[PATCH v2] Makefile: add support for generating JSON compilation database

["Philippe Blain via GitGitGadget" <gitgitgadget@gmail.com>]

From: Philippe Blain <levraiphilippeblain@gmail.com>

Tools based on LibClang [1] can make use of a 'JSON Compilation
Database' [2] that keeps track of the exact options used to compile a set
of source files.

For example, clangd [3], which is a C language server protocol
implementation, can use a JSON compilation database to determine the
flags needed to compile a file so it can provide proper editor
integration.  As a result, editors supporting the language server
protocol (such as VS Code, Emacs, or Vim, with suitable plugins) can
provide better searching, integration, and refactoring tools.

The Clang compiler can generate JSON fragments when compiling [4],
using the `-MJ` flag. These JSON fragments (one per compiled source
file) can then be concatenated to create the compilation database,
commonly called 'compile_commands.json'.

Add support to the Makefile for generating these JSON fragments as well
as the compilation database itself, if the environment variable
'GENERATE_COMPILATION_DATABASE' is set.

If this variable is set, check that $(CC) indeed supports the `-MJ`
flag, following what is done for automatic dependencies.

All JSON fragments are placed in the 'compile_commands/' directory, and
the compilation database 'compile_commands.json' is generated as a
dependency of the 'all' target using a `sed` invocation.

[1] https://clang.llvm.org/docs/Tooling.html
[2] https://clang.llvm.org/docs/JSONCompilationDatabase.html
[3] https://clangd.llvm.org/
[4] https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mj-arg

Helped-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
---
    Add support for generating JSON compilation database
    
    Changes since v1:
    
     * Added a paragraph to the commit message to better explain why this
       feature would help Git developers (Thanks Brian!)
     * Corrected the description of the new GENERATE_COMPILATION_DATABASE 
       Makefile knob to make it reflect its actual behavior (cf. 
       https://lore.kernel.org/git/FC95CFF7-F9DA-4CDA-9923-99C3432DCAD5@gmail.com/
       )
    
    v1: I don't have a lot of knowledge of Make double-colon rules, or
    insight into why they are used for the 'all' target, but I think the
    approach I chose makes sense. In particular, I do not list any
    prerequisite for the 'compile_commands.json' file, but from what I
    tested it is still rebuilt anytime the 'all' target is rebuilt, which is
    what we want.
    
    Note: CMakeLists.txt in contrib/buildsystems does not need to be updated
    to also support this feature because CMake supports it out-of-the-box
    [1].
    
    [1] 
    https://cmake.org/cmake/help/latest/variable/CMAKE_EXPORT_COMPILE_COMMANDS.html

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-714%2Fphil-blain%2Fcompiledb-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-714/phil-blain/compiledb-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/714

Range-diff vs v1:

 1:  411fefaafb ! 1:  da9544e4aa Makefile: add support for generating JSON compilation database
     @@ Commit message
          Database' [2] that keeps track of the exact options used to compile a set
          of source files.
      
     -    The Clang compiler can generate JSON fragments when compiling [3],
     +    For example, clangd [3], which is a C language server protocol
     +    implementation, can use a JSON compilation database to determine the
     +    flags needed to compile a file so it can provide proper editor
     +    integration.  As a result, editors supporting the language server
     +    protocol (such as VS Code, Emacs, or Vim, with suitable plugins) can
     +    provide better searching, integration, and refactoring tools.
     +
     +    The Clang compiler can generate JSON fragments when compiling [4],
          using the `-MJ` flag. These JSON fragments (one per compiled source
          file) can then be concatenated to create the compilation database,
          commonly called 'compile_commands.json'.
     @@ Commit message
      
          [1] https://clang.llvm.org/docs/Tooling.html
          [2] https://clang.llvm.org/docs/JSONCompilationDatabase.html
     -    [3] https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mj-arg
     +    [3] https://clangd.llvm.org/
     +    [4] https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mj-arg
      
     +    Helped-by: brian m. carlson <sandals@crustytoothpaste.net>
          Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
      
       ## .gitignore ##
     @@ Makefile: all::
      +# Define GENERATE_COMPILATION_DATABASE to generate JSON compilation database
      +# entries during compilation if your compiler supports it, using the `-MJ` flag.
      +# The JSON entries will be placed in the `compile_commands/` directory,
     -+# and the JSON compilation database can be created afterwards with
     -+# `make compile_commands.json`.
     ++# and the JSON compilation database 'compile_commands.json' will be created 
     ++# at the root of the repository. 
      +#
       # Define DEVELOPER to enable more compiler warnings. Compiler version
       # and family are auto detected, but could be overridden by defining


 .gitignore |  2 ++
 Makefile   | 52 +++++++++++++++++++++++++++++++++++++++++++++++-----
 2 files changed, 49 insertions(+), 5 deletions(-)

diff --git a/.gitignore b/.gitignore
index ee509a2ad2..f4c51300e0 100644
--- a/.gitignore
+++ b/.gitignore
@@ -197,6 +197,7 @@
 /git.spec
 *.exe
 *.[aos]
+*.o.json
 *.py[co]
 .depend/
 *.gcda
@@ -218,6 +219,7 @@
 /tags
 /TAGS
 /cscope*
+/compile_commands.json
 *.hcc
 *.obj
 *.lib
diff --git a/Makefile b/Makefile
index 65f8cfb236..51cd0f302b 100644
--- a/Makefile
+++ b/Makefile
@@ -462,6 +462,12 @@ all::
 # the global variable _wpgmptr containing the absolute path of the current
 # executable (this is the case on Windows).
 #
+# Define GENERATE_COMPILATION_DATABASE to generate JSON compilation database
+# entries during compilation if your compiler supports it, using the `-MJ` flag.
+# The JSON entries will be placed in the `compile_commands/` directory,
+# and the JSON compilation database 'compile_commands.json' will be created 
+# at the root of the repository. 
+#
 # Define DEVELOPER to enable more compiler warnings. Compiler version
 # and family are auto detected, but could be overridden by defining
 # COMPILER_FEATURES (see config.mak.dev). You can still set
@@ -1258,6 +1264,20 @@ $(error please set COMPUTE_HEADER_DEPENDENCIES to yes, no, or auto \
 endif
 endif
 
+ifdef GENERATE_COMPILATION_DATABASE
+compdb_check = $(shell $(CC) $(ALL_CFLAGS) \
+	-c -MJ /dev/null \
+	-x c /dev/null -o /dev/null 2>&1; \
+	echo $$?)
+ifeq ($(compdb_check),0)
+override GENERATE_COMPILATION_DATABASE = yes
+else
+override GENERATE_COMPILATION_DATABASE = no
+$(warning GENERATE_COMPILATION_DATABASE is set, but your compiler does not \
+support generating compilation database entries)
+endif
+endif
+
 ifdef SANE_TOOL_PATH
 SANE_TOOL_PATH_SQ = $(subst ','\'',$(SANE_TOOL_PATH))
 BROKEN_PATH_FIX = 's|^\# @@BROKEN_PATH_FIX@@$$|git_broken_path_fix "$(SANE_TOOL_PATH_SQ)"|'
@@ -2381,16 +2401,30 @@ missing_dep_dirs =
 dep_args =
 endif
 
+compdb_dir = compile_commands/
+
+ifeq ($(GENERATE_COMPILATION_DATABASE),yes)
+missing_compdb_dir = $(compdb_dir)
+$(missing_compdb_dir):
+	@mkdir -p $@
+
+compdb_file = $(compdb_dir)$(subst .-,,$(subst /,-,$(dir $@)))$(notdir $@).json
+compdb_args = -MJ $(compdb_file)
+else
+missing_compdb_dir =
+compdb_args =
+endif
+
 ASM_SRC := $(wildcard $(OBJECTS:o=S))
 ASM_OBJ := $(ASM_SRC:S=o)
 C_OBJ := $(filter-out $(ASM_OBJ),$(OBJECTS))
 
 .SUFFIXES:
 
-$(C_OBJ): %.o: %.c GIT-CFLAGS $(missing_dep_dirs)
-	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) $<
-$(ASM_OBJ): %.o: %.S GIT-CFLAGS $(missing_dep_dirs)
-	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) $<
+$(C_OBJ): %.o: %.c GIT-CFLAGS $(missing_dep_dirs) $(missing_compdb_dir)
+	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(compdb_args) $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) $<
+$(ASM_OBJ): %.o: %.S GIT-CFLAGS $(missing_dep_dirs) $(missing_compdb_dir)
+	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(compdb_args) $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) $<
 
 %.s: %.c GIT-CFLAGS FORCE
 	$(QUIET_CC)$(CC) -o $@ -S $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) $<
@@ -2413,6 +2447,14 @@ else
 $(OBJECTS): $(LIB_H) $(GENERATED_H)
 endif
 
+ifeq ($(GENERATE_COMPILATION_DATABASE),yes)
+all:: compile_commands.json
+compile_commands.json:
+	@$(RM) $@
+	$(QUIET_GEN)sed -e '1s/^/[/' -e '$$s/,$$/]/' $(compdb_dir)*.o.json > $@+
+	@if test -s $@+; then mv $@+ $@; else $(RM) $@+; fi
+endif
+
 exec-cmd.sp exec-cmd.s exec-cmd.o: GIT-PREFIX
 exec-cmd.sp exec-cmd.s exec-cmd.o: EXTRA_CPPFLAGS = \
 	'-DGIT_EXEC_PATH="$(gitexecdir_SQ)"' \
@@ -3117,7 +3159,7 @@ clean: profile-clean coverage-clean cocciclean
 	$(RM) $(TEST_PROGRAMS)
 	$(RM) $(FUZZ_PROGRAMS)
 	$(RM) $(HCC)
-	$(RM) -r bin-wrappers $(dep_dirs)
+	$(RM) -r bin-wrappers $(dep_dirs) $(compdb_dir) compile_commands.json
 	$(RM) -r po/build/
 	$(RM) *.pyc *.pyo */*.pyc */*.pyo $(GENERATED_H) $(ETAGS_TARGET) tags cscope*
 	$(RM) -r $(GIT_TARNAME) .doc-tmp-dir

base-commit: d9cd4331470f4d9d78677f12dc79063dab832f53
-- 
gitgitgadget