[PATCH/RFC 5/5] Teach Makefile to check header dependencies

[Jonathan Nieder <jrnieder@gmail.com>]

Portability means we cannot completely switch over to
automatically generated dependencies on header files, since some
compilers do not support them.  This would seem to lead to a
dangerous situation in which the hand-written dependency rules
are needed for some configurations but poorly maintained because
most configurations do not use them.

Luckily, there is a way out: as part of testing git, ask the
build system to verify that the hand-written dependency rules are
consistent with the automatically generated ones.  This patch is
a start towards that goal.

The actual patch requires a few steps:

 1. Separate out a USE_COMPUTED_HEADER_DEPENDENCIES option to
    tell make to use the makefile snippets stored in deps/*
    without necessarily regenerating them;

 2. Add a PRINT_HEADER_DEPENDENCIES option to turn on
    USE_COMPUTED_HEADER_DEPENDENCIES and make the %.o: %.c rule
    print its prerequisites instead of compiling anything;

 3. Add a CHECK_HEADER_DEPENDENCIES option to turn off
    USE_COMPUTED_HEADER_DEPENDENCIES and make the %.o: %.c rule
    check that its prerequisites includes all files listed by
    'make -s PRINT_HEADER_DEPENDENCIES=YesPlease $@' instead of
    compiling anything.

With this patch applied,

	echo COMPUTE_HEADER_DEPENDENCIES=YesPlease >> config.mak
	make clean
	make
	make CHECK_HEADER_DEPENDENCIES=YesPlease

produces a useful error message:

	CHECK fast-import.o
	missing dependencies: exec_cmd.h
	make: *** [fast-import.o] Error 1

Probably we should check for missing deps/%.o.d files to avoid
false negatives if some are missing.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
---
I look forward to your thoughts.

This is a bit clunky, but it is useful enough to detect a few problems
with the current dependency rules.  Patches should follow soon.

 Makefile |   74 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++------
 1 files changed, 67 insertions(+), 7 deletions(-)

diff --git a/Makefile b/Makefile
index 578843c..e642a24 100644
--- a/Makefile
+++ b/Makefile
@@ -224,6 +224,8 @@ all::
 #
 # Define COMPUTE_HEADER_DEPENDENCIES if you want to avoid rebuilding objects
 # when an unrelated header file changes and your compiler supports it.
+#
+# Define CHECK_HEADER_DEPENDENCIES after a successful build to find problems.
 
 GIT-VERSION-FILE: FORCE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN
@@ -1064,6 +1066,28 @@ endif
 -include config.mak.autogen
 -include config.mak
 
+ifdef PRINT_HEADER_DEPENDENCIES
+CHECK_HEADER_DEPENDENCIES = YesPlease
+endif
+
+ifdef CHECK_HEADER_DEPENDENCIES
+ifndef COMPUTE_HEADER_DEPENDENCIES
+$(error checking dependencies requires build with COMPUTE_HEADER_DEPENDENCIES)
+endif
+endif
+
+ifdef COMPUTE_HEADER_DEPENDENCIES
+ifdef CHECK_HEADER_DEPENDENCIES
+ifdef PRINT_HEADER_DEPENDENCIES
+USE_COMPUTED_HEADER_DEPENDENCIES = YesPlease
+else
+USE_COMPUTED_HEADER_DEPENDENCIES =
+endif
+else
+USE_COMPUTED_HEADER_DEPENDENCIES = YesPlease
+endif
+endif
+
 ifdef SANE_TOOL_PATH
 SANE_TOOL_PATH_SQ = $(subst ','\'',$(SANE_TOOL_PATH))
 BROKEN_PATH_FIX = 's|^\# @@BROKEN_PATH_FIX@@$$|git_broken_path_fix $(SANE_TOOL_PATH_SQ)|'
@@ -1669,14 +1693,48 @@ endif
 
 .SUFFIXES:
 
+ifdef CHECK_HEADER_DEPENDENCIES
+
+ifdef PRINT_HEADER_DEPENDENCIES
+$(C_OBJ): %.o: %.c FORCE
+	echo $^
+$(ASM_OBJ): %.o: %.S FORCE
+	echo $^
+else
+missing_deps = $(filter-out $^, \
+	$(shell $(MAKE) -s PRINT_HEADER_DEPENDENCIES=1 $@))
+
+$(C_OBJ): %.o: %.c FORCE
+	@set -e; echo CHECK $@; \
+	missing_deps="$(missing_deps)"; \
+	if test "$$missing_deps"; \
+	then \
+		echo missing dependencies: $$missing_deps; \
+		false; \
+	fi
+$(ASM_OBJ): %.o: %.S FORCE
+	@set -e; echo CHECK $@; \
+	missing_deps="$(missing_deps)"; \
+	if test "$$missing_deps"; \
+	then \
+		echo missing dependencies: $$missing_deps; \
+		false; \
+	fi
+endif
+
+else
+
 $(C_OBJ): %.o: %.c GIT-CFLAGS $(dep_dir_dep)
 	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(ALL_CFLAGS) $<
-%.s: %.c GIT-CFLAGS FORCE
-	$(QUIET_CC)$(CC) -S $(ALL_CFLAGS) $<
 $(ASM_OBJ): %.o: %.S GIT-CFLAGS $(dep_dir_dep)
 	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(ALL_CFLAGS) $<
 
-ifdef COMPUTE_HEADER_DEPENDENCIES
+endif
+
+%.s: %.c GIT-CFLAGS FORCE
+	$(QUIET_CC)$(CC) -S $(ALL_CFLAGS) $<
+
+ifdef USE_COMPUTED_HEADER_DEPENDENCIES
 # Take advantage of gcc's dependency generation
 # See <http://gcc.gnu.org/gcc-3.0/features.html>.
 dep_files := $(wildcard $(foreach f,$(OBJECTS),$(dir f)deps/$(notdir $f).d))
@@ -1684,9 +1742,6 @@ dep_files := $(wildcard $(foreach f,$(OBJECTS),$(dir f)deps/$(notdir $f).d))
 ifneq ($(dep_files),)
 include $(dep_files)
 endif
-
-dep_file = $(dir $@)deps/$(notdir $@).d
-dep_args = -MF $(dep_file) -MMD -MP
 else
 $(GIT_OBJS): $(LIB_H)
 http.o http-walker.o http-push.o: http.h
@@ -1695,7 +1750,12 @@ builtin-revert.o wt-status.o: wt-status.h
 
 $(XDIFF_OBJS): xdiff/xinclude.h xdiff/xmacros.h xdiff/xdiff.h xdiff/xtypes.h \
 	xdiff/xutils.h xdiff/xprepare.h xdiff/xdiffi.h xdiff/xemit.h
+endif
 
+ifdef COMPUTE_HEADER_DEPENDENCIES
+dep_file = $(dir $@)deps/$(notdir $@).d
+dep_args = -MF $(dep_file) -MMD -MP
+else
 dep_args =
 endif
 
@@ -1823,7 +1883,7 @@ test-delta$X: diff-delta.o patch-delta.o
 
 test-parse-options$X: parse-options.o
 
-ifndef COMPUTE_HEADER_DEPENDENCIES
+ifndef USE_COMPUTED_HEADER_DEPENDENCIES
 test-parse-options.o: parse-options.h
 endif
 
-- 
1.6.6.rc2