bug#23164: 25.1.50; xref-find-definitions with local tags-file-name fails

[Johan Claesson <johanclaesson@bredband.net>]

Ooops, i see that the formatting of my last email got messed up because 
it includes some form-feeds (^L).  Here is the same message again
without evil form-feeds :)

Hi again,

I tried again this time with the TAGS of Emacs itself (emacs/src/TAGS).
And like you have already established the problem does not occur in this
scenario.  It does occur with other valid TAGS files, though.  

After a little bit of tracing i think the difference that matters is
that emacs/src/TAGS includes two other TAGS file in the very last lines:

^L
../lisp/TAGS,include
^L
../lwlib/TAGS,include

I was not aware of this feature with TAGS files including other TAGS
files before but after invoking xref-find-definitions emacs is visiting
all 3 TAGS files (src/TAGS, lisp/TAGS and lwlib/TAGS).  And
xref-find-definitions loops through them all.

If these last lines in emacs/src/TAGS are deleted the problem occurs.

It also occur with other simple TAGS files that do not include other
TAGS files.  And the TAGS file i was using when observing the problem
the first time was this kind of simple TAGS file.

Same in master and emacs-25.

Regards,

/Johan



> Óscar Fuentes <ofv@wanadoo.es> writes:
>
>> Dmitry Gutov <dgutov@yandex.ru> writes:
>>
>>> Thanks for the report, but that doesn't seem to be the case here. At
>>> least if I replace "/dev/zero" with "~/vc/emacs/src/TAGS", the
>>> scenario doesn't lead to an error (I just get "not found"), and if I
>>> also use e.g. "CALLN" instead of "foo", the jump to the destination
>>> occurs as expected.
>>
>> I can reproduce it here with an existing TAGS file, on emacs -Q.
>>
>> GNU Emacs 25.0.92.3 (x86_64-unknown-linux-gnu, X toolkit) of 2016-04-03
>>
>> at commit fc3cd53900eb5e80b91b6d2615b3800fd2f59c16
>>
>> Debugger entered--Lisp error: (wrong-type-argument stringp nil)
>>   expand-file-name(nil)
>>   tags-expand-table-name(nil)
>>   visit-tags-table-buffer(t)
>>   etags--xref-find-definitions("foo")
>>   #[514 "\300!\207" [etags--xref-find-definitions] 4 "\n\n(fn BACKEND SYMBOL)"](etags "foo")
>>   apply(#[514 "\300!\207" [etags--xref-find-definitions] 4 "\n\n(fn BACKEND SYMBOL)"] etags "foo")
>>   xref-backend-definitions(etags "foo")
>>   xref--find-xrefs("foo" definitions "foo" nil)
>>   xref--find-definitions("foo" nil)
>>   xref-find-definitions("foo")
>>   eval((xref-find-definitions "foo") nil)
>>   elisp--eval-last-sexp(nil)
>>   eval-last-sexp(nil)
>>   funcall-interactively(eval-last-sexp nil)
>>   call-interactively(eval-last-sexp nil nil)
>>   command-execute(eval-last-sexp)