kbuild: let fixdep directly write to .*.cmd files

Currently, fixdep writes dependencies to .*.tmp, which is renamed to
.*.cmd after everything succeeds. This is a very safe way to avoid
corrupted .*.cmd files. The if_changed_dep has carried this safety
mechanism since it was added in 2002.

If fixdep fails for some reasons or a user terminates the build while
fixdep is running, the incomplete output from the fixdep could be
troublesome.

This is my insight about some bad scenarios:

[1] If the compiler succeeds to generate *.o file, but fixdep fails
    to write necessary dependencies to .*.cmd file, Make will miss
    to rebuild the object when headers or CONFIG options are changed.
    In this case, fixdep should not generate .*.cmd file at all so
    that 'arg-check' will surely trigger the rebuild of the object.

[2] A partially constructed .*.cmd file may not be a syntactically
    correct makefile. The next time Make runs, it would include it,
    then fail to parse it. Once this happens, 'make clean' is be the
    only way to fix it.

In fact, [1] is no longer a problem since commit 9c2af1c7 ("kbuild:
add .DELETE_ON_ERROR special target"). Make deletes a target file on
any failure in its recipe. Because fixdep is a part of the recipe of
*.o target, if it fails, the *.o is deleted anyway. However, I am a
bit worried about the slight possibility of [2].

So, here is a solution. Let fixdep directly write to a .*.cmd file,
but allow makefiles to include it only when its corresponding target
exists.

This effectively reverts commit 2982c953 ("kbuild: remove redundant
$(wildcard ...) for cmd_files calculation"), and commit 00d78ab2
("kbuild: remove dead code in cmd_files calculation in top Makefile")
because now we must check the presence of targets.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1043,6 +1043,8 @@ ifdef CONFIG_GDB_SCRIPTS
 endif
 	+$(call if_changed,link-vmlinux)
 
+targets := vmlinux
+
 # Build samples along the rest of the kernel. This needs headers_install.
 ifdef CONFIG_SAMPLES
 vmlinux-dirs += samples
@@ -1758,13 +1760,12 @@ quiet_cmd_depmod = DEPMOD  $(KERNELRELEASE)
 cmd_crmodverdir = $(Q)mkdir -p $(MODVERDIR) \
                   $(if $(KBUILD_MODULES),; rm -f $(MODVERDIR)/*)
 
-# read all saved command lines
-cmd_files := $(wildcard .*.cmd)
+# read saved command lines for existing targets
+existing-targets := $(wildcard $(sort $(targets)))
 
-ifneq ($(cmd_files),)
-  $(cmd_files): ;	# Do not try to update included dependency files
-  include $(cmd_files)
-endif
+cmd_files := $(foreach f,$(existing-targets),$(dir $(f)).$(notdir $(f)).cmd)
+$(cmd_files): ;	# Do not try to update included dependency files
+-include $(cmd_files)
 
 endif   # ifeq ($(config-targets),1)
 endif   # ifeq ($(mixed-targets),1)

scripts/Kbuild.include

@@ -264,9 +264,8 @@ ifndef CONFIG_TRIM_UNUSED_KSYMS
 
 cmd_and_fixdep =                                                             \
 	$(echo-cmd) $(cmd_$(1));                                             \
-	scripts/basic/fixdep $(depfile) $@ '$(make-cmd)' > $(dot-target).tmp;\
-	rm -f $(depfile);                                                    \
-	mv -f $(dot-target).tmp $(dot-target).cmd;
+	scripts/basic/fixdep $(depfile) $@ '$(make-cmd)' > $(dot-target).cmd;\
+	rm -f $(depfile);
 
 else
 
@@ -289,9 +288,8 @@ cmd_and_fixdep =                                                             \
 	$(echo-cmd) $(cmd_$(1));                                             \
 	$(ksym_dep_filter) |                                                 \
 		scripts/basic/fixdep -e $(depfile) $@ '$(make-cmd)'          \
-			> $(dot-target).tmp;	                             \
-	rm -f $(depfile);                                                    \
-	mv -f $(dot-target).tmp $(dot-target).cmd;
+			> $(dot-target).cmd;	                             \
+	rm -f $(depfile);
 
 endif
 

scripts/Makefile.build

@@ -529,18 +529,16 @@ FORCE:
 # optimization, we don't need to read them if the target does not
 # exist, we will rebuild anyway in that case.
 
-cmd_files := $(wildcard $(foreach f,$(sort $(targets)),$(dir $(f)).$(notdir $(f)).cmd))
+existing-targets := $(wildcard $(sort $(targets)))
 
-ifneq ($(cmd_files),)
-  include $(cmd_files)
-endif
+-include $(foreach f,$(existing-targets),$(dir $(f)).$(notdir $(f)).cmd)
 
 ifneq ($(KBUILD_SRC),)
 # Create directories for object files if they do not exist
 obj-dirs := $(sort $(obj) $(patsubst %/,%, $(dir $(targets))))
-# If cmd_files exist, their directories apparently exist.  Skip mkdir.
-exist-dirs := $(sort $(patsubst %/,%, $(dir $(cmd_files))))
-obj-dirs := $(strip $(filter-out $(exist-dirs), $(obj-dirs)))
+# If targets exist, their directories apparently exist. Skip mkdir.
+existing-dirs := $(sort $(patsubst %/,%, $(dir $(existing-targets))))
+obj-dirs := $(strip $(filter-out $(existing-dirs), $(obj-dirs)))
 ifneq ($(obj-dirs),)
 $(shell mkdir -p $(obj-dirs))
 endif