worktree: conditionally allow worktree on VFS-enabled repos (#868)

Add GVFS_SUPPORTS_WORKTREES flag (1<<8) to core.gvfs bitmask. When set,
allow git worktree commands to run on VFS-enabled repos instead of
blocking them with BLOCK_ON_VFS_ENABLED.

Force --no-checkout during worktree add when VFS is active so ProjFS can
be mounted before files are projected.

Support skip-clean-check marker file in worktree gitdir: if
.git/worktrees/<name>/skip-clean-check exists, skip the cleanliness
check during worktree remove. This allows VFSForGit's pre-command hook
to unmount ProjFS after its own status check, then let git proceed
without re-checking (which would fail without the virtual filesystem).

The corresponding change in VFSForGit is
microsoft/VFSForGit#1911

* [x] This change only applies to the virtualization hook and VFS for
Git.

Author: mjcheetham

builtin/worktree.c

@@ -856,6 +856,14 @@ static int add(int ac, const char **av, const char *prefix,
 	if (ac < 1 || ac > 2)
 		usage_with_options(git_worktree_add_usage, options);
 
+	/*
+	 * When the virtual file system is active, skip checkout during
+	 * worktree creation. The VFS layer will handle the checkout
+	 * after the worktree structure is set up.
+	 */
+	if (gvfs_config_is_set(the_repository, GVFS_USE_VIRTUAL_FILESYSTEM))
+		opts.checkout = 0;
+
 	path = prefix_filename(prefix, av[0]);
 	branch = ac < 2 ? "HEAD" : av[1];
 	used_new_branch_options = new_branch || new_branch_force;
@@ -1376,6 +1384,21 @@ static int delete_git_work_tree(struct worktree *wt)
 	return ret;
 }
 
+/*
+ * Check if a pre-command hook has already verified worktree cleanliness
+ * and written a marker file to skip git's own check. VFSForGit uses this
+ * to unmount ProjFS after its own status check; without it, git's status
+ * call would fail because the virtual filesystem is no longer available.
+ */
+static int should_skip_clean_check(struct worktree *wt)
+{
+	char *path = repo_common_path(the_repository,
+		"worktrees/%s/skip-clean-check", wt->id);
+	int skip = file_exists(path);
+	free(path);
+	return skip;
+}
+
 static int remove_worktree(int ac, const char **av, const char *prefix,
 			   struct repository *repo UNUSED)
 {
@@ -1415,7 +1438,7 @@ static int remove_worktree(int ac, const char **av, const char *prefix,
 	strbuf_release(&errmsg);
 
 	if (file_exists(wt->path)) {
-		if (!force)
+		if (!force && !(core_virtualfilesystem && should_skip_clean_check(wt)))
 			check_clean_worktree(wt, av[0]);
 
 		ret |= delete_git_work_tree(wt);
@@ -1487,6 +1510,14 @@ int cmd_worktree(int ac,
 
 	ac = parse_options(ac, av, prefix, options, git_worktree_usage, 0);
 
+	/*
+	 * Block worktree commands when VFS is active unless the VFS layer
+	 * has signaled worktree support via GVFS_SUPPORTS_WORKTREES.
+	 */
+	if (gvfs_config_is_set(the_repository, GVFS_USE_VIRTUAL_FILESYSTEM) &&
+	    !gvfs_config_is_set(the_repository, GVFS_SUPPORTS_WORKTREES))
+		die("'git worktree' is not supported when using the virtual file system");
+
 	prepare_repo_settings(the_repository);
 	the_repository->settings.command_requires_full_index = 0;
 

git.c

@@ -762,7 +762,7 @@ static struct cmd_struct commands[] = {
 #ifndef WITH_BREAKING_CHANGES
 	{ "whatchanged", cmd_whatchanged, RUN_SETUP | DEPRECATED },
 #endif
-	{ "worktree", cmd_worktree, RUN_SETUP | BLOCK_ON_VFS_ENABLED },
+	{ "worktree", cmd_worktree, RUN_SETUP },
 	{ "write-tree", cmd_write_tree, RUN_SETUP },
 };
 

gvfs.h

@@ -27,9 +27,17 @@ struct repository;
 #define GVFS_USE_VIRTUAL_FILESYSTEM                 (1 << 3)
 
 #define GVFS_FETCH_SKIP_REACHABILITY_AND_UPLOADPACK (1 << 4)
+/* Bit 5 was GVFS_LOWER_DEFAULT_SLOP, removed in 2018 (unused). */
 #define GVFS_BLOCK_FILTERS_AND_EOL_CONVERSIONS      (1 << 6)
 #define GVFS_PREFETCH_DURING_FETCH		    (1 << 7)
 
+/*
+ * When set, this flag indicates that the VFS layer supports
+ * git worktrees. This allows `git worktree add/remove` to
+ * operate on VFS-enabled repositories.
+ */
+#define GVFS_SUPPORTS_WORKTREES                     (1 << 8)
+
 #define GVFS_ANY_MASK                               0xFFFFFFFF
 
 int gvfs_config_is_set(struct repository *r, int mask);

t/t0402-block-command-on-gvfs.sh

@@ -27,7 +27,34 @@ not_with_gvfs update-index --index-version 2
 not_with_gvfs update-index --skip-worktree
 not_with_gvfs update-index --no-skip-worktree
 not_with_gvfs update-index --split-index
-not_with_gvfs worktree list
+
+# worktree is conditionally allowed: blocked when VFS enabled without
+# GVFS_SUPPORTS_WORKTREES.
+test_expect_success 'worktree blocked with VFS but without SUPPORTS_WORKTREES' '
+	test_config core.gvfs $((0xffff & ~(1<<8))) && # all bits except GVFS_SUPPORTS_WORKTREES
+	test_must_fail git worktree list 2>err &&
+	test_grep "not supported when using the virtual file system" err
+'
+
+test_expect_success 'worktree operations work when SUPPORTS_WORKTREES is set' '
+	test_commit initial &&
+
+	# Use core.gvfs=true which sets all bits including SUPPORTS_WORKTREES.
+	test_config core.gvfs true &&
+
+	# add: succeeds, forces --no-checkout (no initial.t on disk)
+	git worktree add ../vfs-wt &&
+	test_path_exists ../vfs-wt/.git &&
+	! test_path_exists ../vfs-wt/initial.t &&
+
+	# list: shows the worktree
+	git worktree list >out &&
+	grep "vfs-wt" out &&
+
+	# remove: cleans up
+	git worktree remove --force ../vfs-wt &&
+	! test_path_exists ../vfs-wt
+'
 
 test_expect_success 'test gc --auto succeeds when disabled via config' '
 	test_config core.gvfs true &&