path.c: make get_pathname() call sites return const char *

Before the previous commit, get_pathname returns an array of PATH_MAX
length. Even if git_path() and similar functions does not use the
whole array, git_path() caller can, in theory.

After the commit, get_pathname() may return a buffer that has just
enough room for the returned string and git_path() caller should never
write beyond that.

Make git_path(), mkpath() and git_path_submodule() return a const
buffer to make sure callers do not write in it at all.

This could have been part of the previous commit, but the "const"
conversion is too much distraction from the core changes in path.c.
Convert git_snpath() to strbuf_git_path()

In the previous patch, git_snpath() is modified to allocate a new
strbuf buffer because vsnpath() needs that. But that makes it awkward
because git_snpath() receives a pre-allocated buffer from outside and
has to copy data back. Rename it to strbuf_git_path() and make it
receive strbuf directly.

The conversion from git_snpath() to git_path() in
update_refs_for_switch() is safe because that function does not keep
any pointer to the round-robin buffer pool allocated by
get_pathname().
path.c: rename vsnpath() to do_git_path()

The name vsnpath() gives an impression that this is general path
handling function. It's not. This is the underlying implementation of
git_path(), git_pathdup() and strbuf_git_path() which will prefix
$GIT_DIR in the result string.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
path.c: group git_path(), git_pathdup() and strbuf_git_path() together

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Make git_path() aware of file relocation in $GIT_DIR

We allow the user to relocate certain paths out of $GIT_DIR via
environment variables, e.g. GIT_OBJECT_DIRECTORY, GIT_INDEX_FILE and
GIT_GRAFT_FILE. Callers are not supposed to use git_path() or
git_pathdup() to get those paths. Instead they must use
get_object_directory(), get_index_file() and get_graft_file()
respectively. This is inconvenient and could be missed in review (for
example, there's git_path("objects/info/alternates") somewhere in
sha1_file.c).

This patch makes git_path() and git_pathdup() understand those
environment variables. So if you set GIT_OBJECT_DIRECTORY to /foo/bar,
git_path("objects/abc") should return /foo/bar/abc. The same is done
for the two remaining env variables.

"git rev-parse --git-path" is the wrapper for script use.
*.sh: respect $GIT_INDEX_FILE
reflog: avoid constructing .lock path with git_path

Among pathnames in $GIT_DIR, e.g. "index" or "packed-refs", we want to
automatically and silently map some of them to the $GIT_DIR of the
repository we are borrowing from via $GIT_COMMON_DIR mechanism.  When
we formulate the pathname for its lockfile, we want it to be in the
same location as its final destination.  "index" is not shared and
needs to remain in the borrowing repository, while "packed-refs" is
shared and needs to go to the borrowed repository.

git_path() could be taught about the ".lock" suffix and map
"index.lock" and "packed-refs.lock" the same way their basenames are
mapped, but instead the caller can help by asking where the basename
(e.g. "index") is mapped to git_path() and then appending ".lock"
after the mapping is done.
fast-import: use git_path() for accessing .git dir instead of get_git_dir()

This allows git_path() to redirect info/fast-import to another place
if needed

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
commit: use SEQ_DIR instead of hardcoding "sequencer"

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Add new environment variable $GIT_COMMON_DIR

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

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.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
git-sh-setup.sh: use rev-parse --git-path to get $GIT_DIR/objects

If $GIT_COMMON_DIR is set, $GIT_OBJECT_DIRECTORY should be
$GIT_COMMON_DIR/objects, not $GIT_DIR/objects. Just let rev-parse
--git-path handle it.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
*.sh: avoid hardcoding $GIT_DIR/hooks/...

If $GIT_COMMON_DIR is set, it should be $GIT_COMMON_DIR/hooks/, not
$GIT_DIR/hooks/. Just let rev-parse --git-path handle it.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
git-stash: avoid hardcoding $GIT_DIR/logs/....

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
setup.c: convert is_git_directory() to use strbuf

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
setup.c: detect $GIT_COMMON_DIR in is_git_directory()

If the file "$GIT_DIR/commondir" exists, it contains the value of
$GIT_COMMON_DIR.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
setup.c: convert check_repository_format_gently to use strbuf

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
setup.c: detect $GIT_COMMON_DIR check_repository_format_gently()

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
setup.c: support multi-checkout repo setup

The repo setup procedure is updated to detect $GIT_DIR/commondir and
set $GIT_COMMON_DIR properly.

The core.worktree is ignored when $GIT_COMMON_DIR is set. This is
because the config file is shared in multi-checkout setup, but
checkout directories _are_ different. Making core.worktree effective
in all checkouts mean it's back to a single checkout.
wrapper.c: wrapper to open a file, fprintf then close
use new wrapper write_file() for simple file writing

This fixes common problems in these code about error handling,
forgetting to close the file handle after fprintf() fails, or not
printing out the error string..

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