[PATCH] emacs: Compute build dependencies to fix byte compile issues

[Austin Clements]

Previously, we simply byte compiled each Elisp source file
independently.  This is actually the wrong thing to do and can lead to
issues with macros and performance issues with substitutions because
1) when the byte compiler encounters a (require 'x) form, it will load
x.elc in preference to x.el, even if x.el is newer, and as a result
may load old macro and substitution definitions and 2) if we update a
macro or substitution definition in one file, we currently won't
re-compile other files that depend on the file containing the
definition.

This patch addresses these problems by computing make dependency rules
from the (require 'x) forms in the Elisp source files, which we inject
into make's dependency database.
---
 emacs/Makefile.local |   12 +++++++++
 emacs/make-deps.el   |   66 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 78 insertions(+)
 create mode 100644 emacs/make-deps.el

diff --git a/emacs/Makefile.local b/emacs/Makefile.local
index fb82247..456700a 100644
--- a/emacs/Makefile.local
+++ b/emacs/Makefile.local
@@ -22,6 +22,18 @@ emacs_images := \
 
 emacs_bytecode = $(emacs_sources:.el=.elc)
 
+# Because of defmacro's and defsubst's, we have to account for load
+# dependencies between Elisp files when byte compiling.  Otherwise,
+# the byte compiler may load an old .elc file when processing a
+# "require" or we may fail to rebuild a .elc that depended on a macro
+# from an updated file.
+$(dir)/.eldeps: $(dir)/Makefile.local $(dir)/make-deps.el $(emacs_sources)
+	$(call quiet,EMACS) --directory emacs -batch -l make-deps.el \
+		-f batch-make-deps $(emacs_sources) > $@.tmp && \
+		(cmp -s $@.tmp $@ || mv $@.tmp $@)
+-include $(dir)/.eldeps
+CLEAN+=$(dir)/.eldeps $(dir)/.eldeps.tmp
+
 %.elc: %.el $(global_deps)
 	$(call quiet,EMACS) --directory emacs -batch -f batch-byte-compile $<
 
diff --git a/emacs/make-deps.el b/emacs/make-deps.el
new file mode 100644
index 0000000..a1cd731
--- /dev/null
+++ b/emacs/make-deps.el
@@ -0,0 +1,66 @@
+;; make-deps.el --- compute make dependencies for Elisp sources
+;;
+;; Copyright © Austin Clements
+;;
+;; This file is part of Notmuch.
+;;
+;; Notmuch is free software: you can redistribute it and/or modify it
+;; under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+;;
+;; Notmuch is distributed in the hope that it will be useful, but
+;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;; General Public License for more details.
+;;
+;; You should have received a copy of the GNU General Public License
+;; along with Notmuch.  If not, see <http://www.gnu.org/licenses/>.
+;;
+;; Authors: Austin Clements <aclements at csail.mit.edu>
+
+(defun batch-make-deps ()
+  "Invoke `make-deps' for each file on the command line."
+
+  (setq debug-on-error t)
+  (dolist (file command-line-args-left)
+    (let ((default-directory command-line-default-directory))
+      (find-file-literally file))
+    (make-deps command-line-default-directory))
+  (kill-emacs))
+
+(defun make-deps (&optional dir)
+  "Print make dependencies for the current buffer.
+
+This prints make dependencies to `standard-output' based on the
+top-level `require' expressions in the current buffer.  Paths in
+rules will be given relative to DIR, or `default-directory'."
+
+  (setq dir (or dir default-directory))
+  (save-excursion
+    (goto-char (point-min))
+    (condition-case nil
+	(while t
+	  (let ((form (read (current-buffer))))
+	    ;; Is it a (require 'x) form?
+	    (when (and (listp form) (= (length form) 2)
+		       (eq (car form) 'require)
+		       (listp (cadr form)) (= (length (cadr form)) 2)
+		       (eq (car (cadr form)) 'quote)
+		       (symbolp (cadr (cadr form))))
+	      ;; Find the required library
+	      (let* ((name (cadr (cadr form)))
+		     (fname (locate-library (symbol-name name))))
+		;; Is this file and the library in the same directory?
+		;; If not, assume it's a system library and don't
+		;; bother depending on it.
+		(when (and fname
+			   (string= (file-name-directory (buffer-file-name))
+				    (file-name-directory fname)))
+		  ;; Print the dependency
+		  (princ (format "%s.elc: %s.elc\n"
+				 (file-name-sans-extension
+				  (file-relative-name (buffer-file-name) dir))
+				 (file-name-sans-extension
+				  (file-relative-name fname dir)))))))))
+      (end-of-file nil))))
-- 
1.7.10.4