refactor(external-open): consolidate OS-open dispatch in external-open.el

Phase 4 of utility-consolidation. Three previously-overlapping helpers (system-utils' `cj/identify-external-open-command' and `cj/--open-with-is-launcher-p', plus the dirvish-only `cj/--file-manager-program-for' shipped earlier today) all answered "which OS-open program should I run?".  Pull the answer into one place: external-open.el.

Move and rename:

- `cj/--open-with-is-launcher-p' (system-utils) -> `cj/external-open-launcher-p' (external-open). Public name now matches its module.
- `cj/identify-external-open-command' (system-utils) -> `cj/external-open-command' (external-open). Returns nil for unsupported hosts instead of signaling -- callers that need a command must handle nil explicitly. The wrapper `cj/xdg-open' (also moved into external-open) converts nil to a `user-error' with a clear message, preserving the user-facing failure shape.
- Delete dirvish's `cj/--file-manager-program-for' helper. `cj/dirvish-open-file-manager-here' now calls `cj/external-open-command' directly. The shell-command fallback for nil-program preserves the previous escape hatch.

Break the system-utils <-> external-open recursive require by moving `cj/xdg-open' (the only system-utils function that external-open used) into external-open along with the dispatch.

Tests reorganized to match the move. Two new test files (`test-external-open-command.el', `test-external-open-launcher-p.el') replace the two system-utils-named test files. The dirvish file-manager-program test goes away with the helper. 11 tests covering Normal/Boundary/Error for the dispatch (plus the new "unsupported host returns nil" contract).

Add `(require \='external-open)' to system-utils.el and `(require \='system-lib)' to external-open.el (for `cj/file-from-context' which xdg-open uses).

3 files changed, 50 insertions, 57 deletions

diff --git a/modules/dirvish-config.el b/modules/dirvish-config.el
index 89f6c397..d5834dac 100644
--- a/modules/dirvish-config.el
+++ b/modules/dirvish-config.el
@@ -246,21 +246,6 @@ Examples:
 
 ;;; ----------------------- Dirvish Open File Manager Here ----------------------
 
-(defun cj/--file-manager-program-for (has-xdg-open-p system-type)
-  "Return the file-manager command for HAS-XDG-OPEN-P + SYSTEM-TYPE, or nil.
-
-Pure helper used by `cj/dirvish-open-file-manager-here'.  When
-HAS-XDG-OPEN-P is non-nil, returns \"xdg-open\" regardless of
-SYSTEM-TYPE -- xdg-open works on Linux and many ported environments.
-Without xdg-open, falls back to `darwin' -> \"open\", `windows-nt' ->
-\"explorer\", everything else -> nil so the caller can shell-command
-its way out."
-  (cond
-   (has-xdg-open-p "xdg-open")
-   ((eq system-type 'darwin) "open")
-   ((eq system-type 'windows-nt) "explorer")
-   (t nil)))
-
 (defun cj/dirvish-open-file-manager-here ()
   "Open system's default file manager in the current dired/dirvish directory.
 Always opens the file manager in the directory currently being displayed,
@@ -271,9 +256,8 @@ regardless of what file or subdirectory the point is on."
         (progn
           (message "Opening file manager in %s..." current-dir)
           ;; Use pipe instead of pty for the async call-process below.
-          (let* ((process-connection-type nil)
-                 (program (cj/--file-manager-program-for
-                           (executable-find "xdg-open") system-type)))
+          (let ((process-connection-type nil)
+                (program (cj/external-open-command)))
             (if program
                 (call-process program nil 0 nil current-dir)
               (shell-command (format "xdg-open %s &"
diff --git a/modules/external-open.el b/modules/external-open.el
index 9eddf352..c9b5f1f6 100644
--- a/modules/external-open.el
+++ b/modules/external-open.el
@@ -21,8 +21,8 @@
 ;;
 ;;; Code:
 
-(require 'system-utils) ;; for xdg-open and others
 (require 'host-environment) ;; environment information functions
+(require 'system-lib) ;; for cj/file-from-context
 (require 'cl-lib)
 
 ;; Declare platform-specific functions
@@ -89,6 +89,51 @@
   :type '(repeat (regexp :tag "File extension regexp"))
   :group 'external-open)
 
+;; ----------------------- External-Open Command Resolution -------------------
+
+(defun cj/external-open-command ()
+  "Return the OS-default \"open\" command for this host, or nil if unsupported.
+Returns one of \"xdg-open\" (Linux), \"open\" (macOS), \"start\" (Windows).
+Callers that require a command should error on nil with a contextual
+message so the user sees what feature is unavailable."
+  (cond
+   ((env-linux-p)   "xdg-open")
+   ((env-macos-p)   "open")
+   ((env-windows-p) "start")
+   (t nil)))
+
+(defun cj/external-open-launcher-p (command)
+  "Return non-nil when COMMAND is a desktop launcher.
+Launchers (xdg-open, open, start) need to be called with `call-process'
+and a zero BUFFER argument so they fully detach from Emacs.  Other
+commands get `start-process-shell-command' so their output is visible."
+  (and (stringp command)
+       (member command '("xdg-open" "open" "start"))
+       t))
+
+(defun cj/xdg-open (&optional filename)
+  "Open FILENAME (or the file at point) with the OS default handler.
+Logs output and exit code to buffer *external-open.log*."
+  (interactive)
+  (let* ((file  (expand-file-name
+                 (or (cj/file-from-context filename)
+                     (user-error "No file associated with this buffer"))))
+         (cmd   (or (cj/external-open-command)
+                    (user-error "External-open: unsupported host environment")))
+         (logbuf (get-buffer-create "*external-open.log*")))
+    (with-current-buffer logbuf
+      (goto-char (point-max))
+      (insert (format-time-string "[%Y-%m-%d %H:%M:%S] "))
+      (insert (format "Opening: %s\n" file)))
+    (cond
+     ((env-windows-p)
+      (w32-shell-execute "open" file))
+     (t
+      (call-process cmd nil 0 nil file)
+      (with-current-buffer logbuf
+        (insert "  → Launched asynchronously\n"))))
+    nil))
+
 ;; ------------------------------- Open File With ------------------------------
 
 (defun cj/open-this-file-with (command)
diff --git a/modules/system-utils.el b/modules/system-utils.el
index e266cd15..008a5396 100644
--- a/modules/system-utils.el
+++ b/modules/system-utils.el
@@ -24,6 +24,7 @@
 ;;; Code:
 
 (require 'system-lib)
+(require 'external-open)
 
 (declare-function dired-get-file-for-visit "dired" ())
 (declare-function dired-file-name-at-point "dired" ())
@@ -57,13 +58,6 @@
 ;;; ------------------------------- Open File With ------------------------------
 ;; TASK: Favor this method over cj/open-this-file-with and add to custom buffer funcs
 
-(defun cj/--open-with-is-launcher-p (command)
-  "Return non-nil if COMMAND is a desktop launcher.
-Launchers (xdg-open, open, start) need to be called with `call-process'
-and a zero BUFFER argument so they fully detach from Emacs.  Other
-commands get `start-process-shell-command' so their output is visible."
-  (and (member command '("xdg-open" "open" "start")) t))
-
 (defun cj/open-file-with-command (command)
   "Open the current file with COMMAND.
 Works in both Dired buffers and regular file buffers.  Prompts for a
@@ -74,7 +68,7 @@ detached from Emacs."
 				   (read-file-name "File to open: "))))
 	(unless (and file (file-exists-p file))
 	  (error "No valid file found or selected"))
-	(if (cj/--open-with-is-launcher-p command)
+	(if (cj/external-open-launcher-p command)
 		(progn
 		  (call-process command nil 0 nil file)
 		  (message "Opening %s with %s..."
@@ -90,36 +84,6 @@ detached from Emacs."
 		(message "Running %s on %s..."
 				 (file-name-nondirectory file) command)))))
 
-(defun cj/identify-external-open-command ()
-  "Return the OS-default \"open\" command for this host.
-Signals an error if the host is unsupported."
-  (cond
-   ((env-linux-p)   "xdg-open")
-   ((env-macos-p)   "open")
-   ((env-windows-p) "start")
-   (t (error "External-open: unsupported host environment"))))
-
-(defun cj/xdg-open (&optional filename)
-  "Open FILENAME (or the file at point) with the OS default handler.
-Logs output and exit code to buffer *external-open.log*."
-  (interactive)
-  (let* ((file  (expand-file-name
-				 (or (cj/file-from-context filename)
-					 (user-error "No file associated with this buffer"))))
-		 (cmd   (cj/identify-external-open-command))
-		 (logbuf (get-buffer-create "*external-open.log*")))
-	(with-current-buffer logbuf
-	  (goto-char (point-max))
-	  (insert (format-time-string "[%Y-%m-%d %H:%M:%S] "))
-	  (insert (format "Opening: %s\n" file)))
-	(cond
-	 ((env-windows-p)
-	  (w32-shell-execute "open" file))
-	 (t
-	  (call-process cmd nil 0 nil file)
-	  (with-current-buffer logbuf
-		(insert "  → Launched asynchronously\n"))))
-	nil))
 
 ;;; ------------------------------ Server Shutdown ------------------------------