worktree.c: add validate_worktree()

This function is later used by "worktree move" and "worktree remove"
to ensure that we have a good connection between the repository and
the worktree. For example, if a worktree is moved manually, the
worktree location recorded in $GIT_DIR/worktrees/.../gitdir is
incorrect and we should not move that one.

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

worktree.c

@@ -254,6 +254,78 @@ const char *is_worktree_locked(struct worktree *wt)
 	return wt->lock_reason;
 }
 
+/* convenient wrapper to deal with NULL strbuf */
+static void strbuf_addf_gently(struct strbuf *buf, const char *fmt, ...)
+{
+	va_list params;
+
+	if (!buf)
+		return;
+
+	va_start(params, fmt);
+	strbuf_vaddf(buf, fmt, params);
+	va_end(params);
+}
+
+int validate_worktree(const struct worktree *wt, struct strbuf *errmsg)
+{
+	struct strbuf wt_path = STRBUF_INIT;
+	char *path = NULL;
+	int err, ret = -1;
+
+	strbuf_addf(&wt_path, "%s/.git", wt->path);
+
+	if (is_main_worktree(wt)) {
+		if (is_directory(wt_path.buf)) {
+			ret = 0;
+			goto done;
+		}
+		/*
+		 * Main worktree using .git file to point to the
+		 * repository would make it impossible to know where
+		 * the actual worktree is if this function is executed
+		 * from another worktree. No .git file support for now.
+		 */
+		strbuf_addf_gently(errmsg,
+				   _("'%s' at main working tree is not the repository directory"),
+				   wt_path.buf);
+		goto done;
+	}
+
+	/*
+	 * Make sure "gitdir" file points to a real .git file and that
+	 * file points back here.
+	 */
+	if (!is_absolute_path(wt->path)) {
+		strbuf_addf_gently(errmsg,
+				   _("'%s' file does not contain absolute path to the working tree location"),
+				   git_common_path("worktrees/%s/gitdir", wt->id));
+		goto done;
+	}
+
+	if (!file_exists(wt_path.buf)) {
+		strbuf_addf_gently(errmsg, _("'%s' does not exist"), wt_path.buf);
+		goto done;
+	}
+
+	path = xstrdup_or_null(read_gitfile_gently(wt_path.buf, &err));
+	if (!path) {
+		strbuf_addf_gently(errmsg, _("'%s' is not a .git file, error code %d"),
+				   wt_path.buf, err);
+		goto done;
+	}
+
+	ret = fspathcmp(path, real_path(git_common_path("worktrees/%s", wt->id)));
+
+	if (ret)
+		strbuf_addf_gently(errmsg, _("'%s' does not point back to '%s'"),
+				   wt->path, git_common_path("worktrees/%s", wt->id));
+done:
+	free(path);
+	strbuf_release(&wt_path);
+	return ret;
+}
+
 int is_worktree_being_rebased(const struct worktree *wt,
 			      const char *target)
 {

worktree.h

@@ -3,6 +3,8 @@
 
 #include "refs.h"
 
+struct strbuf;
+
 struct worktree {
 	char *path;
 	char *id;
@@ -59,6 +61,13 @@ extern int is_main_worktree(const struct worktree *wt);
  */
 extern const char *is_worktree_locked(struct worktree *wt);
 
+/*
+ * Return zero if the worktree is in good condition. Error message is
+ * returned if "errmsg" is not NULL.
+ */
+extern int validate_worktree(const struct worktree *wt,
+			     struct strbuf *errmsg);
+
 /*
  * Free up the memory for worktree(s)
  */