               ((symbol-function 'call-interactively)
-               (lambda (cmd) (setq called cmd))))
+               (lambda (cmd &rest _) (setq called cmd))))
       (cj/system-command-menu))
     (should (eq called 'cj/system-cmd-lock))))