vim-patch:9.0.0389: crash when 'tagfunc' closes the window

Problem:    Crash when 'tagfunc' closes the window.
Solution:   Bail out when the window was closed.

ccfde4d028

Add docs for E1299 from Vim runtime.

Co-authored-by: Bram Moolenaar <Bram@vim.org>

runtime/doc/tagsrch.txt

@@ -909,6 +909,8 @@ If the function returns |v:null| instead of a List, a standard tag lookup will
 be performed instead.
 
 It is not allowed to change the tagstack from inside 'tagfunc'.  *E986* 
+It is not allowed to close a window or change window from inside 'tagfunc'.
+*E1299*
 
 The following is a hypothetical example of a function used for 'tagfunc'.  It
 uses the output of |taglist()| to generate the result: a list of tags in the

src/nvim/tag.c

@@ -106,6 +106,8 @@ static char_u *recurmsg
   = (char_u *)N_("E986: cannot modify the tag stack within tagfunc");
 static char_u *tfu_inv_ret_msg
   = (char_u *)N_("E987: invalid return value from tagfunc");
+static char e_window_unexpectedly_close_while_searching_for_tags[]
+  = N_("E1299: Window unexpectedly closed while searching for tags");
 
 static char *tagmatchname = NULL;   // name of last used tag
 
@@ -501,6 +503,15 @@ void do_tag(char *tag, int type, int count, int forceit, int verbose)
                                    // found: all matches found.
       }
 
+      // A tag function may do anything, which may cause various
+      // information to become invalid.  At least check for the tagstack
+      // to still be the same.
+      if (tagstack != curwin->w_tagstack) {
+        emsg(_(e_window_unexpectedly_close_while_searching_for_tags));
+        FreeWild(new_num_matches, new_matches);
+        break;
+      }
+
       // If there already were some matches for the same name, move them
       // to the start.  Avoids that the order changes when using
       // ":tnext" and jumping to another file.

src/nvim/testdir/test_tagfunc.vim

@@ -285,4 +285,17 @@ func Test_tagfunc_wipes_buffer()
   set tagfunc=
 endfunc
 
+func Test_tagfunc_closes_window()
+  split any
+  func MytagfuncClose(pat, flags, info)
+    close
+    return [{'name' : 'mytag', 'filename' : 'Xtest', 'cmd' : '1'}]
+  endfunc
+  set tagfunc=MytagfuncClose
+  call assert_fails('tag xyz', 'E1299:')
+
+  set tagfunc=
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab