$GIT_COMMON_DIR: a new environment variable

This variable is intended to support multiple working directories
attached to a repository. Such a repository may have a main working
directory, created by either "git init" or "git clone" and one or more
linked working directories. These working directories and the main
repository share the same repository directory.

In linked working directories, $GIT_COMMON_DIR must be defined to point
to the real repository directory and $GIT_DIR points to an unused
subdirectory inside $GIT_COMMON_DIR. File locations inside the
repository are reorganized from the linked worktree view point:

 - worktree-specific such as HEAD, logs/HEAD, index, other top-level
   refs and unrecognized files are from $GIT_DIR.

 - the rest like objects, refs, info, hooks, packed-refs, shallow...
   are from $GIT_COMMON_DIR (except info/sparse-checkout, but that's
   a separate patch)

Scripts are supposed to retrieve paths in $GIT_DIR with "git rev-parse
--git-path", which will take care of "$GIT_DIR vs $GIT_COMMON_DIR"
business.

The redirection is done by git_path(), git_pathdup() and
strbuf_git_path(). The selected list of paths goes to $GIT_COMMON_DIR,
not the other way around in case a developer adds a new
worktree-specific file and it's accidentally promoted to be shared
across repositories (this includes unknown files added by third party
commands)

The list of known files that belong to $GIT_DIR are:

ADD_EDIT.patch BISECT_ANCESTORS_OK BISECT_EXPECTED_REV BISECT_LOG
BISECT_NAMES CHERRY_PICK_HEAD COMMIT_MSG FETCH_HEAD HEAD MERGE_HEAD
MERGE_MODE MERGE_RR NOTES_EDITMSG NOTES_MERGE_WORKTREE ORIG_HEAD
REVERT_HEAD SQUASH_MSG TAG_EDITMSG fast_import_crash_* logs/HEAD
next-index-* rebase-apply rebase-merge rsync-refs-* sequencer/*
shallow_*

Path mapping is NOT done for git_path_submodule(). Multi-checkouts are
not supported as submodules.

Helped-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 34 insertions, 0 deletions

diff --git a/path.c b/path.c
index 4910783..94db501 100644
--- a/path.c
+++ b/path.c
@@ -90,6 +90,38 @@ static void replace_dir(struct strbuf *buf, int len, const char *newdir)
 		buf->buf[newlen] = '/';
 }
 
+static const char *common_list[] = {
+	"/branches", "/hooks", "/info", "/logs", "/lost-found", "/modules",
+	"/objects", "/refs", "/remotes", "/rr-cache", "/svn",
+	"config", "gc.pid", "packed-refs", "shallow",
+	NULL
+};
+
+static void update_common_dir(struct strbuf *buf, int git_dir_len)
+{
+	char *base = buf->buf + git_dir_len;
+	const char **p;
+
+	if (is_dir_file(base, "logs", "HEAD"))
+		return;	/* keep this in $GIT_DIR */
+	for (p = common_list; *p; p++) {
+		const char *path = *p;
+		int is_dir = 0;
+		if (*path == '/') {
+			path++;
+			is_dir = 1;
+		}
+		if (is_dir && dir_prefix(base, path)) {
+			replace_dir(buf, git_dir_len, get_git_common_dir());
+			return;
+		}
+		if (!is_dir && !strcmp(base, path)) {
+			replace_dir(buf, git_dir_len, get_git_common_dir());
+			return;
+		}
+	}
+}
+
 static void adjust_git_path(struct strbuf *buf, int git_dir_len)
 {
 	const char *base = buf->buf + git_dir_len;
@@ -101,6 +133,8 @@ static void adjust_git_path(struct strbuf *buf, int git_dir_len)
 			      get_index_file(), strlen(get_index_file()));
 	else if (git_db_env && dir_prefix(base, "objects"))
 		replace_dir(buf, git_dir_len + 7, get_object_directory());
+	else if (git_common_dir_env)
+		update_common_dir(buf, git_dir_len);
 }
 
 static void do_git_path(struct strbuf *buf, const char *fmt, va_list args)