find-tag定位tag受到很多因素的影响，有时候希望它能首先定位到当前buffer的tag，但是它却跳到其它地方去了。如http://debbugs.gnu.org/cgi/bugreport.cgi?bug=2544描述的static函数是一个例子，另外就是为不同体系结构或者操作系统实现接口时使用相同的名字。如果我们注意到这种情况，那么只是操作上麻烦一点，倒还好说；如果没注意到，那么可能完全读错了代码，我在读Android Dalvik garbage collection的实现时就吃到了苦头。
为了解决这个问题，我对find-tag做了一点小小的改进，使其首先定位当前buffer的tag，这样就不会造成困惑。
方法很简单，首先从etags.el从复制出三个函数：find-tag、find-tag-noselect、以及find-tag-in-order，然后把他们重新命名为wl-etags-find-tag、wl-etags-find-tag-noselect和wl-etags-find-tag-in-order，并修改相应的调用点。最后把wl-etags-find-tag-in-order修改为下面的样子：
(defun wl-etags-get-from-buffer-file-name ()
(let ((marker (ring-ref find-tag-marker-ring 0)))
(with-current-buffer (marker-buffer marker)
buffer-file-name)))
(defun wl-etags-goto-file (file)
(let (beg)
(while (and (search-forward "\f\n" nil t)
(progn
(setq beg (point))
(end-of-line)
(skip-chars-backward "^," beg)
(or (looking-at "include$")
(not
(string-equal
file
(expand-file-name
(convert-standard-filename
(buffer-substring beg (1- (point)))))))))))))
;;; prefer tag in the same buffer as source buffer
(defun wl-etags-find-tag-in-order (pattern
search-forward-func
order
next-line-after-failure-p
matching
first-search)
"Internal tag-finding function.
PATTERN is a string to pass to arg SEARCH-FORWARD-FUNC, and to any
member of the function list ORDER. If ORDER is nil, use saved state
to continue a previous search.
Arg NEXT-LINE-AFTER-FAILURE-P is non-nil if after a failed match,
point should be moved to the next line.
Arg MATCHING is a string, an English `-ing' word, to be used in an
error message."
;; Algorithm is as follows:
;; For each qualifier-func in ORDER, go to beginning of tags file, and
;; perform inner loop: for each naive match for PATTERN found using
;; SEARCH-FORWARD-FUNC, qualify the naive match using qualifier-func. If
;; it qualifies, go to the specified line in the specified source file
;; and return. Qualified matches are remembered to avoid repetition.
;; State is saved so that the loop can be continued.
(let (file ;name of file containing tag
tag-info ;where to find the tag in FILE
(first-table t)
(tag-order order)
(match-marker (make-marker))
goto-func
(case-fold-search (if (memq tags-case-fold-search '(nil t))
tags-case-fold-search
case-fold-search))
)
(save-excursion
(if first-search
;; This is the start of a search for a fresh tag.
;; Clear the list of tags matched by the previous search.
;; find-tag-noselect has already put us in the first tags table
;; buffer before we got called.
(setq tag-lines-already-matched nil)
;; Continuing to search for the tag specified last time.
;; tag-lines-already-matched lists locations matched in previous
;; calls so we don't visit the same tag twice if it matches twice
;; during two passes with different qualification predicates.
;; Switch to the current tags table buffer.
(visit-tags-table-buffer 'same))
;; Get a qualified match.
(catch 'qualified-match-found
;; Iterate over the list of tags tables.
(while (or first-table
(visit-tags-table-buffer t))
(and first-search first-table
;; Start at beginning of tags file.
(goto-char (point-min)))
;; Iterate over the list of ordering predicates.
(while order
;; Only give one chance to tags in the from buffer here.
;; They can be found later.
(when (and first-search first-table)
(let ((file (wl-etags-get-from-buffer-file-name)))
(when file
(let* ((file-beg-pos (progn
(wl-etags-goto-file file)
(beginning-of-line)
(point)))
(file-end-pos (progn
(search-forward "\f\n" nil t)
(point))))
(goto-char file-beg-pos)
(while (funcall search-forward-func pattern file-end-pos t)
;; Naive match found. Qualify the match.
(and (funcall (car order) pattern)
;; Make sure it is not a previous qualified match.
(not (member (set-marker match-marker (point-at-bol))
tag-lines-already-matched))
(throw 'qualified-match-found nil))
(if next-line-after-failure-p
(forward-line 1)))
;; Nothing found. Restart from the beginning.
(goto-char (point-min))))))
(while (funcall search-forward-func pattern nil t)
;; Naive match found. Qualify the match.
(and (funcall (car order) pattern)
;; Make sure it is not a previous qualified match.
(not (member (set-marker match-marker (point-at-bol))
tag-lines-already-matched))
(throw 'qualified-match-found nil))
(if next-line-after-failure-p
(forward-line 1)))
;; Try the next flavor of match.
(setq order (cdr order))
(goto-char (point-min)))
(setq first-table nil)
(setq order tag-order))
;; We throw out on match, so only get here if there were no matches.
;; Clear out the markers we use to avoid duplicate matches so they
;; don't slow down editing and are immediately available for GC.
(while tag-lines-already-matched
(set-marker (car tag-lines-already-matched) nil nil)
(setq tag-lines-already-matched (cdr tag-lines-already-matched)))
(set-marker match-marker nil nil)
(error "No %stags %s %s" (if first-search "" "more ")
matching pattern))
;; Found a tag; extract location info.
(beginning-of-line)
(setq tag-lines-already-matched (cons match-marker
tag-lines-already-matched))
;; Expand the filename, using the tags table buffer's default-directory.
;; We should be able to search for file-name backwards in file-of-tag:
;; the beginning-of-line is ok except when positioned on a "file-name" tag.
(setq file (expand-file-name
(if (memq (car order) '(tag-exact-file-name-match-p
tag-file-name-match-p
tag-partial-file-name-match-p))
(save-excursion (forward-line 1)
(file-of-tag))
(file-of-tag)))
tag-info (funcall snarf-tag-function))
;; Get the local value in the tags table buffer before switching buffers.
(setq goto-func goto-tag-location-function)
(tag-find-file-of-tag-noselect file)
(widen)
(push-mark)
(funcall goto-func tag-info)
;; Return the buffer where the tag was found.
(current-buffer))))
从实现上来说，并非完美的方案，有值得商榷的地方；从使用上来说，完美解决了我目前碰到的问题。:-)
emacs有一个defadvice机制，可以提供在某个函数的任意代码位置插入一段代码。
如此，那这个hack就不至少需要全拷贝，维护起来也不会与base版本相差太久。
建议，向find-tag的作者提交patch, 这样大家就可以获利了。
defadvice只有before, after和around三种方式，任意位置恐怕不行。
patch的事我也在考虑，先体验一段，看看有没有什么bug或者还可以改进的地方。