misc/emacs: Use patch output of gofmt instead of replacing the buffer.

This uses the patch output of gofmt (-d option) and applies each
chunk to the buffer, instead of replacing the whole buffer.  The
main advantage is that the undo history is kept across gofmt'ings,
so it can really be used as a before-save-hook.

R=sameer, sameer
CC=golang-dev
https://golang.org/cl/6198047

misc/emacs/go-mode.el

@@ -746,39 +746,66 @@ Replace the current buffer on success; display errors on failure."
 
   (interactive)
   (let ((currconf (current-window-configuration)))
-    (let ((srcbuf (current-buffer)))
-      (with-temp-buffer
-        (let ((outbuf (current-buffer))
-              (errbuf (get-buffer-create "*Gofmt Errors*"))
+    (let ((srcbuf (current-buffer))
+          (filename buffer-file-name)
+          (patchbuf (get-buffer-create "*Gofmt patch*")))
+      (with-current-buffer patchbuf
+        (let ((errbuf (get-buffer-create "*Gofmt Errors*"))
               (coding-system-for-read 'utf-8)    ;; use utf-8 with subprocesses
               (coding-system-for-write 'utf-8))
-          (with-current-buffer errbuf (erase-buffer))
+          (with-current-buffer errbuf
+            (toggle-read-only 0)
+            (erase-buffer))
           (with-current-buffer srcbuf
             (save-restriction
               (let (deactivate-mark)
                 (widen)
-                (if (= 0 (shell-command-on-region (point-min) (point-max) "gofmt"
-                                                  outbuf nil errbuf))
-                    ;; restore window config
-                    ;; gofmt succeeded: replace the current buffer with outbuf,
-                    ;; restore the mark and point, and discard errbuf.
-                    (let ((old-mark (mark t))
-                          (old-point (point))
-                          (old-start (window-start)))
-                      (erase-buffer)
-                      (insert-buffer-substring outbuf)
-                      (set-window-configuration currconf)
-                      (set-window-start (selected-window) (min old-start (point-max)))
-                      (goto-char (min old-point (point-max)))
-                      (if old-mark (push-mark (min old-mark (point-max)) t))
-                      (kill-buffer errbuf))
+                (if (= 0 (shell-command-on-region (point-min) (point-max) "gofmt -d"
+                                                  patchbuf nil errbuf))
+                    ; gofmt succeeded: apply patch hunks.
+                    (progn
+                      (kill-buffer errbuf)
+                      (gofmt-apply-patch filename srcbuf patchbuf)
+                      (set-window-configuration currconf))
 
                   ;; gofmt failed: display the errors
-                  (display-buffer errbuf)))))
+                  (gofmt-process-errors filename errbuf)))))
 
           ;; Collapse any window opened on outbuf if shell-command-on-region
           ;; displayed it.
-          (delete-windows-on outbuf))))))
+          (delete-windows-on patchbuf)))
+      (kill-buffer patchbuf))))
+
+(defconst gofmt-stdin-tag "<standard input>")
+
+(defun gofmt-apply-patch (filename srcbuf patchbuf)
+  (require 'diff-mode)
+  ;; apply all the patch hunks and restore the mark and point
+  (let ((old-point (point))
+        (old-mark (mark t)))
+    (with-current-buffer patchbuf
+      (let ((filename (file-name-nondirectory filename))
+            (min (point-min)))
+        (replace-string gofmt-stdin-tag  filename nil min (point-max))
+        (replace-regexp "^--- /tmp/gofmt[0-9]*" (concat "--- /tmp/" filename)
+                        nil min (point-max)))
+      (condition-case nil
+          (while t
+            (diff-hunk-next)
+            (diff-apply-hunk))
+        ;; When there's no more hunks, diff-hunk-next signals an error, ignore it
+        (error nil)))
+    (goto-char (min old-point (point-max)))
+    (if old-mark (push-mark (min old-mark (point-max)) t))))
+
+(defun gofmt-process-errors (filename errbuf)
+  ;; Convert the gofmt stderr to something understood by the compilation mode.
+  (with-current-buffer errbuf
+    (beginning-of-buffer)
+    (insert "gofmt errors:\n")
+    (replace-string gofmt-stdin-tag (file-name-nondirectory filename) nil (point-min) (point-max))
+    (display-buffer errbuf)
+    (compilation-mode)))
 
 ;;;###autoload
 (defun gofmt-before-save ()