[RFC PATCH 1/4] Makefile: add a style-all targets for .clang-format testing

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Extend "style" rule added in 2118805b929 (Makefile: add style build
rule, 2017-08-14) with a set of "style-all" rules. These rules all
apply our ".clang-format" added in 6134de6ac18 (clang-format: outline
the git project's coding style, 2017-08-14) to all our tracked files,
rather than using "git-clang-format", which only applies the rules to
files that differ from those that are tracked.

This allows for testing and improving the .clang-format itself, as it
should as closely as possible mirror out stated
Documentation/CodingGuidelines, or in cases where we don't have
explicit guidelines it should match the prevailing preferred style in
the project.

Let's apply one such change, in 6134de6ac18 the "AlignEscapedNewlines"
configuration was set to "Left", but as setting it to "DontAlign"
shows our more common pattern is to not align "\"'s across newlines in
macro definitions.

Before & after the .clang-format change, running:

	git checkout -f '*.[ch]' && make -k style-all-diff-ok -k; make style-all-diff-apply

Will yield, respectively:

	578 files changed, 32191 insertions(+), 29944 deletions(-)
	579 files changed, 32065 insertions(+), 29818 deletions(-)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 .clang-format |  8 +++---
 Makefile      | 67 +++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 71 insertions(+), 4 deletions(-)

diff --git a/.clang-format b/.clang-format
index c592dda681f..3f536c49f24 100644
--- a/.clang-format
+++ b/.clang-format
@@ -32,12 +32,12 @@ AlignConsecutiveAssignments: false
 # double b = 3.14;
 AlignConsecutiveDeclarations: false
 
-# Align escaped newlines as far left as possible
-# #define A   \
+# Do not align escaped newlines in macro definitions
+# #define A \
 #   int aaaa; \
-#   int b;    \
+#   int b; \
 #   int cccccccc;
-AlignEscapedNewlines: Left
+AlignEscapedNewlines: DontAlign
 
 # Align operands of binary and ternary expressions
 # int aaa = bbbbbbbbbbb +
diff --git a/Makefile b/Makefile
index 04d0fd1fe60..67306820002 100644
--- a/Makefile
+++ b/Makefile
@@ -3108,10 +3108,77 @@ $(HCO): %.hco: %.hcc FORCE
 .PHONY: hdr-check $(HCO)
 hdr-check: $(HCO)
 
+##
+## clang-format targets
+##
+
+### style: apply a clang-format "diff" based on the changes in the
+### working tree
 .PHONY: style
 style:
 	git clang-format --style file --diff --extensions c,h
 
+#### style: common variables
+STYLE_SOURCES = $(filter-out $(THIRD_PARTY_SOURCES) $(EXCEPT_HDRS),\
+	$(FOUND_C_SOURCES) $(FOUND_H_SOURCES))
+
+#### style-all: generate clang-format output for everything. Copying
+#### the file to */in/* is so we can apply it with "git apply" (so one
+#### side won't have a .build/style-all/* prefix). If only "git apply"
+#### had a "-p" that applied only to one of "a/" or "b/" ...
+STYLE_SOURCES_IN = $(addprefix .build/style-all/in/,$(STYLE_SOURCES))
+$(STYLE_SOURCES_IN): .build/style-all/in/% : %
+	$(call mkdir_p_parent_template)
+	$(QUIET_LNCP)cp $< $@
+
+STYLE_SOURCES_OUT = $(addprefix .build/style-all/out/,$(STYLE_SOURCES))
+$(STYLE_SOURCES_OUT): .clang-format
+$(STYLE_SOURCES_OUT): .build/style-all/out/%: .build/style-all/in/%
+	$(call mkdir_p_parent_template)
+	$(QUIET_GEN)clang-format $< >$@
+.PHONY: style-all
+style-all: $(STYLE_SOURCES_GEN)
+
+#### style-all-diff: generate diffs of clang-format output
+#### v.s. checked-in files
+STYLE_SOURCES_DIFF = $(addprefix .build/style-all/diff/,$(STYLE_SOURCES))
+$(STYLE_SOURCES_DIFF): .build/style-all/diff/%: .build/style-all/out/%
+	$(call mkdir_p_parent_template)
+	$(QUIET_GEN)git \
+		-P \
+		diff --no-index \
+		$(<:.build/style-all/out/%=.build/style-all/in/%) $< >$@ || \
+	test -s $@
+
+.PHONY: style-all-diff
+style-all-diff: $(STYLE_SOURCES_DIFF)
+
+#### style-all-diff-ok: fail on files that have pending clang-format
+#### changes
+STYLE_SOURCES_DIFF_OK = $(addsuffix .ok,$(STYLE_SOURCES_DIFF))
+$(STYLE_SOURCES_DIFF_OK): %.ok: %
+	$(QUIET_GEN)! test -s $< && \
+	>$@
+.PHONY: style-all-diff-ok
+style-all-diff-ok: $(STYLE_SOURCES_DIFF_OK)
+
+.build/style-all.diff: $(STYLE_SOURCES_DIFF)
+	$(QUIET_GEN)cat $^ >$@
+
+#### style-all-diff-apply: apply the proposed clang-format changes. We
+#### need to have no local changes here, and use FORCE as the
+#### dependency graph of this rule is circular. I.e. we'll modify
+#### git.c, but eventually depend on it as well.
+STYLE_ALL_DIFF_APPLY_EXCLUDE =
+STYLE_ALL_DIFF_APPLY_EXCLUDE += ':!/Makefile'
+STYLE_ALL_DIFF_APPLY_EXCLUDE += ':!/.clang-format'
+style-all-diff-apply: FORCE
+style-all-diff-apply: .build/style-all.diff
+	git diff --quiet HEAD -- $(STYLE_ALL_DIFF_APPLY_EXCLUDE) && \
+	git diff --quiet --cached -- $(STYLE_ALL_DIFF_APPLY_EXCLUDE) && \
+	git apply -p4 $< && \
+	git -P diff --shortstat -- $(STYLE_ALL_DIFF_APPLY_EXCLUDE)
+
 .PHONY: check
 check: $(GENERATED_H)
 	@if sparse; \
-- 
2.37.0.913.g189dca38629