Makefile.tracer

## Usage
#
# TLDR:
# 	Run `make -f <path_to_this_makefile>`
#   and take a look at your 'auto-tracer' branch
#
# Long:
# 	1. Include this Makefile at the end of your project Makefile
# 	2. Add `$(call git_commit, "commit message")` to your target
# 	3. Run `make` and your modifications will be traced when your target is run
#   See also the attached `Makefile` for an example

## Configuration
#
# TRACER_BRANCH will be the branch name, change it to any name you like :)
TRACER_BRANCH = auto-tracer
# GITFLAGS is the flags for git commit, without commit message
GITFLAGS = -q --author='auto-tracer <auto-tracer@saves.me>' --no-verify --allow-empty

THIS_PATH = $(abspath $(lastword $(MAKEFILE_LIST)))

# PROJ_HOME defaluts to the toplevel dir of current Git repo, which will be $PWD of following `git` runs
PROJ_HOME = $(abspath $(shell git rev-parse --show-toplevel))
# GIT_DIR is the `.git` dir of current Git repo, this should work fine with submodules
GIT_DIR   = $(abspath $(shell git rev-parse --git-dir))

# WORK_BRANCH is the previous branch we were working on, or a commit hash if detached
# this is used to checkout back to where we depart, it cannot be empty or just literally HEAD 
WORK_BRANCH = $(shell git symbolic-ref --short HEAD 2>/dev/null || git rev-parse HEAD)

INDEX_LOCK = $(GIT_DIR)/index.lock
CURR_INDEX = $(GIT_DIR)/index
WORK_INDEX = $(GIT_DIR)/index-$(WORK_BRANCH)

# Default target of this Makefile
trace:
	$(call git_commit, "tracer called")

# prototype: git_soft_checkout(branch)
define git_soft_checkout
	git checkout --detach -q && git reset --soft $(1) -q -- && git checkout $(1) -q --
endef

# prototype: git_commit(msg)
define git_commit
	-@flock $(GIT_DIR) $(MAKE) -C $(PROJ_HOME) -f $(THIS_PATH) .git_commit MSG='$(1)'
	-@sync $(GIT_DIR)
endef

.git_commit:
	-@while (test -e $(INDEX_LOCK)); do sleep 0.1; done;                  `# wait for other git instances`
	-@git branch $(TRACER_BRANCH) -q 2>/dev/null || true                  `# create tracer branch if not existent`
	-@cp -a $(CURR_INDEX) $(WORK_INDEX)                                   `# backup git index`
	-@$(call git_soft_checkout, $(TRACER_BRANCH))                         `# switch to tracer branch`
	-@git add . -A --ignore-errors                                        `# add files to commit`
	-@(echo "> $(MSG)" && echo "$$(whoami) $$(pwd)" && uname -a && uptime `# generate commit msg`) \
	                | git commit -F - $(GITFLAGS)                         `# commit changes in tracer branch`
	-@$(call git_soft_checkout, $(WORK_BRANCH))                           `# switch to work branch`
	-@mv $(WORK_INDEX) $(CURR_INDEX)                                      `# restore git index`

.clean_index:
	rm -f $(WORK_INDEX)

.PHONY: .git_commit .clean_index trace