From 7949cd6ebce3cf985e283cff28db56b4eebac5c8 Mon Sep 17 00:00:00 2001
From: Stefan Hajnoczi <stefanha@redhat.com>
Date: Wed, 17 Apr 2019 11:21:29 +0100
Subject: [PATCH] build: turn COMMIT into a file dependency

Makefile uses $(shell) to build a git commit string.  Unfortunately this
means make targets cannot be rebuilt when COMMIT changes value.  We need
to reflect this string value into files on which make can process
dependencies.

I stole a solution from QEMU's Makefile:
1. Print the string into .git-commit.tmp
2. If .git-commit.tmp differs from .git-commit, copy it to .git-commit
3. Depend on .git-commit from all targets that need $COMMIT

This way targets are only rebuilt if the commit string value actually
changes.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 .gitignore |  2 ++
 Makefile   | 22 +++++++++++++++-------
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/.gitignore b/.gitignore
index 7b6eee072d..79ceb23872 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,6 +1,8 @@
 *.o
 *.patch
 *.swp
+.git-commit
+.git-commit.tmp
 /cli/config/configuration-fc.toml
 /cli/config/configuration-qemu.toml
 /cli/config-generated.go
diff --git a/Makefile b/Makefile
index 2137bb9783..e68912f6f4 100644
--- a/Makefile
+++ b/Makefile
@@ -182,8 +182,16 @@ SHIMV2_DIR = $(CLI_DIR)/$(SHIMV2)
 
 SOURCES := $(shell find . 2>&1 | grep -E '.*\.(c|h|go)$$')
 VERSION := ${shell cat ./VERSION}
-COMMIT_NO := $(shell git rev-parse HEAD 2> /dev/null || true)
-COMMIT := $(if $(shell git status --porcelain --untracked-files=no),${COMMIT_NO}-dirty,${COMMIT_NO})
+
+# Targets that depend on .git-commit can use $(shell cat .git-commit) to get a
+# git revision string.  They will only be rebuilt if the revision string
+# actually changes.
+.PHONY: .git-commit.tmp
+.git-commit: .git-commit.tmp
+	@cmp $< $@ >/dev/null 2>&1 || cp $< $@
+.git-commit.tmp:
+	@echo -n "$$(git rev-parse HEAD 2>/dev/null)" >$@
+	@test -n "$$(git status --porcelain --untracked-files=no)" && echo -n "-dirty" >>$@ || true
 
 # List of configuration files to build and install
 CONFIGS =
@@ -414,9 +422,9 @@ $(TARGET).coverage: $(SOURCES) $(GENERATED_FILES) $(MAKEFILE_LIST)
 
 GENERATED_FILES += $(CONFIGS)
 
-$(GENERATED_FILES): %: %.in $(MAKEFILE_LIST) VERSION
+$(GENERATED_FILES): %: %.in $(MAKEFILE_LIST) VERSION .git-commit
 	$(QUIET_GENERATE)$(SED) \
-		-e "s|@COMMIT@|$(COMMIT)|g" \
+		-e "s|@COMMIT@|$(shell cat .git-commit)|g" \
 		-e "s|@VERSION@|$(VERSION)|g" \
 		-e "s|@CONFIG_QEMU_IN@|$(CONFIG_QEMU_IN)|g" \
 		-e "s|@CONFIG_FC_IN@|$(CONFIG_FC_IN)|g" \
@@ -510,7 +518,7 @@ install-completions:
 	$(QUIET_INST)install --mode 0644 -D  $(BASH_COMPLETIONS) $(DESTDIR)/$(BASH_COMPLETIONSDIR)/$(notdir $(BASH_COMPLETIONS));
 
 clean:
-	$(QUIET_CLEAN)rm -f $(TARGET) $(SHIMV2) $(NETMON_TARGET) $(CONFIGS) $(GENERATED_GO_FILES) $(GENERATED_FILES)
+	$(QUIET_CLEAN)rm -f $(TARGET) $(SHIMV2) $(NETMON_TARGET) $(CONFIGS) $(GENERATED_FILES) .git-commit .git-commit.tmp
 
 show-usage: show-header
 	@printf "• Overview:\n"
@@ -549,8 +557,8 @@ show-variables:
           "$(foreach v,$(sort $(USER_VARS)),$(shell printf "\\t$(v)='$($(v))'\\\n"))"
 	@printf "\n"
 
-show-header:
-	@printf "%s - version %s (commit %s)\n\n" $(TARGET) $(VERSION) $(COMMIT)
+show-header: .git-commit
+	@printf "%s - version %s (commit %s)\n\n" $(TARGET) $(VERSION) $(shell cat .git-commit)
 
 show-arches: show-header
 	@printf "Supported architectures (possible values for ARCH variable):\n\n"