If an explicit GIT_DIR is given without a working tree, we
implicitly assume that the current working directory should
be used as the working tree. E.g.,:

  GIT_DIR=/some/repo.git git status

would compare against the cwd.

Unfortunately, we fool this rule for sub-invocations of git
by setting GIT_DIR internally ourselves. For example:

  git init foo
  cd foo/.git
  git status ;# fails, as we expect
  git config alias.st status
  git status ;# does not fail, but should

What happens is that we run setup_git_directory when doing
alias lookup (since we need to see the config), set GIT_DIR
as a result, and then leave GIT_WORK_TREE blank (because we
do not have one). Then when we actually run the status
command, we do setup_git_directory again, which sees our
explicit GIT_DIR and uses the cwd as an implicit worktree.

It's tempting to argue that we should be suppressing that
second invocation of setup_git_directory, as it could use
the values we already found in memory. However, the problem
still exists for sub-processes (e.g., if "git status" were
an external command).

You can see another example with the "--bare" option, which
sets GIT_DIR explicitly. For example:

  git init foo
  cd foo/.git
  git status ;# fails
  git --bare status ;# does NOT fail

We need some way of telling sub-processes "even though
GIT_DIR is set, do not use cwd as an implicit working tree".
We could do it by putting a special token into
GIT_WORK_TREE, but the obvious choice (an empty string) has
some portability problems, and could potentially be
triggered accidentally by a user.

Instead, we add a new boolean variable, GIT_IMPLICIT_WORK_TREE,
which suppresses the use of cwd as a working tree when
GIT_DIR is set. We trigger the new variable when we know we
are in a bare setting.

The variable is left intentionally undocumented, as this is
an internal detail (for now, anyway). If somebody comes up
with a good alternate use for it, and once we are confident
we have shaken any bugs out of it, we can consider promoting
it further.

Signed-off-by: Jeff King <p...@peff.net>
---
So I think this just ends up being a cleaner and smaller change than
trying to support $GIT_BARE. I think $GIT_BARE could allow more
flexibility, but it's flexibility nobody is particularly asking for, and
there are lots of nasty corner cases around it. I'm pretty sure this is
doing the right thing.

Having written this, I'm still tempted to signal the same thing by
putting /dev/null into GIT_WORK_TREE (Junio's suggestion from the old
thread). This one works OK because we only check GIT_WORK_TREE_IMPLICIT
_after_ exhausting all of the other working tree options, so it is
always subordinate to a later setting of GIT_WORK_TREE. But it seems a
little cleaner for somebody setting GIT_WORK_TREE To clear this
"implicit" flag automatically.

At the same time, I would wonder how other git implementations would
react to GIT_WORK_TREE=/dev/null. Would they try to chdir() there and
barf, when they could happily exist without a working tree? Doing it
this way seems a bit safer from regressions (those other implementations
do not get the _benefit_ of this patch unless they support
GIT_WORK_TREE_IMPLICIT, of course, but at least we are not breaking
them).

 cache.h               |  1 +
 git.c                 |  1 +
 setup.c               |  8 ++++++++
 t/t1510-repo-setup.sh | 19 +++++++++++++++++++
 4 files changed, 29 insertions(+)

diff --git a/cache.h b/cache.h
index e493563..070169a 100644
--- a/cache.h
+++ b/cache.h
@@ -344,6 +344,7 @@ 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 GIT_IMPLICIT_WORK_TREE_ENVIRONMENT "GIT_IMPLICIT_WORK_TREE"
 #define DEFAULT_GIT_DIR_ENVIRONMENT ".git"
 #define DB_ENVIRONMENT "GIT_OBJECT_DIRECTORY"
 #define INDEX_ENVIRONMENT "GIT_INDEX_FILE"
diff --git a/git.c b/git.c
index b10c18b..24b7984 100644
--- a/git.c
+++ b/git.c
@@ -125,6 +125,7 @@ static int handle_options(const char ***argv, int *argc, 
int *envchanged)
                        static char git_dir[PATH_MAX+1];
                        is_bare_repository_cfg = 1;
                        setenv(GIT_DIR_ENVIRONMENT, getcwd(git_dir, 
sizeof(git_dir)), 0);
+                       setenv(GIT_IMPLICIT_WORK_TREE_ENVIRONMENT, "0", 1);
                        if (envchanged)
                                *envchanged = 1;
                } else if (!strcmp(cmd, "-c")) {
diff --git a/setup.c b/setup.c
index 1dee47e..6c87660 100644
--- a/setup.c
+++ b/setup.c
@@ -523,6 +523,12 @@ static const char *setup_explicit_git_dir(const char 
*gitdirenv,
                        set_git_work_tree(core_worktree);
                }
        }
+       else if (!git_env_bool(GIT_IMPLICIT_WORK_TREE_ENVIRONMENT, 1)) {
+               /* #16d */
+               set_git_dir(gitdirenv);
+               free(gitfile);
+               return NULL;
+       }
        else /* #2, #10 */
                set_git_work_tree(".");
 
@@ -601,6 +607,8 @@ static const char *setup_bare_git_dir(char *cwd, int 
offset, int len, int *nongi
        if (check_repository_format_gently(".", nongit_ok))
                return NULL;
 
+       setenv(GIT_IMPLICIT_WORK_TREE_ENVIRONMENT, "0", 1);
+
        /* --work-tree is set without --git-dir; use discovered one */
        if (getenv(GIT_WORK_TREE_ENVIRONMENT) || git_work_tree_cfg) {
                const char *gitdir;
diff --git a/t/t1510-repo-setup.sh b/t/t1510-repo-setup.sh
index 80aedfc..cf2ee78 100755
--- a/t/t1510-repo-setup.sh
+++ b/t/t1510-repo-setup.sh
@@ -517,6 +517,25 @@ test_expect_success '#16c: bare .git has no worktree' '
                "$here/16c/.git" "(null)" "$here/16c/sub" "(null)"
 '
 
+test_expect_success '#16d: bareness preserved across alias' '
+       setup_repo 16d unset "" unset &&
+       (
+               cd 16d/.git &&
+               test_must_fail git status &&
+               git config alias.st status &&
+               test_must_fail git st
+       )
+'
+
+test_expect_success '#16e: bareness preserved by --bare' '
+       setup_repo 16e unset "" unset &&
+       (
+               cd 16e/.git &&
+               test_must_fail git status &&
+               test_must_fail git --bare status
+       )
+'
+
 test_expect_success '#17: GIT_WORK_TREE without explicit GIT_DIR is accepted 
(bare case)' '
        # Just like #16.
        setup_repo 17a unset "" true &&
-- 
1.8.2.rc2.4.g3e774bb

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