[PATCH] Record dependencies during build instead of before

diff --git a/10/183b357e5826d2aec9f62141a1a06fa684a4f5 b/10/183b357e5826d2aec9f62141a1a06fa684a4f5
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+From: Austin Clements <amdragon@MIT.EDU>\r
+To: notmuch@notmuchmail.org\r
+Subject: [PATCH] Record dependencies during build instead of before\r
+Date: Sun,  8 Apr 2012 00:01:50 -0400\r
+Message-Id: <1333857710-24511-1-git-send-email-amdragon@mit.edu>\r
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+\r
+Previously, the makefile created dependency files in a separate, first\r
+pass.  In particular, include-ing the dependency files would cause\r
+make to attempt to rebuild those files using the dependency-generation\r
+rules in the makefile.  Unfortunately, this approach required obtuse\r
+rules and silently delayed the start of the build process (by quite a\r
+bit on a clean tree without any dependency files).  Worse, this\r
+required the dependency files to themselves depend on all of the\r
+headers the source file depended on, which meant that, if a header\r
+file was removed, the depedency file could not be updated because of a\r
+missing dependency (!), which would cause make to silently fail.\r
+\r
+This patch eliminates the dependency generation rules and instead\r
+generates dependency files as a side-effect of the regular build rule.\r
+On the first build, we don't need to know the dependencies beforehand;\r
+the object file doesn't exist, so it will be built anyway.  On\r
+subsequent builds, if a header file is updated, the dependency rules\r
+generated by the previous build will force a rebuild.  If a source\r
+file is updated, the dependency rules may be stale, but it doesn't\r
+matter because the updated source file will force a rebuild.\r
+\r
+In the final case above, the stale dependency rules may refer to a\r
+header file that no longer exists but is also no longer needed.  In\r
+order to prevent this from breaking the build, we also pass gcc the\r
+-MP option, which generates phony targets for every depended-on header\r
+file, so make won't complain if it can't find them during a later\r
+build.\r
+---\r
+The description is way more complicated than the patch.\r
+\r
+ Makefile.local |   18 ++++--------------\r
+ 1 files changed, 4 insertions(+), 14 deletions(-)\r
+\r
+diff --git a/Makefile.local b/Makefile.local\r
+index 1131dea..525eda0 100644\r
+--- a/Makefile.local\r
++++ b/Makefile.local\r
+@@ -256,23 +256,12 @@ endif\r
+ quiet ?= $($(shell echo $1 | sed -e s'/ .*//'))\r
+ \r
+ %.o: %.cc $(global_deps)\r
++      @mkdir -p .deps/$(@D)\r
++      $(call quiet,CXX $(CXXFLAGS)) -c $(FINAL_CXXFLAGS) $< -o $@ -MD -MP -MF .deps/$*.d\r
+-      $(call quiet,CXX $(CXXFLAGS)) -c $(FINAL_CXXFLAGS) $< -o $@\r
+ \r
+ %.o: %.c $(global_deps)\r
++      @mkdir -p .deps/$(@D)\r
++      $(call quiet,CC $(CFLAGS)) -c $(FINAL_CFLAGS) $< -o $@ -MD -MP -MF .deps/$*.d\r
+-      $(call quiet,CC $(CFLAGS)) -c $(FINAL_CFLAGS) $< -o $@\r
+-\r
+-.deps/%.d: %.c $(global_deps)\r
+-      @set -e; rm -f $@; mkdir -p $$(dirname $@) ; \\r
+-      $(CC) -M $(CPPFLAGS) $(FINAL_CFLAGS) $< > $@.$$$$ 2>/dev/null ; \\r
+-      sed 's,'$$(basename $*)'\.o[ :]*,$*.o $@ : ,g' < $@.$$$$ > $@; \\r
+-      rm -f $@.$$$$\r
+-\r
+-.deps/%.d: %.cc $(global_deps)\r
+-      @set -e; rm -f $@; mkdir -p $$(dirname $@) ; \\r
+-      $(CXX) -M $(CPPFLAGS) $(FINAL_CXXFLAGS) $< > $@.$$$$ 2>/dev/null ; \\r
+-      sed 's,'$$(basename $*)'\.o[ :]*,$*.o $@ : ,g' < $@.$$$$ > $@; \\r
+-      rm -f $@.$$$$\r
+ \r
+ .PHONY : clean\r
+ clean:\r
+-- \r
+1.7.9.1\r
+\r