read-cache: add index.version config variable

Add a config variable that allows setting the default index version when
initializing a new index file.  Similar to the GIT_INDEX_VERSION
environment variable this only affects new index files.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1601,6 +1601,10 @@ imap::
 	The configuration variables in the 'imap' section are described
 	in linkgit:git-imap-send[1].
 
+index.version::
+	Specify the version with which new index files should be
+	initialized.  This does not affect existing repositories.
+
 init.templatedir::
 	Specify the directory from which templates will be copied.
 	(See the "TEMPLATE DIRECTORY" section of linkgit:git-init[1].)

read-cache.c

@@ -1219,23 +1219,40 @@ static struct cache_entry *refresh_cache_entry(struct cache_entry *ce, int reall
 
 #define INDEX_FORMAT_DEFAULT 3
 
+static int index_format_config(const char *var, const char *value, void *cb)
+{
+	unsigned int *version = cb;
+	if (!strcmp(var, "index.version")) {
+		*version = git_config_int(var, value);
+		return 0;
+	}
+	return 1;
+}
+
 static unsigned int get_index_format_default(void)
 {
 	char *envversion = getenv("GIT_INDEX_VERSION");
-	if (!envversion) {
-		return INDEX_FORMAT_DEFAULT;
-	} else {
-		char *endp;
-		unsigned int version = strtoul(envversion, &endp, 10);
+	char *endp;
+	unsigned int version = INDEX_FORMAT_DEFAULT;
 
-		if (*endp ||
-		    version < INDEX_FORMAT_LB || INDEX_FORMAT_UB < version) {
-			warning(_("GIT_INDEX_VERSION set, but the value is invalid.\n"
+	if (!envversion) {
+		git_config(index_format_config, &version);
+		if (version < INDEX_FORMAT_LB || INDEX_FORMAT_UB < version) {
+			warning(_("index.version set, but the value is invalid.\n"
 				  "Using version %i"), INDEX_FORMAT_DEFAULT);
-			version = INDEX_FORMAT_DEFAULT;
+			return INDEX_FORMAT_DEFAULT;
 		}
 		return version;
 	}
+
+	version = strtoul(envversion, &endp, 10);
+	if (*endp ||
+	    version < INDEX_FORMAT_LB || INDEX_FORMAT_UB < version) {
+		warning(_("GIT_INDEX_VERSION set, but the value is invalid.\n"
+			  "Using version %i"), INDEX_FORMAT_DEFAULT);
+		version = INDEX_FORMAT_DEFAULT;
+	}
+	return version;
 }
 
 /*

t/t1600-index.sh

@@ -46,4 +46,31 @@ test_expect_success 'no warning with bogus GIT_INDEX_VERSION and existing index'
 	)
 '
 
+test_expect_success 'out of bounds index.version issues warning' '
+	(
+		sane_unset GIT_INDEX_VERSION &&
+		rm -f .git/index &&
+		git config --add index.version 1 &&
+		git add a 2>&1 | sed "s/[0-9]//" >actual.err &&
+		sed -e "s/ Z$/ /" <<-\EOF >expect.err &&
+			warning: index.version set, but the value is invalid.
+			Using version Z
+		EOF
+		test_i18ncmp expect.err actual.err
+	)
+'
+
+test_expect_success 'GIT_INDEX_VERSION takes precedence over config' '
+	(
+		rm -f .git/index &&
+		GIT_INDEX_VERSION=4 &&
+		export GIT_INDEX_VERSION &&
+		git config --add index.version 2 &&
+		git add a 2>&1 &&
+		echo 4 >expect &&
+		test-index-version <.git/index >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done