From 7f3d63908cd05fb34347d942e435c2964cd8b249 Mon Sep 17 00:00:00 2001
From: Glenn Morris <rgm@gnu.org>
Date: Sat, 6 May 2017 17:58:20 -0700
Subject: [PATCH] Write autoloads file atomically

* lisp/emacs-lisp/autoload.el (autoload--save-buffer):
New function, to save buffer atomically.
(autoload-save-buffers, update-directory-autoloads):
Use autoload--save-buffer.
* lisp/Makefile.in ($(lisp)/loaddefs.el):
No longer write to a temp file by hand.
---
 lisp/Makefile.in            | 13 +------------
 lisp/emacs-lisp/autoload.el | 22 ++++++++++++++++++----
 2 files changed, 19 insertions(+), 16 deletions(-)

diff --git a/lisp/Makefile.in b/lisp/Makefile.in
index cbbea78a00f..1da8814370a 100644
--- a/lisp/Makefile.in
+++ b/lisp/Makefile.in
@@ -203,28 +203,17 @@ $(lisp)/finder-inf.el:
 # batch-update-autoloads, which only updates the autoloads whose
 # sources have changed.
 
-# Write to a temporary file in case we're doing a parallel build and a
-# CANNOT_DUMP-mode Emacs needs to read loaddefs at startup.
-# (FIXME: This seems like something that batch-update-autoloads should
-# do internally, then it would Just Work for all loaddefs files.)
-# We start by copying an existing loaddefs.el to loaddefs.tmp to avoid
-# regenerating the entire file anew, which is slow; starting from an
-# almost-correct content will enable the "only update where necessary"
-# feature of batch-update-autoloads.
-
 # Use expand-file-name rather than $abs_scrdir so that Emacs does not
 # get confused when it compares file-names for equality.
 
 autoloads .PHONY: $(lisp)/loaddefs.el
 $(lisp)/loaddefs.el: gen-lisp $(LOADDEFS)
 	@echo Directories for loaddefs: ${SUBDIRS_ALMOST}
-	@if test -f $@ ; then cp $@ $(lisp)/loaddefs.tmp ; fi
 	$(AM_V_GEN)$(emacs) -l autoload \
 	    --eval '(setq autoload-ensure-writable t)' \
 	    --eval '(setq autoload-builtin-package-versions t)' \
-	    --eval '(setq generated-autoload-file (expand-file-name (unmsys--file-name "$(lisp)/loaddefs.tmp")))' \
+	    --eval '(setq generated-autoload-file (expand-file-name (unmsys--file-name "$@")))' \
 	    -f batch-update-autoloads ${SUBDIRS_ALMOST}
-	$(top_srcdir)/build-aux/move-if-change $(lisp)/loaddefs.tmp $@
 
 # autoloads only runs when loaddefs.el is nonexistent, although it
 # generates a number of different files. Provide a force option to enable
diff --git a/lisp/emacs-lisp/autoload.el b/lisp/emacs-lisp/autoload.el
index 9865b3198b2..8ad5e6b823d 100644
--- a/lisp/emacs-lisp/autoload.el
+++ b/lisp/emacs-lisp/autoload.el
@@ -866,11 +866,26 @@ FILE's modification time."
      (error "%s:0:0: error: %s: %s" file (car err) (cdr err)))
     ))
 
+;; For parallel builds, to stop another process reading a half-written file.
+(defun autoload--save-buffer ()
+  "Save current buffer to its file, atomically."
+  ;; Copied from byte-compile-file.
+  (let* ((version-control 'never)
+         (tempfile (make-temp-name buffer-file-name))
+         (kill-emacs-hook
+          (cons (lambda () (ignore-errors (delete-file tempfile)))
+                kill-emacs-hook)))
+    (write-region (point-min) (point-max) tempfile nil 1)
+    (backup-buffer)
+    (rename-file tempfile buffer-file-name t)
+    (set-buffer-modified-p nil)
+    (set-visited-file-modtime)
+    (or noninteractive (message "Wrote %s" buffer-file-name))))
+
 (defun autoload-save-buffers ()
   (while autoload-modified-buffers
     (with-current-buffer (pop autoload-modified-buffers)
-      (let ((version-control 'never))
-	(save-buffer)))))
+      (autoload--save-buffer))))
 
 ;; FIXME This command should be deprecated.
 ;; See http://debbugs.gnu.org/22213#41
@@ -1110,8 +1125,7 @@ write its autoloads into the specified file instead."
       ;; dependencies don't trigger unnecessarily.
       (if (not changed)
           (set-buffer-modified-p nil)
-        (let ((version-control 'never))
-          (save-buffer)))
+        (autoload--save-buffer))
 
       ;; In case autoload entries were added to other files because of
       ;; file-local autoload-generated-file settings.
-- 
2.39.2