refactor: extract shared format-region helper into system-lib

prog-json and prog-yaml each carried a byte-identical cj/--<lang>-format-region that runs a formatter over the buffer via call-process-region and replaces it on exit 0. Hoist it to system-lib as cj/format-region-with-program with a generic output buffer, and point both formatters at it. Adds the first direct unit coverage of the helper (Normal, Boundary, Error).

4 files changed, 100 insertions, 54 deletions

diff --git a/modules/prog-json.el b/modules/prog-json.el
index 953b5f79b..e7abd1828 100644
--- a/modules/prog-json.el
+++ b/modules/prog-json.el
@@ -9,7 +9,7 @@
 ;; Eager reason: none necessary; currently eager but should load by JSON major
 ;;   mode (Phase 6 deferral candidate).
 ;; Top-level side effects: one add-hook, package configuration via use-package.
-;; Runtime requires: none (configures packages via use-package).
+;; Runtime requires: system-lib (cj/format-region-with-program).
 ;; Direct test load: yes.
 ;;
 ;; JSON editing with tree-sitter highlighting, one-key formatting, and
@@ -27,6 +27,8 @@
 
 ;;; Code:
 
+(require 'system-lib)
+
 (defvar json-ts-mode-map)
 
 ;; -------------------------------- JSON Mode ----------------------------------
@@ -41,38 +43,13 @@
 ;; -------------------------------- Formatting ---------------------------------
 ;; pretty-print with sorted keys, bound to standard format key
 
-(defun cj/--json-format-region (program &rest args)
-  "Replace the buffer with PROGRAM ARGS run over its contents, via argv.
-Runs PROGRAM (with ARGS) on the whole buffer through
-`call-process-region' — no shell, so no quoting or word-splitting.
-The buffer is replaced only when PROGRAM exits zero; on a non-zero
-exit the buffer is left untouched and an error is signalled with
-the program's stderr text.  Point is preserved as closely as the
-reformatted size allows.  Returns t on success."
-  (let* ((point (point))
-         (src (current-buffer))
-         (out (generate-new-buffer " *json-format-out*"))
-         (status (apply #'call-process-region
-                        (point-min) (point-max) program
-                        nil out nil args)))
-    (unwind-protect
-        (if (and (integerp status) (zerop status))
-            (progn
-              (with-current-buffer src
-                (replace-buffer-contents out)
-                (goto-char (min point (point-max))))
-              t)
-          (user-error "%s failed: %s" program
-                      (string-trim (with-current-buffer out (buffer-string)))))
-      (kill-buffer out))))
-
 (defun cj/json-format-buffer ()
   "Format the current JSON buffer with sorted keys.
 Uses jq if available for reliable formatting, otherwise falls
 back to the built-in `json-pretty-print-buffer-ordered'."
   (interactive)
   (if (executable-find "jq")
-      (cj/--json-format-region "jq" "--sort-keys" ".")
+      (cj/format-region-with-program "jq" "--sort-keys" ".")
     (json-pretty-print-buffer-ordered)))
 
 (defun cj/json-setup ()
diff --git a/modules/prog-yaml.el b/modules/prog-yaml.el
index c2bb559b1..e07cf510e 100644
--- a/modules/prog-yaml.el
+++ b/modules/prog-yaml.el
@@ -9,7 +9,7 @@
 ;; Eager reason: none necessary; currently eager but should load by YAML major
 ;;   mode (Phase 6 deferral candidate).
 ;; Top-level side effects: one add-hook, package configuration via use-package.
-;; Runtime requires: none (configures packages via use-package).
+;; Runtime requires: system-lib (cj/format-region-with-program).
 ;; Direct test load: yes.
 ;;
 ;; YAML editing with tree-sitter highlighting and one-key formatting.
@@ -24,6 +24,8 @@
 
 ;;; Code:
 
+(require 'system-lib)
+
 ;; -------------------------------- YAML Mode ----------------------------------
 ;; tree-sitter mode for YAML files (built-in, Emacs 29+)
 ;; NOTE: No :mode directive — treesit-auto (in prog-general.el) handles
@@ -36,37 +38,12 @@
 ;; -------------------------------- Formatting ---------------------------------
 ;; normalize indentation and style, bound to standard format key
 
-(defun cj/--yaml-format-region (program &rest args)
-  "Replace the buffer with PROGRAM ARGS run over its contents, via argv.
-Runs PROGRAM (with ARGS) on the whole buffer through
-`call-process-region' — no shell, so no quoting or word-splitting.
-The buffer is replaced only when PROGRAM exits zero; on a non-zero
-exit the buffer is left untouched and an error is signalled with
-the program's stderr text.  Point is preserved as closely as the
-reformatted size allows.  Returns t on success."
-  (let* ((point (point))
-         (src (current-buffer))
-         (out (generate-new-buffer " *yaml-format-out*"))
-         (status (apply #'call-process-region
-                        (point-min) (point-max) program
-                        nil out nil args)))
-    (unwind-protect
-        (if (and (integerp status) (zerop status))
-            (progn
-              (with-current-buffer src
-                (replace-buffer-contents out)
-                (goto-char (min point (point-max))))
-              t)
-          (user-error "%s failed: %s" program
-                      (string-trim (with-current-buffer out (buffer-string)))))
-      (kill-buffer out))))
-
 (defun cj/yaml-format-buffer ()
   "Format the current YAML buffer with prettier.
 Preserves point position as closely as possible."
   (interactive)
   (if (executable-find "prettier")
-      (cj/--yaml-format-region "prettier" "--parser" "yaml")
+      (cj/format-region-with-program "prettier" "--parser" "yaml")
     (user-error "prettier not found; install with: npm install -g prettier")))
 
 (defun cj/yaml-setup ()
diff --git a/modules/system-lib.el b/modules/system-lib.el
index ed98a476e..49bb6cd1a 100644
--- a/modules/system-lib.el
+++ b/modules/system-lib.el
@@ -164,5 +164,29 @@ contributes its own modes regardless of load order."
     (setq font-lock-global-modes
           (cj/--font-lock-global-modes-excluding font-lock-global-modes mode))))
 
+(defun cj/format-region-with-program (program &rest args)
+  "Replace the current buffer with PROGRAM ARGS run over its contents, via argv.
+Runs PROGRAM (with ARGS) on the whole buffer through `call-process-region'
+-- no shell, so no quoting or word-splitting.  The buffer is replaced only
+when PROGRAM exits zero; on a non-zero exit the buffer is left untouched and
+a `user-error' is signalled with the program's stderr text.  Point is
+preserved as closely as the reformatted size allows.  Returns t on success."
+  (let* ((point (point))
+         (src (current-buffer))
+         (out (generate-new-buffer " *format-out*"))
+         (status (apply #'call-process-region
+                        (point-min) (point-max) program
+                        nil out nil args)))
+    (unwind-protect
+        (if (and (integerp status) (zerop status))
+            (progn
+              (with-current-buffer src
+                (replace-buffer-contents out)
+                (goto-char (min point (point-max))))
+              t)
+          (user-error "%s failed: %s" program
+                      (string-trim (with-current-buffer out (buffer-string)))))
+      (kill-buffer out))))
+
 (provide 'system-lib)
 ;;; system-lib.el ends here
diff --git a/tests/test-system-lib--format-region-with-program.el b/tests/test-system-lib--format-region-with-program.el
new file mode 100644
index 000000000..29b392b84
--- /dev/null
+++ b/tests/test-system-lib--format-region-with-program.el
@@ -0,0 +1,68 @@
+;;; test-system-lib--format-region-with-program.el --- Tests for cj/format-region-with-program -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; `cj/format-region-with-program' runs an external formatter over the whole
+;; buffer via `call-process-region' (argv, no shell) and replaces the buffer
+;; only when the program exits zero.  Extracted from the byte-identical
+;; per-language helpers in prog-json.el / prog-yaml.el, so this is the first
+;; direct unit coverage of the logic.  call-process-region is mocked at the
+;; boundary (the established pattern in test-prog-json--json-format-buffer.el).
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'system-lib)
+
+(ert-deftest test-system-lib-format-region-with-program-replaces-on-success ()
+  "Normal: on exit 0 the buffer is replaced with the program's output, returns t."
+  (cl-letf (((symbol-function 'call-process-region)
+             (lambda (_start _end _prog &rest rest)
+               (with-current-buffer (nth 1 rest) (insert "FORMATTED"))
+               0)))
+    (with-temp-buffer
+      (insert "raw")
+      (should (eq t (cj/format-region-with-program "fmt")))
+      (should (equal "FORMATTED" (buffer-string))))))
+
+(ert-deftest test-system-lib-format-region-with-program-forwards-argv ()
+  "Normal: PROGRAM and ARGS reach call-process-region as argv (no shell)."
+  (let (got-prog got-args)
+    (cl-letf (((symbol-function 'call-process-region)
+               (lambda (_start _end prog &rest rest)
+                 (setq got-prog prog
+                       got-args (nthcdr 3 rest))
+                 (with-current-buffer (nth 1 rest) (insert "x"))
+                 0)))
+      (with-temp-buffer
+        (cj/format-region-with-program "jq" "--sort-keys" ".")))
+    (should (equal "jq" got-prog))
+    (should (equal '("--sort-keys" ".") got-args))))
+
+(ert-deftest test-system-lib-format-region-with-program-empty-output ()
+  "Boundary: empty program output empties the buffer and still returns t."
+  (cl-letf (((symbol-function 'call-process-region)
+             (lambda (_start _end _prog &rest _rest) 0)))   ; writes nothing
+    (with-temp-buffer
+      (insert "raw")
+      (should (eq t (cj/format-region-with-program "fmt")))
+      (should (equal "" (buffer-string))))))
+
+(ert-deftest test-system-lib-format-region-with-program-nonzero-untouched ()
+  "Error: a non-zero exit leaves the buffer untouched and signals user-error
+carrying the program's stderr text."
+  (cl-letf (((symbol-function 'call-process-region)
+             (lambda (_start _end _prog &rest rest)
+               (with-current-buffer (nth 1 rest) (insert "boom: bad input"))
+               1)))
+    (with-temp-buffer
+      (insert "raw")
+      (let ((err (should-error (cj/format-region-with-program "fmt")
+                               :type 'user-error)))
+        (should (string-match-p "boom: bad input" (error-message-string err))))
+      (should (equal "raw" (buffer-string))))))
+
+(provide 'test-system-lib--format-region-with-program)
+;;; test-system-lib--format-region-with-program.el ends here