[PATCH 11/13] Makefile: auto-build C strings from make variables

[Jeff King <peff@peff.net>]

We already insert the values of some make variables into
files in MAKE/*. We can therefore build a simple pattern
rule for converting such a value into a C string in a header
file, which can then be included and used as normal.

The new system is demonstrated on version.c, where it can
replace the use of "-D" on the command-line. Note that we
still have to manually specify a dependency in the
Makefile. In an ideal world, we would auto-detect the header
dependency; however, there are two holdups:

  1. We cannot rely on having automatic header dependency
     generation on all platforms.

  2. Even when we do generate the dependencies, we rely on
     being able to compile the file once, which means our
     system cannot handle generated headers without a manual
     dependency to bootstrap it.

Signed-off-by: Jeff King <peff@peff.net>
---
This is a technique that I have used in other projects with great
success, as the make variable dependencies are represented as file
dependencies (which is the only type of dependency make knows about).

_But_ it ends up a lot less nice here because of the way we do
auto-dependencies. I'm totally open to revamping the way we handle our
header dependencies, but I didn't do that in this series.

 Makefile         | 11 +++++++----
 script/mkcstring | 18 ++++++++++++++++++
 version.c        |  6 ++++--
 3 files changed, 29 insertions(+), 6 deletions(-)
 create mode 100644 script/mkcstring

diff --git a/Makefile b/Makefile
index cd07a70..203171d 100644
--- a/Makefile
+++ b/Makefile
@@ -1593,6 +1593,11 @@ MAKE/$1: FORCE
 	fi
 endef
 
+MAKE/%-string.h: MAKE/% script/mkcstring
+	$(QUIET_GEN)$(SHELL_PATH) script/mkcstring \
+		$(subst -,_,$*) <$< >$@+ && \
+		mv $@+ $@
+
 LIBS = $(GITLIBS) $(EXTLIBS)
 
 BASIC_CFLAGS += -DSHA1_HEADER=$(call sq,$(SHA1_HEADER)) \
@@ -1614,6 +1619,7 @@ BASIC_CFLAGS += -DSHELL_PATH=$(call scq,$(SHELL_PATH))
 endif
 
 $(eval $(call make-var,USER-AGENT,user agent string,$(GIT_USER_AGENT)))
+$(eval $(call make-var,VERSION,version,$(GIT_VERSION)))
 
 ifdef DEFAULT_HELP_FORMAT
 BASIC_CFLAGS += -DDEFAULT_HELP_FORMAT='"$(DEFAULT_HELP_FORMAT)"'
@@ -1713,10 +1719,7 @@ builtin/help.sp builtin/help.s builtin/help.o: EXTRA_CPPFLAGS = \
 	-DGIT_MAN_PATH=$(call scq,$(mandir_relative)) \
 	-DGIT_INFO_PATH=$(call scq,$(infodir_relative))
 
-version.sp version.s version.o: GIT-VERSION-FILE MAKE/USER-AGENT
-version.sp version.s version.o: EXTRA_CPPFLAGS = \
-	-DGIT_VERSION=$(call scq,$(GIT_VERSION)) \
-	-DGIT_USER_AGENT=$(call scq,$(GIT_USER_AGENT))
+version.sp version.s version.o: MAKE/VERSION-string.h MAKE/USER-AGENT-string.h
 
 $(BUILT_INS): git$X
 	$(QUIET_BUILT_IN)$(RM) $@ && \
diff --git a/script/mkcstring b/script/mkcstring
new file mode 100644
index 0000000..c01f430
--- /dev/null
+++ b/script/mkcstring
@@ -0,0 +1,18 @@
+#!/bin/sh
+
+name=$1; shift
+
+c_quote() {
+	sed 's/\\/\\\\/g; s/"/\\"/'
+}
+
+cat <<-EOF
+#ifndef MAKE_${name}_H
+#define MAKE_${name}_H
+
+/* Auto-generated by mkcstring */
+
+#define MAKE_${name} "$(c_quote)"
+
+#endif /* MAKE_${name}_H */
+EOF
diff --git a/version.c b/version.c
index 6106a80..f68a93b 100644
--- a/version.c
+++ b/version.c
@@ -1,8 +1,10 @@
 #include "git-compat-util.h"
 #include "version.h"
 #include "strbuf.h"
+#include "MAKE/USER-AGENT-string.h"
+#include "MAKE/VERSION-string.h"
 
-const char git_version_string[] = GIT_VERSION;
+const char git_version_string[] = MAKE_VERSION;
 
 const char *git_user_agent(void)
 {
@@ -11,7 +13,7 @@ const char *git_user_agent(void)
 	if (!agent) {
 		agent = getenv("GIT_USER_AGENT");
 		if (!agent)
-			agent = GIT_USER_AGENT;
+			agent = MAKE_USER_AGENT;
 	}
 
 	return agent;
-- 
1.8.5.2.500.g8060133