fix: make the in-process execution mode explicit in specs and contract checks

Two coupled holes surfaced in the Phase 0-3 review. duet--transfer-spec copied only :argv, :default-directory, and :process-environment out of a backend's command result, dropping the :tramp marker, so a TRAMP-routed spec arrived as :argv nil with nothing telling the executor to copy in process. And duet--check-command leaned on listp, where nil is a list in Elisp, so a command builder returning nil or a bare :argv nil passed the minimum tier.

Both turn on the same idea, so they share a fix. duet--command-spec-executable-p defines a runnable spec: a non-empty plist with either a non-empty :argv of strings (a CLI backend) or a declared in-process mode such as :tramp. The contract checker rejects anything else, and transfer-spec now carries :tramp through, so the TRAMP fallback has a positive execution signal rather than an ambiguous nil argv.

The legitimate TRAMP backend keeps passing because it declares its mode. A broken backend that forgets argv no longer slips through.

1 files changed, 19 insertions, 3 deletions

diff --git a/duet.el b/duet.el
index e8ae8c4..28d45f2 100644
--- a/duet.el
+++ b/duet.el
@@ -268,15 +268,30 @@ normalizer."
       (unless (or (null score) (numberp score))
         "handles must return a number or nil"))))
 
+(defun duet--command-spec-executable-p (spec)
+  "Return non-nil when process SPEC has a recognized, runnable execution shape.
+A SPEC is runnable when it is a non-empty plist carrying either a non-empty
+:argv whose elements are all strings (a CLI backend) or an explicit
+in-process mode such as :tramp (a backend that copies in process).  A nil
+spec, a bare nil argv, or a shell string is not runnable."
+  (and (listp spec) spec
+       (not (plist-get spec :shell-command))
+       (or (plist-get spec :tramp)
+           (let ((argv (plist-get spec :argv)))
+             (and (consp argv) (cl-every #'stringp argv))))))
+
 (defun duet--check-command (backend src dst opts)
   "Return a minimum-tier command violation for BACKEND on SRC/DST/OPTS, or nil."
   (if (not (functionp (duet-backend-command backend)))
       "command must be a function"
     (let ((spec (funcall (duet-backend-command backend) src dst opts)))
       (cond
-       ((not (listp spec)) "command must return a process-spec plist")
-       ((plist-get spec :shell-command) "command must not build a shell string; use :argv")
-       ((not (listp (plist-get spec :argv))) "command :argv must be an argument list")))))
+       ((not (and (listp spec) spec))
+        "command must return a non-empty process-spec plist")
+       ((plist-get spec :shell-command)
+        "command must not build a shell string; use :argv")
+       ((not (duet--command-spec-executable-p spec))
+        "command must return a runnable spec: a non-empty :argv of strings, or a declared in-process mode such as :tramp")))))
 
 (defun duet--check-redaction (backend)
   "Return a minimum-tier redaction violation for BACKEND, or nil.
@@ -494,6 +509,7 @@ backend's command builder.  Return nil when no backend handles the pair."
               :backend (duet-backend-name backend)
               :route (duet--transfer-route src dst opts)
               :argv (plist-get cmd :argv)
+              :tramp (plist-get cmd :tramp)
               :default-directory (plist-get cmd :default-directory)
               :process-environment (plist-get cmd :process-environment)
               :async (if (plist-member opts :async) (plist-get opts :async) t))))))