Allow creation of loaddefs files without timestamps.

* lisp/emacs-lisp/autoload.el (autoload-timestamps): New variable.
(autoload-generate-file-autoloads, update-directory-autoloads):
If autoload-timestamps is nil, write "t" instead of file timestamp.
(autoload-find-destination, update-directory-autoloads):
If timestamp is "t", use the modtime of the output file instead.

; * etc/NEWS: Mention this.

2 files changed, 54 insertions, 11 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 232f4970cc9..8907f7ad453 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -68,6 +68,9 @@ in these situations.
 
 * Lisp Changes in Emacs 25.2
 
+** Autoload files can be generated without timestamps,
+by setting `autoload-timestamps' to nil.
+
 
 * Changes in Emacs 25.2 on Non-Free Operating Systems
 
diff --git a/lisp/emacs-lisp/autoload.el b/lisp/emacs-lisp/autoload.el
index 12d0a94127f..98b05a382aa 100644
--- a/lisp/emacs-lisp/autoload.el
+++ b/lisp/emacs-lisp/autoload.el
@@ -87,6 +87,21 @@ that text will be copied verbatim to `generated-autoload-file'.")
 (defconst generate-autoload-section-continuation ";;;;;; "
   "String to add on each continuation of the section header form.")
 
+(defvar autoload-timestamps t
+  "Non-nil means insert a timestamp for each input file into the output.
+We use these in incremental updates of the output file to decide
+if we need to rescan an input file.  If you set this to nil,
+then we use the timestamp of the output file instead.  As a result:
+ - for fixed inputs, the output will be the same every time
+ - incremental updates of the output file might not be correct if:
+   i) the timestamp of the output file cannot be trusted (at least
+     relative to that of the input files)
+   ii) any of the input files can be modified during the time it takes
+      to create the output
+   iii) only a subset of the input files are scanned
+   These issues are unlikely to happen in practice, and would arguably
+   represent bugs in the build system.")
+
 (defvar autoload-modified-buffers)      ;Dynamically scoped var.
 
 (defun make-autoload (form file &optional expansion)
@@ -624,7 +639,9 @@ FILE's modification time."
                                       ;; We'd really want to just use
                                       ;; `emacs-internal' instead.
                                       nil nil 'emacs-mule-unix)
-                               (nth 5 (file-attributes relfile))))
+                               (if autoload-timestamps
+                                   (nth 5 (file-attributes relfile))
+                                 t)))
                             (insert ";;; Generated autoloads from " relfile "\n")))
                         (insert generate-autoload-section-trailer))))
                   (or noninteractive
@@ -688,6 +705,9 @@ removes any prior now out-of-date autoload entries."
   (catch 'up-to-date
     (let* ((buf (current-buffer))
            (existing-buffer (if buffer-file-name buf))
+           (output-file (autoload-generated-file))
+           (output-time (if (file-exists-p output-file)
+                            (nth 5 (file-attributes output-file))))
            (found nil))
       (with-current-buffer (autoload-find-generated-file)
         ;; This is to make generated-autoload-file have Unix EOLs, so
@@ -712,16 +732,26 @@ removes any prior now out-of-date autoload entries."
                          (file-time (nth 5 (file-attributes file))))
                      (if (and (or (null existing-buffer)
                                   (not (buffer-modified-p existing-buffer)))
-                              (or
+                              (cond
                                ;; last-time is the time-stamp (specifying
                                ;; the last time we looked at the file) and
                                ;; the file hasn't been changed since.
-                               (and (listp last-time) (= (length last-time) 2)
-                                    (not (time-less-p last-time file-time)))
+                               ((listp last-time)
+                                (not (time-less-p last-time file-time)))
+                               ;; FIXME? Arguably we should throw a
+                               ;; user error, or some kind of warning,
+                               ;; if we were called from update-file-autoloads,
+                               ;; which can update only a single input file.
+                               ;; It's not appropriate to use the output
+                               ;; file modtime in such a case,
+                               ;; if there are multiple input files
+                               ;; contributing to the output.
+                               ((and output-time (eq t last-time))
+                                (not (time-less-p output-time file-time)))
                                ;; last-time is an MD5 checksum instead.
-                               (and (stringp last-time)
-                                    (equal last-time
-                                           (md5 buf nil nil 'emacs-mule)))))
+                               ((stringp last-time)
+                                (equal last-time
+                                       (md5 buf nil nil 'emacs-mule)))))
                          (throw 'up-to-date nil)
                        (autoload-remove-section begin)
                        (setq found t))))
@@ -781,7 +811,10 @@ write its autoloads into the specified file instead."
          (generated-autoload-file
           (if (called-interactively-p 'interactive)
               (read-file-name "Write autoload definitions to file: ")
-            generated-autoload-file)))
+            generated-autoload-file))
+         (output-time
+          (if (file-exists-p generated-autoload-file)
+              (nth 5 (file-attributes generated-autoload-file)))))
 
     (with-current-buffer (autoload-find-generated-file)
       (save-excursion
@@ -799,6 +832,8 @@ write its autoloads into the specified file instead."
                    ;; Remove the obsolete section.
                    (autoload-remove-section (match-beginning 0))
                    (setq last-time (nth 4 form))
+                   (if (equal t last-time)
+                       (setq last-time output-time))
                    (dolist (file file)
                      (let ((file-time (nth 5 (file-attributes file))))
                        (when (and file-time
@@ -814,7 +849,10 @@ write its autoloads into the specified file instead."
                        (member (expand-file-name file) autoload-excludes))
                    ;; Remove the obsolete section.
                    (autoload-remove-section (match-beginning 0)))
-                  ((not (time-less-p (nth 4 form)
+                  ((not (time-less-p (let ((oldtime (nth 4 form)))
+                                       (if (equal t oldtime)
+                                           output-time
+                                         oldtime))
                                      (nth 5 (file-attributes file))))
                    ;; File hasn't changed.
                    nil)
@@ -830,7 +868,7 @@ write its autoloads into the specified file instead."
       (let ((no-autoloads-time (or last-time '(0 0 0 0))) file-time)
         (dolist (file files)
           (cond
-           ((member (expand-file-name file) autoload-excludes) nil)
+           ((member (expand-file-name file) autoload-excludes nil))
            ;; Passing nil as second argument forces
            ;; autoload-generate-file-autoloads to look for the right
            ;; spot where to insert each autoloads section.
@@ -847,7 +885,9 @@ write its autoloads into the specified file instead."
           (goto-char (point-max))
           (search-backward "\f" nil t)
           (autoload-insert-section-header
-           (current-buffer) nil nil no-autoloads no-autoloads-time)
+           (current-buffer) nil nil no-autoloads (if autoload-timestamps
+                                                     no-autoloads-time
+                                                   t))
           (insert generate-autoload-section-trailer)))
 
       (let ((version-control 'never))