version: add git_user_agent function

This is basically a fancy way of saying "git/$GIT_VERSION",
except that it is overridable at build-time and through the
environment. Which means that people who don't want to
advertise their git version (for privacy or security
reasons) can tweak it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -296,6 +296,9 @@ all::
 # the diff algorithm.  It gives a nice speedup if your processor has
 # fast unaligned word loads.  Does NOT work on big-endian systems!
 # Enabled by default on x86_64.
+#
+# Define GIT_USER_AGENT if you want to change how git identifies itself during
+# network interactions.  The default is "git/$(GIT_VERSION)".
 
 GIT-VERSION-FILE: FORCE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN
@@ -905,6 +908,8 @@ BUILTIN_OBJS += builtin/write-tree.o
 GITLIBS = $(LIB_FILE) $(XDIFF_LIB)
 EXTLIBS =
 
+GIT_USER_AGENT = git/$(GIT_VERSION)
+
 #
 # Platform specific tweaks
 #
@@ -1916,6 +1921,11 @@ SHELL_PATH_CQ_SQ = $(subst ','\'',$(SHELL_PATH_CQ))
 BASIC_CFLAGS += -DSHELL_PATH='$(SHELL_PATH_CQ_SQ)'
 endif
 
+GIT_USER_AGENT_SQ = $(subst ','\'',$(GIT_USER_AGENT))
+GIT_USER_AGENT_CQ = "$(subst ",\",$(subst \,\\,$(GIT_USER_AGENT)))"
+GIT_USER_AGENT_CQ_SQ = $(subst ','\'',$(GIT_USER_AGENT_CQ))
+BASIC_CFLAGS += -DGIT_USER_AGENT='$(GIT_USER_AGENT_CQ_SQ)'
+
 ALL_CFLAGS += $(BASIC_CFLAGS)
 ALL_LDFLAGS += $(BASIC_LDFLAGS)
 
@@ -2000,6 +2010,7 @@ sed -e '1s|#!.*/sh|#!$(SHELL_PATH_SQ)|' \
     -e 's|@SHELL_PATH@|$(SHELL_PATH_SQ)|' \
     -e 's|@@DIFF@@|$(DIFF_SQ)|' \
     -e 's/@@GIT_VERSION@@/$(GIT_VERSION)/g' \
+    -e 's|@@GIT_USER_AGENT@@|$(GIT_USER_AGENT_SQ)|g' \
     -e 's|@@LOCALEDIR@@|$(localedir_SQ)|g' \
     -e 's/@@NO_CURL@@/$(NO_CURL)/g' \
     -e 's/@@USE_GETTEXT_SCHEME@@/$(USE_GETTEXT_SCHEME)/g' \

version.c

@@ -2,3 +2,16 @@
 #include "version.h"
 
 const char git_version_string[] = GIT_VERSION;
+
+const char *git_user_agent(void)
+{
+	static const char *agent = NULL;
+
+	if (!agent) {
+		agent = getenv("GIT_USER_AGENT");
+		if (!agent)
+			agent = GIT_USER_AGENT;
+	}
+
+	return agent;
+}

version.h

@@ -3,4 +3,6 @@
 
 extern const char git_version_string[];
 
+const char *git_user_agent(void);
+
 #endif /* VERSION_H */