[PATCH] simple submodule checkout support

@@ -37,6 +37,7 @@ git-diff
 git-diff-files
 git-diff-index
 git-diff-tree
+git-dirlink-checkout
 git-describe
 git-fast-import
 git-fetch

@@ -196,7 +196,7 @@ SCRIPT_SH = \
 	git-merge-one-file.sh git-mergetool.sh git-parse-remote.sh \
 	git-pull.sh git-rebase.sh \
 	git-repack.sh git-request-pull.sh git-reset.sh \
-	git-sh-setup.sh \
+	git-sh-setup.sh git-dirlink-checkout.sh \
 	git-tag.sh git-verify-tag.sh \
 	git-applymbox.sh git-applypatch.sh git-am.sh \
 	git-merge.sh git-merge-stupid.sh git-merge-octopus.sh \

@@ -1,5 +1,6 @@
 #include "cache.h"
 #include "blob.h"
+#include "run-command.h"
 
 static void create_directories(const char *path, const struct checkout *state)
 {

@@ -79,6 +80,13 @@ static int write_entry(struct cache_entry *ce, char *path, const struct checkout
 {
 	int fd;
 	long wrote;
+	const char *dirlink_checkout[] = {
+		"dirlink-checkout", path, sha1_to_hex(ce->sha1), NULL
+	};
+	struct child_process cmd = {
+		.argv = dirlink_checkout,
+		.git_cmd = 1
+	};
 
 	switch (ntohl(ce->ce_mode) & S_IFMT) {
 		char *buf, *new;

@@ -148,7 +156,7 @@ static int write_entry(struct cache_entry *ce, char *path, const struct checkout
 	case S_IFDIRLNK:
 		if (to_tempfile)
 			return error("git-checkout-index: cannot create temporary subproject %s", path);
-		if (mkdir(path, 0777) < 0)
+		if (run_command(&cmd) != 0)
 			return error("git-checkout-index: cannot create subproject directory %s", path);
 		break;
 	default:

@@ -192,10 +200,8 @@ int checkout_entry(struct cache_entry *ce, const struct checkout *state, char *t
 		 * just do the right thing)
 		 */
 		unlink(path);
-		if (S_ISDIR(st.st_mode)) {
-			/* If it is a gitlink, leave it alone! */
-			if (S_ISDIRLNK(ntohl(ce->ce_mode)))
-				return 0;
+		if (S_ISDIR(st.st_mode) &&
+		    access(mkpath("%s/.git/HEAD", path), R_OK)!=0) {
 			if (!state->force)
 				return error("%s is a directory", path);
 			remove_subtree(path);

@@ -0,0 +1,27 @@
+#!/bin/sh -e
+
+USAGE="<dirlink-path> <new-ref>"
+
+path="$1"
+sha1="$2"
+
+test -n "$sha1" || usage
+unset GIT_DIR
+
+if test ! -d "$path/.git"; then
+	mkdir "$path"
+	cd "$path"
+	# FIXME talk about what the user can do here
+	echo "This git submodule has not been populated yet." > README.git
+	exit 0
+fi
+
+cd "$path"
+. git-sh-setup
+
+if git rev-parse --verify "$sha1" >/dev/null 2>&1; then
+	git checkout "$sha1"
+else
+	# FIXME try to fetch it
+	echo >&2 "Cannot checkout $sha1 for $path."
+fi

@@ -0,0 +1,39 @@
+#!/bin/sh
+
+test_description='submodule checkout'
+. ./test-lib.sh
+
+test_expect_success 'submodule creation' \
+    '(mkdir A && cd A &&
+      git init &&
+      echo 1 > a &&
+      git add a &&
+      git commit -m "create submodule" || exit $? )'
+
+test_expect_success 'Super module creation' \
+    'git add A &&
+     git commit -m "supermodule creation" &&
+     git branch one'
+
+test_expect_success 'submodule change' \
+    '(cd A &&
+      echo 2 > a &&
+      git add a &&
+      git commit -m "create submodule" || exit $? )'
+
+test_expect_success 'supermodule change' \
+    'git add A &&
+     git commit -m "supermodule creation"'
+
+test_expect_success 'supermodule switching branch' \
+    'git checkout one &&
+     echo 1 > expected &&
+     git diff expected A/a'
+
+test_expect_success 'supermodule reset' \
+    'git reset --hard master &&
+     echo 2 > expected &&
+     git diff expected A/a'
+
+
+test_done