bug#37518: [PATCH] Search upward from current dir for the default TAGS file

[Hong Xu <hong@topbug.net>]

* lisp/progmodes/etags.el (tags--find-default-tags-dir)
(tags--find-default-tags-dir-impl, visit-tags-table): Search
upward from current dir for the default TAGS file
---
  lisp/progmodes/etags.el | 38 ++++++++++++++++++++++++++++++++------
  1 file changed, 32 insertions(+), 6 deletions(-)

diff --git a/lisp/progmodes/etags.el b/lisp/progmodes/etags.el
index a03516100087..81f0f135c577 100644
--- a/lisp/progmodes/etags.el
+++ b/lisp/progmodes/etags.el
@@ -274,6 +274,29 @@ tags-table-mode
    (setq buffer-undo-list t)
    (initialize-new-tags-table))
  
+(defun tags--find-default-tags-dir-impl (current-dir)
+  "Implements finding the directory that hosts the default TAGS file.
+It finds the first directory that contains a file named TAGS encountered
+searching upward from CURRENT-DIR."
+  (let ((tag-filename (expand-file-name "TAGS" current-dir)))
+    (if (file-exists-p tag-filename)
+        current-dir
+      (let ((parent-dir
+             (file-name-directory (directory-file-name current-dir))))
+        (if (string= parent-dir current-dir)  ;; root dir is reached
+            nil
+          (tags--find-default-tags-dir-impl parent-dir))))))
+
+(defun tags--find-default-tags-dir ()
+  "Find the directory that hosts the default TAGS file.
+It is the first directory that contains a file named TAGS encountered
+searching upward from `default-directory'."
+  (let ((default-tag-dir
+          (tags--find-default-tags-dir-impl default-directory)))
+    (if default-tag-dir
+        default-tag-dir
+      default-directory)))
+
  ;;;###autoload
  (defun visit-tags-table (file &optional local)
    "Tell tags commands to use tags table file FILE.
@@ -286,12 +309,15 @@ visit-tags-table
  When you find a tag with \\[find-tag], the buffer it finds the tag
  in is given a local value of this variable which is the name of the tags
  file the tag was in."
-  (interactive (list (read-file-name "Visit tags table (default TAGS): "
-				     default-directory
-				     (expand-file-name "TAGS"
-						       default-directory)
-				     t)
-		     current-prefix-arg))
+  (interactive
+   (let ((default-tag-dir (tags--find-default-tags-dir)))
+     (list (read-file-name
+            "Visit tags table (default TAGS): "
+            ;; default to TAGS from default-directory up to root.
+            default-tag-dir
+            (expand-file-name "TAGS" default-tag-dir)
+            t)))
+           current-prefix-arg)
    (or (stringp file) (signal 'wrong-type-argument (list 'stringp file)))
    ;; Bind tags-file-name so we can control below whether the local or
    ;; global value gets set.
-- 
2.20.1