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+;;; test-system-lib-process-output-or-error.el --- Tests for cj/process-output-or-error and cj/git-output-or-error -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; `cj/process-output-or-error' is the generic argv-based runner: take
+;; a program name and its argument list, run it via `process-file', and
+;; return stdout on success or signal a clear `user-error' on non-zero
+;; exit (with program/status/output in the message).
+;;
+;; `cj/git-output-or-error' is the thin wrapper that supplies "git" as
+;; the program.  Both helpers stay deterministic in tests by stubbing
+;; `process-file'.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'system-lib)
+
+(defmacro test-system-lib-runner--with-stub (status output &rest body)
+  "Run BODY with `process-file' stubbed to return STATUS and write OUTPUT."
+  (declare (indent 2))
+  `(cl-letf (((symbol-function 'process-file)
+              (lambda (_program _infile destination _display &rest _args)
+                (when destination
+                  (let ((buf (cond
+                              ((eq destination t) (current-buffer))
+                              ((bufferp destination) destination)
+                              ((consp destination) (car destination)))))
+                    (when (bufferp buf)
+                      (with-current-buffer buf
+                        (insert ,output)))))
+                ,status)))
+     ,@body))
+
+(ert-deftest test-cj-process-output-or-error-success-returns-stdout ()
+  "Normal: zero exit returns stdout."
+  (test-system-lib-runner--with-stub 0 "hello world\n"
+    (should (equal (cj/process-output-or-error "echo" "hello" "world")
+                   "hello world\n"))))
+
+(ert-deftest test-cj-process-output-or-error-no-args-runs ()
+  "Boundary: no args is a valid invocation."
+  (test-system-lib-runner--with-stub 0 "ok\n"
+    (should (equal (cj/process-output-or-error "true") "ok\n"))))
+
+(ert-deftest test-cj-process-output-or-error-non-zero-signals-user-error ()
+  "Error: non-zero exit signals user-error."
+  (test-system-lib-runner--with-stub 1 "boom\n"
+    (should-error (cj/process-output-or-error "false")
+                  :type 'user-error)))
+
+(ert-deftest test-cj-process-output-or-error-message-names-program-and-status ()
+  "Error: the user-error message names the program, the exit status,
+and the (trimmed) output so the user can see what went wrong."
+  (test-system-lib-runner--with-stub 128 "fatal: not a git repo\n"
+    (condition-case err
+        (cj/process-output-or-error "git" "status")
+      (user-error
+       (let ((message (error-message-string err)))
+         (should (string-match-p "git" message))
+         (should (string-match-p "128" message))
+         (should (string-match-p "fatal: not a git repo" message)))))))
+
+(ert-deftest test-cj-git-output-or-error-uses-git-program ()
+  "Normal: git wrapper passes its args through with `git' as the program."
+  (let (captured-program captured-args)
+    (cl-letf (((symbol-function 'process-file)
+               (lambda (program _infile destination _display &rest args)
+                 (setq captured-program program
+                       captured-args args)
+                 (when destination
+                   (let ((buf (cond ((eq destination t) (current-buffer))
+                                    ((bufferp destination) destination)
+                                    ((consp destination) (car destination)))))
+                     (when (bufferp buf)
+                       (with-current-buffer buf (insert "abcdef\n")))))
+                 0)))
+      (should (equal (cj/git-output-or-error "rev-parse" "HEAD") "abcdef\n"))
+      (should (equal captured-program "git"))
+      (should (equal captured-args '("rev-parse" "HEAD"))))))
+
+(ert-deftest test-cj-git-output-or-error-non-zero-signals-user-error ()
+  "Error: git wrapper raises the same user-error shape on failure."
+  (test-system-lib-runner--with-stub 1 "fatal: bad ref\n"
+    (should-error (cj/git-output-or-error "rev-parse" "bogus")
+                  :type 'user-error)))
+
+(provide 'test-system-lib-process-output-or-error)
+;;; test-system-lib-process-output-or-error.el ends here