ref namespaces: infrastructure

Add support for dividing the refs of a single repository into multiple
namespaces, each of which can have its own branches, tags, and HEAD.
Git can expose each namespace as an independent repository to pull from
and push to, while sharing the object store, and exposing all the refs
to operations such as git-gc.

Storing multiple repositories as namespaces of a single repository
avoids storing duplicate copies of the same objects, such as when
storing multiple branches of the same source.  The alternates mechanism
provides similar support for avoiding duplicates, but alternates do not
prevent duplication between new objects added to the repositories
without ongoing maintenance, while namespaces do.

To specify a namespace, set the GIT_NAMESPACE environment variable to
the namespace.  For each ref namespace, git stores the corresponding
refs in a directory under refs/namespaces/.  For example,
GIT_NAMESPACE=foo will store refs under refs/namespaces/foo/.  You can
also specify namespaces via the --namespace option to git.

Note that namespaces which include a / will expand to a hierarchy of
namespaces; for example, GIT_NAMESPACE=foo/bar will store refs under
refs/namespaces/foo/refs/namespaces/bar/.  This makes paths in
GIT_NAMESPACE behave hierarchically, so that cloning with
GIT_NAMESPACE=foo/bar produces the same result as cloning with
GIT_NAMESPACE=foo and cloning from that repo with GIT_NAMESPACE=bar.  It
also avoids ambiguity with strange namespace paths such as
foo/refs/heads/, which could otherwise generate directory/file conflicts
within the refs directory.

Add the infrastructure for ref namespaces: handle the GIT_NAMESPACE
environment variable and --namespace option, and support iterating over
refs in a namespace.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Jamey Sharp <jamey@minilop.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -379,6 +379,7 @@ static inline enum object_type object_type(unsigned int mode)
 }
 
 #define GIT_DIR_ENVIRONMENT "GIT_DIR"
+#define GIT_NAMESPACE_ENVIRONMENT "GIT_NAMESPACE"
 #define GIT_WORK_TREE_ENVIRONMENT "GIT_WORK_TREE"
 #define DEFAULT_GIT_DIR_ENVIRONMENT ".git"
 #define DB_ENVIRONMENT "GIT_OBJECT_DIRECTORY"
@@ -419,6 +420,8 @@ extern char *get_object_directory(void);
 extern char *get_index_file(void);
 extern char *get_graft_file(void);
 extern int set_git_dir(const char *path);
+extern const char *get_git_namespace(void);
+extern const char *strip_namespace(const char *namespaced_ref);
 extern const char *get_git_work_tree(void);
 extern const char *read_gitfile_gently(const char *path);
 extern void set_git_work_tree(const char *tree);

contrib/completion/git-completion.bash

@@ -2639,6 +2639,7 @@ _git ()
 			--exec-path
 			--html-path
 			--work-tree=
+			--namespace=
 			--help
 			"
 			;;

environment.c

@@ -8,6 +8,7 @@
  * are.
  */
 #include "cache.h"
+#include "refs.h"
 
 char git_default_email[MAX_GITNAME];
 char git_default_name[MAX_GITNAME];
@@ -65,6 +66,9 @@ int core_preload_index = 0;
 char *git_work_tree_cfg;
 static char *work_tree;
 
+static const char *namespace;
+static size_t namespace_len;
+
 static const char *git_dir;
 static char *git_object_dir, *git_index_file, *git_graft_file;
 
@@ -86,6 +90,27 @@ const char * const local_repo_env[LOCAL_REPO_ENV_SIZE + 1] = {
 	NULL
 };
 
+static char *expand_namespace(const char *raw_namespace)
+{
+	struct strbuf buf = STRBUF_INIT;
+	struct strbuf **components, **c;
+
+	if (!raw_namespace || !*raw_namespace)
+		return xstrdup("");
+
+	strbuf_addstr(&buf, raw_namespace);
+	components = strbuf_split(&buf, '/');
+	strbuf_reset(&buf);
+	for (c = components; *c; c++)
+		if (strcmp((*c)->buf, "/") != 0)
+			strbuf_addf(&buf, "refs/namespaces/%s", (*c)->buf);
+	strbuf_list_free(components);
+	if (check_ref_format(buf.buf) != CHECK_REF_FORMAT_OK)
+		die("bad git namespace path \"%s\"", raw_namespace);
+	strbuf_addch(&buf, '/');
+	return strbuf_detach(&buf, NULL);
+}
+
 static void setup_git_env(void)
 {
 	git_dir = getenv(GIT_DIR_ENVIRONMENT);
@@ -111,6 +136,8 @@ static void setup_git_env(void)
 		git_graft_file = git_pathdup("info/grafts");
 	if (getenv(NO_REPLACE_OBJECTS_ENVIRONMENT))
 		read_replace_refs = 0;
+	namespace = expand_namespace(getenv(GIT_NAMESPACE_ENVIRONMENT));
+	namespace_len = strlen(namespace);
 }
 
 int is_bare_repository(void)
@@ -131,6 +158,20 @@ const char *get_git_dir(void)
 	return git_dir;
 }
 
+const char *get_git_namespace(void)
+{
+	if (!namespace)
+		setup_git_env();
+	return namespace;
+}
+
+const char *strip_namespace(const char *namespaced_ref)
+{
+	if (prefixcmp(namespaced_ref, get_git_namespace()) != 0)
+		return NULL;
+	return namespaced_ref + namespace_len;
+}
+
 static int git_work_tree_initialized;
 
 /*

git.c

@@ -7,8 +7,8 @@
 
 const char git_usage_string[] =
 	"git [--version] [--exec-path[=<path>]] [--html-path] [--man-path] [--info-path]\n"
-	"           [-p|--paginate|--no-pager] [--no-replace-objects]\n"
-	"           [--bare] [--git-dir=<path>] [--work-tree=<path>]\n"
+	"           [-p|--paginate|--no-pager] [--no-replace-objects] [--bare]\n"
+	"           [--git-dir=<path>] [--work-tree=<path>] [--namespace=<name>]\n"
 	"           [-c name=value] [--help]\n"
 	"           <command> [<args>]";
 
@@ -127,6 +127,20 @@ static int handle_options(const char ***argv, int *argc, int *envchanged)
 			setenv(GIT_DIR_ENVIRONMENT, cmd + 10, 1);
 			if (envchanged)
 				*envchanged = 1;
+		} else if (!strcmp(cmd, "--namespace")) {
+			if (*argc < 2) {
+				fprintf(stderr, "No namespace given for --namespace.\n" );
+				usage(git_usage_string);
+			}
+			setenv(GIT_NAMESPACE_ENVIRONMENT, (*argv)[1], 1);
+			if (envchanged)
+				*envchanged = 1;
+			(*argv)++;
+			(*argc)--;
+		} else if (!prefixcmp(cmd, "--namespace=")) {
+			setenv(GIT_NAMESPACE_ENVIRONMENT, cmd + 12, 1);
+			if (envchanged)
+				*envchanged = 1;
 		} else if (!strcmp(cmd, "--work-tree")) {
 			if (*argc < 2) {
 				fprintf(stderr, "No directory given for --work-tree.\n" );

refs.c

@@ -782,6 +782,31 @@ int for_each_replace_ref(each_ref_fn fn, void *cb_data)
 	return do_for_each_ref(NULL, "refs/replace/", fn, 13, 0, cb_data);
 }
 
+int head_ref_namespaced(each_ref_fn fn, void *cb_data)
+{
+	struct strbuf buf = STRBUF_INIT;
+	int ret = 0;
+	unsigned char sha1[20];
+	int flag;
+
+	strbuf_addf(&buf, "%sHEAD", get_git_namespace());
+	if (resolve_ref(buf.buf, sha1, 1, &flag))
+		ret = fn(buf.buf, sha1, flag, cb_data);
+	strbuf_release(&buf);
+
+	return ret;
+}
+
+int for_each_namespaced_ref(each_ref_fn fn, void *cb_data)
+{
+	struct strbuf buf = STRBUF_INIT;
+	int ret;
+	strbuf_addf(&buf, "%srefs/", get_git_namespace());
+	ret = do_for_each_ref(NULL, buf.buf, fn, 0, 0, cb_data);
+	strbuf_release(&buf);
+	return ret;
+}
+
 int for_each_glob_ref_in(each_ref_fn fn, const char *pattern,
 	const char *prefix, void *cb_data)
 {

refs.h

@@ -36,6 +36,9 @@ extern int for_each_tag_ref_submodule(const char *submodule, each_ref_fn fn, voi
 extern int for_each_branch_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data);
 extern int for_each_remote_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data);
 
+extern int head_ref_namespaced(each_ref_fn fn, void *cb_data);
+extern int for_each_namespaced_ref(each_ref_fn fn, void *cb_data);
+
 static inline const char *has_glob_specials(const char *pattern)
 {
 	return strpbrk(pattern, "?*[");