Add a place for (not) sharing stuff between worktrees

When multiple worktrees are used, we need rules to determine if
something belongs to one worktree or all of them. Instead of keeping
adding rules when new stuff comes (*), have a generic rule:

- Inside $GIT_DIR, which is per-worktree by default, add
  $GIT_DIR/common which is always shared. New features that want to
  share stuff should put stuff under this directory.

- Inside refs/, which is shared by default except refs/bisect, add
  refs/worktree/ which is per-worktree. We may eventually move
  refs/bisect to this new location and remove the exception in refs
  code.

(*) And it may also include stuff from external commands which will
    have no way to modify common/per-worktree rules.

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

2 files changed, 27 insertions, 3 deletions

diff --git a/Documentation/git-worktree.txt b/Documentation/git-worktree.txt
index e2ee9fc21b..a50fbf8094 100644
--- a/Documentation/git-worktree.txt
+++ b/Documentation/git-worktree.txt
@@ -204,6 +204,22 @@ working trees, it can be used to identify worktrees. For example if
 you only have two working trees, at "/abc/def/ghi" and "/abc/def/ggg",
 then "ghi" or "def/ghi" is enough to point to the former working tree.
 
+REFS
+----
+In multiple working trees, some refs may be shared between all working
+trees, some refs are local. One example is HEAD is different for all
+working trees. This section is about the sharing rules.
+
+In general, all pseudo refs are per working tree and all refs starting
+with "refs/" are shared. Pseudo refs are ones like HEAD which are
+directly under GIT_DIR instead of inside GIT_DIR/refs. There are one
+exception to this: refs inside refs/bisect and refs/worktree is not
+shared.
+
+To access refs, it's best not to look inside GIT_DIR directly. Instead
+use commands such as linkgit:git-revparse[1] or linkgit:git-update-ref[1]
+which will handle refs correctly.
+
 DETAILS
 -------
 Each linked working tree has a private sub-directory in the repository's
@@ -228,7 +244,8 @@ linked working tree `git rev-parse --git-path HEAD` returns
 `/path/other/test-next/.git/HEAD` or `/path/main/.git/HEAD`) while `git
 rev-parse --git-path refs/heads/master` uses
 $GIT_COMMON_DIR and returns `/path/main/.git/refs/heads/master`,
-since refs are shared across all working trees.
+since refs are shared across all working trees, except refs/bisect and
+refs/worktree.
 
 See linkgit:gitrepository-layout[5] for more information. The rule of
 thumb is do not make any assumption about whether a path belongs to
diff --git a/Documentation/gitrepository-layout.txt b/Documentation/gitrepository-layout.txt
index e85148f05e..89b616e049 100644
--- a/Documentation/gitrepository-layout.txt
+++ b/Documentation/gitrepository-layout.txt
@@ -95,8 +95,10 @@ refs::
 	References are stored in subdirectories of this
 	directory.  The 'git prune' command knows to preserve
 	objects reachable from refs found in this directory and
-	its subdirectories. This directory is ignored if $GIT_COMMON_DIR
-	is set and "$GIT_COMMON_DIR/refs" will be used instead.
+	its subdirectories.
+	This directory is ignored (except refs/bisect and
+	refs/worktree) if $GIT_COMMON_DIR is set and
+	"$GIT_COMMON_DIR/refs" will be used instead.
 
 refs/heads/`name`::
 	records tip-of-the-tree commit objects of branch `name`
@@ -165,6 +167,11 @@ hooks::
 	each hook. This directory is ignored if $GIT_COMMON_DIR is set
 	and "$GIT_COMMON_DIR/hooks" will be used instead.
 
+common::
+	When multiple working trees are used, most of files in
+	$GIT_DIR are per-worktree with a few known exceptions. All
+	files under 'common' however will be shared between all
+	working trees.
 
 index::
 	The current index file for the repository.  It is