[PATCH] emacs: Avoid rebuilding .eldeps even when there's nothing to do

diff --git a/21/5dfc3d02bfa2bd7d3a16cade152c32cad94667 b/21/5dfc3d02bfa2bd7d3a16cade152c32cad94667
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+From: Austin Clements <amdragon@MIT.EDU>\r
+To: notmuch@notmuchmail.org\r
+Subject: [PATCH] emacs: Avoid rebuilding .eldeps even when there's nothing to\r
+       do\r
+Date: Wed, 19 Feb 2014 23:24:24 -0500\r
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+\r
+Previously, we updated .eldeps only if the file contents actually\r
+needed to change.  This was done to avoid unnecessary make restarts\r
+(if the .eldeps rule changes the mtime of .eldeps, make has to restart\r
+to collect the new dependencies).  However, this meant that, after a\r
+modification to any .el file that did not change dependencies, .eldeps\r
+would always be out of date, so every make invocation would run the\r
+.eldeps rule, which is both expensive because it starts up Emacs and\r
+noisy.  This was true even when there was nothing to do.  E.g.,\r
+\r
+$ make clean && make\r
+...\r
+$ touch emacs/notmuch-lib.el && make\r
+...\r
+$ make\r
+Use "make V=1" to see the verbose compile lines.\r
+EMACS emacs/.eldeps\r
+make: Nothing to be done for `all'.\r
+$ make\r
+Use "make V=1" to see the verbose compile lines.\r
+EMACS emacs/.eldeps\r
+make: Nothing to be done for `all'.\r
+\r
+Fix this by replacing .eldeps with two files with identical content.\r
+One tracks the mtime of the dependency information and triggers the\r
+Emacs call to rebuild dependencies only when it may be necessary.  The\r
+other tracks the content only; this rule over-triggers in the same way\r
+the old rule did, but this rule is cheap and quiet.\r
+---\r
+ emacs/Makefile.local | 15 ++++++++++++---\r
+ 1 file changed, 12 insertions(+), 3 deletions(-)\r
+\r
+diff --git a/emacs/Makefile.local b/emacs/Makefile.local\r
+index 42bfbd9..6a39b32 100644\r
+--- a/emacs/Makefile.local\r
++++ b/emacs/Makefile.local\r
+@@ -33,10 +33,19 @@ ifeq ($(HAVE_EMACS),1)\r
+ $(dir)/.eldeps: $(dir)/Makefile.local $(dir)/make-deps.el $(emacs_sources)\r
+       $(call quiet,EMACS) --directory emacs -batch -l make-deps.el \\r
+               -f batch-make-deps $(emacs_sources) > $@.tmp && \\r
+-              (cmp -s $@.tmp $@ || mv $@.tmp $@)\r
+--include $(dir)/.eldeps\r
++              mv $@.tmp $@\r
++# We could include .eldeps directly, but that would cause a make\r
++# restart whenever any .el file was modified, even if dependencies\r
++# didn't change, because the mtime of .eldeps will change.  Instead,\r
++# we include a second file, .eldeps.x, which we ensure always has the\r
++# same content as .eldeps, but its mtime only changes when dependency\r
++# information changes, in which case a make restart is necessary\r
++# anyway.\r
++$(dir)/.eldeps.x: $(dir)/.eldeps\r
++      @cmp -s $^ $@ || cp $^ $@\r
++-include $(dir)/.eldeps.x\r
+ endif\r
+-CLEAN+=$(dir)/.eldeps $(dir)/.eldeps.tmp\r
++CLEAN+=$(dir)/.eldeps $(dir)/.eldeps.tmp $(dir)/.eldeps.x\r
+ \r
+ ifeq ($(HAVE_EMACS),1)\r
+ %.elc: %.el $(global_deps)\r
+-- \r
+1.8.4.rc3\r
+\r