vim-patch:8.1.1228: not possible to process tags with a function

Problem:    Not possible to process tags with a function.
Solution:   Add tagfunc() (Christian Brabandt, Andy Massimino, closes vim/vim#4010)
https://github.com/vim/vim/commit/45e18cbdc40afd8144d20dcc07ad2d981636f4c9

14 files changed, 482 insertions, 69 deletions

diff --git a/src/nvim/buffer.c b/src/nvim/buffer.c
index b81ffd09e1..1d5aa8ba9b 100644
--- a/src/nvim/buffer.c
+++ b/src/nvim/buffer.c
@@ -1947,6 +1947,7 @@ void free_buf_options(buf_T *buf, int free_p_ff)
   clear_string_option(&buf->b_p_path);
   clear_string_option(&buf->b_p_tags);
   clear_string_option(&buf->b_p_tc);
+  clear_string_option(&buf->b_p_tfu);
   clear_string_option(&buf->b_p_dict);
   clear_string_option(&buf->b_p_tsr);
   clear_string_option(&buf->b_p_qe);
diff --git a/src/nvim/buffer_defs.h b/src/nvim/buffer_defs.h
index 16c7804be0..ca740dea21 100644
--- a/src/nvim/buffer_defs.h
+++ b/src/nvim/buffer_defs.h
@@ -119,10 +119,11 @@ typedef uint16_t disptick_T;  // display tick type
  * The taggy struct is used to store the information about a :tag command.
  */
 typedef struct taggy {
-  char_u      *tagname;         /* tag name */
-  fmark_T fmark;                /* cursor position BEFORE ":tag" */
-  int cur_match;                /* match number */
-  int cur_fnum;                 /* buffer number used for cur_match */
+  char_u      *tagname;         // tag name
+  fmark_T fmark;                // cursor position BEFORE ":tag"
+  int cur_match;                // match number
+  int cur_fnum;                 // buffer number used for cur_match
+  char_u *user_data;            // used with tagfunc
 } taggy_T;
 
 typedef struct buffblock buffblock_T;
@@ -647,6 +648,7 @@ struct file_buffer {
   char_u *b_p_cpt;              ///< 'complete'
   char_u *b_p_cfu;              ///< 'completefunc'
   char_u *b_p_ofu;              ///< 'omnifunc'
+  char_u *b_p_tfu;              ///< 'tagfunc'
   int b_p_eol;                  ///< 'endofline'
   int b_p_fixeol;               ///< 'fixendofline'
   int b_p_et;                   ///< 'expandtab'
diff --git a/src/nvim/edit.c b/src/nvim/edit.c
index 16c4882975..a4b4e0d980 100644
--- a/src/nvim/edit.c
+++ b/src/nvim/edit.c
@@ -4047,12 +4047,14 @@ static int ins_compl_get_exp(pos_T *ini)
 
       // Find up to TAG_MANY matches.  Avoids that an enormous number
       // of matches is found when compl_pattern is empty
+      g_tag_at_cursor = true;
       if (find_tags(compl_pattern, &num_matches, &matches,
                     TAG_REGEXP | TAG_NAMES | TAG_NOIC | TAG_INS_COMP
                     | (l_ctrl_x_mode != CTRL_X_NORMAL ? TAG_VERBOSE : 0),
                     TAG_MANY, curbuf->b_ffname) == OK && num_matches > 0) {
         ins_compl_add_matches(num_matches, matches, p_ic);
       }
+      g_tag_at_cursor = false;
       p_ic = save_p_ic;
       break;
 
diff --git a/src/nvim/ex_cmds.c b/src/nvim/ex_cmds.c
index 00de7a3d66..2e8bd79c81 100644
--- a/src/nvim/ex_cmds.c
+++ b/src/nvim/ex_cmds.c
@@ -4905,7 +4905,7 @@ int find_help_tags(const char_u *arg, int *num_matches, char_u ***matches,
 
   *matches = NULL;
   *num_matches = 0;
-  int flags = TAG_HELP | TAG_REGEXP | TAG_NAMES | TAG_VERBOSE;
+  int flags = TAG_HELP | TAG_REGEXP | TAG_NAMES | TAG_VERBOSE | TAG_NO_TAGFUNC;
   if (keep_lang) {
     flags |= TAG_KEEP_LANG;
   }
diff --git a/src/nvim/globals.h b/src/nvim/globals.h
index 5237c621f9..c3d1a4d40b 100644
--- a/src/nvim/globals.h
+++ b/src/nvim/globals.h
@@ -787,7 +787,11 @@ EXTERN int postponed_split_flags INIT(= 0);       /* args for win_split() */
 EXTERN int postponed_split_tab INIT(= 0);       /* cmdmod.tab */
 EXTERN int g_do_tagpreview INIT(= 0);       /* for tag preview commands:
                                                height of preview window */
-EXTERN int replace_offset INIT(= 0);        /* offset for replace_push() */
+EXTERN int g_tag_at_cursor INIT(= false);  // whether the tag command comes
+                                           // from the command line (0) or was
+                                           // invoked as a normal command (1)
+
+EXTERN int replace_offset INIT(= 0);        // offset for replace_push()
 
 EXTERN char_u   *escape_chars INIT(= (char_u *)" \t\\\"|");
 /* need backslash in cmd line */
diff --git a/src/nvim/normal.c b/src/nvim/normal.c
index e0dc9d4f23..28183ffa1d 100644
--- a/src/nvim/normal.c
+++ b/src/nvim/normal.c
@@ -4929,7 +4929,9 @@ static void nv_ident(cmdarg_T *cap)
     add_to_history(HIST_SEARCH, (char_u *)buf, true, NUL);
     (void)normal_search(cap, cmdchar == '*' ? '/' : '?', (char_u *)buf, 0);
   } else {
+    g_tag_at_cursor = true;
     do_cmdline_cmd(buf);
+    g_tag_at_cursor = false;
   }
 
   xfree(buf);
diff --git a/src/nvim/option.c b/src/nvim/option.c
index 22f7b85133..20351d3908 100644
--- a/src/nvim/option.c
+++ b/src/nvim/option.c
@@ -133,6 +133,7 @@ static char_u   *p_cms;
 static char_u   *p_cpt;
 static char_u   *p_cfu;
 static char_u   *p_ofu;
+static char_u   *p_tfu;
 static int p_eol;
 static int p_fixeol;
 static int p_et;
@@ -2273,6 +2274,7 @@ void check_buf_options(buf_T *buf)
   check_string_option(&buf->b_p_ep);
   check_string_option(&buf->b_p_path);
   check_string_option(&buf->b_p_tags);
+  check_string_option(&buf->b_p_tfu);
   check_string_option(&buf->b_p_tc);
   check_string_option(&buf->b_p_dict);
   check_string_option(&buf->b_p_tsr);
@@ -5590,6 +5592,7 @@ static char_u *get_varp_scope(vimoption_T *p, int opt_flags)
     case PV_INC:  return (char_u *)&(curbuf->b_p_inc);
     case PV_DICT: return (char_u *)&(curbuf->b_p_dict);
     case PV_TSR:  return (char_u *)&(curbuf->b_p_tsr);
+    case PV_TFU:  return (char_u *)&(curbuf->b_p_tfu);
     case PV_STL:  return (char_u *)&(curwin->w_p_stl);
     case PV_UL:   return (char_u *)&(curbuf->b_p_ul);
     case PV_LW:   return (char_u *)&(curbuf->b_p_lw);
@@ -5742,6 +5745,7 @@ static char_u *get_varp(vimoption_T *p)
   case PV_SPF:    return (char_u *)&(curwin->w_s->b_p_spf);
   case PV_SPL:    return (char_u *)&(curwin->w_s->b_p_spl);
   case PV_SW:     return (char_u *)&(curbuf->b_p_sw);
+  case PV_TFU:    return (char_u *)&(curbuf->b_p_tfu);
   case PV_TS:     return (char_u *)&(curbuf->b_p_ts);
   case PV_TW:     return (char_u *)&(curbuf->b_p_tw);
   case PV_UDF:    return (char_u *)&(curbuf->b_p_udf);
@@ -6004,6 +6008,7 @@ void buf_copy_options(buf_T *buf, int flags)
       buf->b_p_cpt = vim_strsave(p_cpt);
       buf->b_p_cfu = vim_strsave(p_cfu);
       buf->b_p_ofu = vim_strsave(p_ofu);
+      buf->b_p_tfu = vim_strsave(p_tfu);
       buf->b_p_sts = p_sts;
       buf->b_p_sts_nopaste = p_sts_nopaste;
       buf->b_p_com = vim_strsave(p_com);
diff --git a/src/nvim/option_defs.h b/src/nvim/option_defs.h
index 67cb53ce02..e5a3c0bd95 100644
--- a/src/nvim/option_defs.h
+++ b/src/nvim/option_defs.h
@@ -780,6 +780,7 @@ enum {
   , BV_SUA
   , BV_SW
   , BV_SWF
+  , BV_TFU
   , BV_TAGS
   , BV_TC
   , BV_TS
diff --git a/src/nvim/options.lua b/src/nvim/options.lua
index 52e788944b..e96b3f8e02 100644
--- a/src/nvim/options.lua
+++ b/src/nvim/options.lua
@@ -2389,6 +2389,14 @@ return {
       defaults={if_true={vi=""}}
     },
     {
+      full_name='tagfunc', abbreviation='tfu',
+      type='string', scope={'buffer'},
+      vim=true,
+      vi_def=true,
+      varname='p_tfu',
+      defaults={if_true={vi=""}}
+    },
+    {
       full_name='tabline', abbreviation='tal',
       type='string', scope={'global'},
       vi_def=true,
diff --git a/src/nvim/tag.c b/src/nvim/tag.c
index 880c467d30..872fc0f279 100644
--- a/src/nvim/tag.c
+++ b/src/nvim/tag.c
@@ -51,17 +51,20 @@
  * Structure to hold pointers to various items in a tag line.
  */
 typedef struct tag_pointers {
-  /* filled in by parse_tag_line(): */
-  char_u      *tagname;         /* start of tag name (skip "file:") */
-  char_u      *tagname_end;     /* char after tag name */
-  char_u      *fname;           /* first char of file name */
-  char_u      *fname_end;       /* char after file name */
-  char_u      *command;         /* first char of command */
-  /* filled in by parse_match(): */
-  char_u      *command_end;     /* first char after command */
-  char_u      *tag_fname;       /* file name of the tags file */
-  char_u      *tagkind;         /* "kind:" value */
-  char_u      *tagkind_end;     /* end of tagkind */
+  // filled in by parse_tag_line():
+  char_u  *tagname;  // start of tag name (skip "file:")
+  char_u  *tagname_end;  // char after tag name
+  char_u  *fname;   // first char of file name
+  char_u  *fname_end;  // char after file name
+  char_u  *command;  // first char of command
+  // filled in by parse_match():
+  char_u  *command_end;  // first char after command
+  char_u  *tag_fname;  // file name of the tags file. This is used
+  // when 'tr' is set.
+  char_u  *tagkind;  // "kind:" value
+  char_u  *tagkind_end;  // end of tagkind
+  char_u  *user_data;  // user_data string
+  char_u  *user_data_end;  // end of user_data
 } tagptrs_T;
 
 /*
@@ -102,6 +105,10 @@ static char_u   *nofile_fname = NULL;   /* fname for NOTAGFILE error */
 
 static char_u *bottommsg = (char_u *)N_("E555: at bottom of tag stack");
 static char_u *topmsg = (char_u *)N_("E556: at top of tag stack");
+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_u   *tagmatchname = NULL;   /* name of last used tag */
 
@@ -109,7 +116,12 @@ static char_u   *tagmatchname = NULL;   /* name of last used tag */
  * Tag for preview window is remembered separately, to avoid messing up the
  * normal tagstack.
  */
-static taggy_T ptag_entry = { NULL, { { 0, 0, 0 }, 0, 0, NULL }, 0, 0 };
+static taggy_T ptag_entry = { NULL, { { 0, 0, 0 }, 0, 0, NULL }, 0, 0, NULL };
+
+static int tfu_in_use = false;  // disallow recursive call of tagfunc
+
+// Used instead of NUL to separate tag fields in the growarrays.
+#define TAG_SEP 0x02
 
 /*
  * Jump to tag; handling of tag commands and tag stack
@@ -161,6 +173,7 @@ do_tag(
   int use_tagstack;
   int skip_msg = false;
   char_u *buf_ffname = curbuf->b_ffname;  // name for priority computation
+  int use_tfu = 1;
 
   /* remember the matches for the last used tag */
   static int num_matches = 0;
@@ -168,6 +181,11 @@ do_tag(
   static char_u       **matches = NULL;
   static int flags;
 
+  if (tfu_in_use) {
+    EMSG(_(recurmsg));
+    return false;
+  }
+
 #ifdef EXITFREE
   if (type == DT_FREE) {
     /* remove the list of matches */
@@ -181,6 +199,7 @@ do_tag(
   if (type == DT_HELP) {
     type = DT_TAG;
     no_regexp = true;
+    use_tfu = 0;
   }
 
   prev_num_matches = num_matches;
@@ -196,7 +215,7 @@ do_tag(
     use_tagstack = false;
     new_tag = true;
     if (g_do_tagpreview != 0) {
-      xfree(ptag_entry.tagname);
+      tagstack_clear_entry(&ptag_entry);
       ptag_entry.tagname = vim_strsave(tag);
     }
   } else {
@@ -220,7 +239,7 @@ do_tag(
           cur_match = ptag_entry.cur_match;
           cur_fnum = ptag_entry.cur_fnum;
         } else {
-          xfree(ptag_entry.tagname);
+          tagstack_clear_entry(&ptag_entry);
           ptag_entry.tagname = vim_strsave(tag);
         }
       } else {
@@ -228,16 +247,18 @@ do_tag(
          * If the last used entry is not at the top, delete all tag
          * stack entries above it.
          */
-        while (tagstackidx < tagstacklen)
-          xfree(tagstack[--tagstacklen].tagname);
+        while (tagstackidx < tagstacklen) {
+          tagstack_clear_entry(&tagstack[--tagstacklen]);
+        }
 
         /* if the tagstack is full: remove oldest entry */
         if (++tagstacklen > TAGSTACKSIZE) {
           tagstacklen = TAGSTACKSIZE;
-          xfree(tagstack[0].tagname);
-          for (i = 1; i < tagstacklen; ++i)
+          tagstack_clear_entry(&tagstack[0]);
+          for (i = 1; i < tagstacklen; i++) {
             tagstack[i - 1] = tagstack[i];
-          --tagstackidx;
+          }
+          tagstackidx--;
         }
 
         // put the tag name in the tag stack
@@ -447,15 +468,22 @@ do_tag(
       } else
         flags = TAG_NOIC;
 
-      if (type == DT_CSCOPE)
+      if (type == DT_CSCOPE) {
         flags = TAG_CSCOPE;
-      if (verbose)
+      }
+      if (verbose) {
         flags |= TAG_VERBOSE;
+      }
+      if (!use_tfu) {
+        flags |= TAG_NO_TAGFUNC;
+      }
+
       if (find_tags(name, &new_num_matches, &new_matches, flags,
-              max_num_matches, buf_ffname) == OK
-          && new_num_matches < max_num_matches)
-        max_num_matches = MAXCOL;         /* If less than max_num_matches
-                                             found: all matches found. */
+                    max_num_matches, buf_ffname) == OK
+          && new_num_matches < max_num_matches) {
+        max_num_matches = MAXCOL;  // If less than max_num_matches
+                                   // found: all matches found.
+      }
 
       /* If there already were some matches for the same name, move them
        * to the start.  Avoids that the order changes when using
@@ -543,9 +571,20 @@ do_tag(
         cur_match = num_matches - 1;
       }
       if (use_tagstack) {
+        tagptrs_T   tagp2;
+
         tagstack[tagstackidx].cur_match = cur_match;
         tagstack[tagstackidx].cur_fnum = cur_fnum;
-        ++tagstackidx;
+
+        // store user-provided data originating from tagfunc
+        if (use_tfu && parse_match(matches[cur_match], &tagp2) == OK
+            && tagp2.user_data) {
+          XFREE_CLEAR(tagstack[tagstackidx].user_data);
+          tagstack[tagstackidx].user_data = vim_strnsave(
+              tagp2.user_data, tagp2.user_data_end - tagp2.user_data);
+        }
+
+        tagstackidx++;
       } else if (g_do_tagpreview != 0)   {
         ptag_entry.cur_match = cur_match;
         ptag_entry.cur_fnum = cur_fnum;
@@ -1083,6 +1122,220 @@ static void prepare_pats(pat_T *pats, int has_re)
     pats->regmatch.regprog = NULL;
 }
 
+//
+// Call the user-defined function to generate a list of tags used by
+// find_tags().
+//
+// Return OK if at least 1 tag has been successfully found,
+// NOTDONE if the function returns v:null, and FAIL otherwise.
+//
+static int find_tagfunc_tags(
+    char_u  *pat,        // pattern supplied to the user-defined function
+    garray_T  *ga,       // the tags will be placed here
+    int   *match_count,  // here the number of tags found will be placed
+    int   flags,         // flags from find_tags (TAG_*)
+    char_u *buf_ffname)  // name of buffer for priority
+{
+  pos_T       save_pos;
+  list_T      *taglist;
+  int         ntags = 0;
+  int         result = FAIL;
+  typval_T  args[4];
+  typval_T  rettv;
+  char_u      flagString[3];
+  dict_T  *d;
+  taggy_T *tag = &curwin->w_tagstack[curwin->w_tagstackidx];
+
+  if (*curbuf->b_p_tfu == NUL) {
+    return FAIL;
+  }
+
+  args[0].v_type = VAR_STRING;
+  args[0].vval.v_string = pat;
+  args[1].v_type = VAR_STRING;
+  args[1].vval.v_string = flagString;
+
+  // create 'info' dict argument
+  d = tv_dict_alloc();
+  if (tag->user_data != NULL) {
+    tv_dict_add_str(d, S_LEN("user_data"), (const char *)tag->user_data);
+  }
+  if (buf_ffname != NULL) {
+    tv_dict_add_str(d, S_LEN("buf_ffname"), (const char *)buf_ffname);
+  }
+
+  d->dv_refcount++;
+  args[2].v_type = VAR_DICT;
+  args[2].vval.v_dict = d;
+
+  args[3].v_type = VAR_UNKNOWN;
+
+  vim_snprintf((char *)flagString, sizeof(flagString),
+               "%s%s",
+               g_tag_at_cursor      ? "c": "",
+               flags & TAG_INS_COMP ? "i": "");
+
+  save_pos = curwin->w_cursor;
+  result = call_vim_function(curbuf->b_p_tfu, 3, args, &rettv);
+  curwin->w_cursor = save_pos;  // restore the cursor position
+  d->dv_refcount--;
+
+  if (result == FAIL) {
+    return FAIL;
+  }
+  if (rettv.v_type == VAR_SPECIAL && rettv.vval.v_number == VV_NULL) {
+    tv_clear(&rettv);
+    return NOTDONE;
+  }
+  if (rettv.v_type != VAR_LIST || !rettv.vval.v_list) {
+    tv_clear(&rettv);
+    EMSG(_(tfu_inv_ret_msg));
+    return FAIL;
+  }
+  taglist = rettv.vval.v_list;
+
+  TV_LIST_ITER_CONST(taglist, li, {
+    char_u    *mfp;
+    char_u    *res_name;
+    char_u    *res_fname;
+    char_u    *res_cmd;
+    char_u    *res_kind;
+    int   len;
+    int   has_extra = 0;
+    int   name_only = flags & TAG_NAMES;
+
+    if (TV_LIST_ITEM_TV(li)->v_type != VAR_DICT) {
+      EMSG(_(tfu_inv_ret_msg));
+      break;
+    }
+
+    len = 2;
+    res_name = NULL;
+    res_fname = NULL;
+    res_cmd = NULL;
+    res_kind = NULL;
+
+    TV_DICT_ITER(TV_LIST_ITEM_TV(li)->vval.v_dict, di, {
+      const char_u *dict_key = di->di_key;
+      typval_T *tv = &di->di_tv;
+
+      if (tv->v_type != VAR_STRING || tv->vval.v_string == NULL) {
+        continue;
+      }
+
+      len += STRLEN(tv->vval.v_string) + 1;   // Space for "\tVALUE"
+      if (!STRCMP(dict_key, "name")) {
+        res_name = tv->vval.v_string;
+        continue;
+      }
+      if (!STRCMP(dict_key, "filename")) {
+        res_fname = tv->vval.v_string;
+        continue;
+      }
+      if (!STRCMP(dict_key, "cmd")) {
+        res_cmd = tv->vval.v_string;
+        continue;
+      }
+      has_extra = 1;
+      if (!STRCMP(dict_key, "kind")) {
+        res_kind = tv->vval.v_string;
+        continue;
+      }
+      // Other elements will be stored as "\tKEY:VALUE"
+      // Allocate space for the key and the colon
+      len += STRLEN(dict_key) + 1;
+    });
+
+    if (has_extra) {
+      len += 2;  // need space for ;"
+    }
+
+    if (!res_name || !res_fname || !res_cmd) {
+      EMSG(_(tfu_inv_ret_msg));
+      break;
+    }
+
+    if (name_only) {
+      mfp = vim_strsave(res_name);
+    } else {
+      mfp = (char_u *)xmalloc((int)sizeof(char_u) + len + 1);
+    }
+
+    if (mfp == NULL) {
+      continue;
+    }
+
+    if (!name_only) {
+      char_u *p = mfp;
+
+      *p++ = MT_GL_OTH + 1;   // mtt
+      *p++ = TAG_SEP;     // no tag file name
+
+      STRCPY(p, res_name);
+      p += STRLEN(p);
+
+      *p++ = TAB;
+      STRCPY(p, res_fname);
+      p += STRLEN(p);
+
+      *p++ = TAB;
+      STRCPY(p, res_cmd);
+      p += STRLEN(p);
+
+      if (has_extra) {
+        STRCPY(p, ";\"");
+        p += STRLEN(p);
+
+        if (res_kind) {
+          *p++ = TAB;
+          STRCPY(p, res_kind);
+          p += STRLEN(p);
+        }
+
+        TV_DICT_ITER(TV_LIST_ITEM_TV(li)->vval.v_dict, di, {
+          const char_u *dict_key = di->di_key;
+          typval_T *tv = &di->di_tv;
+          if (tv->v_type != VAR_STRING || tv->vval.v_string == NULL) {
+            continue;
+          }
+
+          if (!STRCMP(dict_key, "name")) {
+            continue;
+          }
+          if (!STRCMP(dict_key, "filename")) {
+            continue;
+          }
+          if (!STRCMP(dict_key, "cmd")) {
+            continue;
+          }
+          if (!STRCMP(dict_key, "kind")) {
+            continue;
+          }
+
+          *p++ = TAB;
+          STRCPY(p, dict_key);
+          p += STRLEN(p);
+          STRCPY(p, ":");
+          p += STRLEN(p);
+          STRCPY(p, tv->vval.v_string);
+          p += STRLEN(p);
+        });
+      }
+    }
+
+    // Add all matches because tagfunc should do filtering.
+    ga_grow(ga, 1);
+    ((char_u **)(ga->ga_data))[ga->ga_len++] = mfp;
+    ntags++;
+    result = OK;
+  });
+
+  tv_clear(&rettv);
+
+  *match_count = ntags;
+  return result;
+}
+
 /*
  * find_tags() - search for tags in tags files
  *
@@ -1108,6 +1361,7 @@ static void prepare_pats(pat_T *pats, int has_re)
  * TAG_NOIC	  don't always ignore case
  * TAG_KEEP_LANG  keep language
  * TAG_CSCOPE	  use cscope results for tags
+ * TAG_NO_TAGFUNC do not call the 'tagfunc' function
  */
 int
 find_tags(
@@ -1198,6 +1452,7 @@ find_tags(
   int get_it_again = FALSE;
   int use_cscope = (flags & TAG_CSCOPE);
   int verbose = (flags & TAG_VERBOSE);
+  int use_tfu = ((flags & TAG_NO_TAGFUNC) == 0);
   int save_p_ic = p_ic;
 
   // Change the value of 'ignorecase' according to 'tagcase' for the
@@ -1275,6 +1530,16 @@ find_tags(
   // uninitialised.
   memset(&search_info, 0, 1);  // -V512
 
+  if (*curbuf->b_p_tfu != NUL && use_tfu && !tfu_in_use) {
+    tfu_in_use = true;
+    retval = find_tagfunc_tags(pat, &ga_match[0], &match_count,
+                               flags, buf_ffname);
+    tfu_in_use = false;
+    if (retval != NOTDONE) {
+      goto findtag_end;
+    }
+  }
+
   /*
    * When finding a specified number of matches, first try with matching
    * case, so binary search can be used, and try ignore-case matches in a
@@ -1856,7 +2121,6 @@ parse_line:
               }
             }
           } else {
-#define TAG_SEP 0x02
             size_t tag_fname_len = STRLEN(tag_fname);
             // Save the tag in a buffer.
             // Use 0x02 to separate fields (Can't use NUL, because the
@@ -2040,9 +2304,7 @@ void free_tag_stuff(void)
   do_tag(NULL, DT_FREE, 0, 0, 0);
   tag_freematch();
 
-  if (ptag_entry.tagname) {
-    XFREE_CLEAR(ptag_entry.tagname);
-  }
+  tagstack_clear_entry(&ptag_entry);
 }
 
 #endif
@@ -2283,6 +2545,7 @@ parse_match(
       tagp);
 
   tagp->tagkind = NULL;
+  tagp->user_data = NULL;
   tagp->command_end = NULL;
 
   if (retval == OK) {
@@ -2300,13 +2563,17 @@ parse_match(
         while (ASCII_ISALPHA(*p)) {
           if (STRNCMP(p, "kind:", 5) == 0) {
             tagp->tagkind = p + 5;
+          } else if (STRNCMP(p, "user_data:", 10) == 0) {
+            tagp->user_data = p + 10;
+          }
+          if (tagp->tagkind != NULL && tagp->user_data != NULL) {
             break;
           }
+
           pc = vim_strchr(p, ':');
           pt = vim_strchr(p, '\t');
           if (pc == NULL || (pt != NULL && pc > pt)) {
             tagp->tagkind = p;
-            break;
           }
           if (pt == NULL)
             break;
@@ -2320,6 +2587,12 @@ parse_match(
         ;
       tagp->tagkind_end = p;
     }
+    if (tagp->user_data != NULL) {
+      for (p = tagp->user_data;
+           *p && *p != '\t' && *p != '\r' && *p != '\n'; p++) {
+      }
+      tagp->user_data_end = p;
+    }
   }
   return retval;
 }
@@ -2770,6 +3043,15 @@ static int find_extra(char_u **pp)
   return FAIL;
 }
 
+//
+// Free a single entry in a tag stack
+//
+static void tagstack_clear_entry(taggy_T *item)
+{
+  XFREE_CLEAR(item->tagname);
+  XFREE_CLEAR(item->user_data);
+}
+
 int 
 expand_tags (
     int tagnames,                   /* expand tag names */
@@ -2789,14 +3071,16 @@ expand_tags (
     tagnmflag = TAG_NAMES;
   else
     tagnmflag = 0;
-  if (pat[0] == '/')
+  if (pat[0] == '/') {
     ret = find_tags(pat + 1, num_file, file,
-        TAG_REGEXP | tagnmflag | TAG_VERBOSE,
-        TAG_MANY, curbuf->b_ffname);
-  else
+                    TAG_REGEXP | tagnmflag | TAG_VERBOSE | TAG_NO_TAGFUNC,
+                    TAG_MANY, curbuf->b_ffname);
+  } else {
     ret = find_tags(pat, num_file, file,
-        TAG_REGEXP | tagnmflag | TAG_VERBOSE | TAG_NOIC,
-        TAG_MANY, curbuf->b_ffname);
+                    TAG_REGEXP | tagnmflag | TAG_VERBOSE
+                    | TAG_NO_TAGFUNC | TAG_NOIC,
+                    TAG_MANY, curbuf->b_ffname);
+  }
   if (ret == OK && !tagnames) {
     /* Reorganize the tags for display and matching as strings of:
      * "<tagname>\0<kind>\0<filename>\0"
@@ -2958,6 +3242,9 @@ static void get_tag_details(taggy_T *tag, dict_T *retdict)
   tv_dict_add_str(retdict, S_LEN("tagname"), (const char *)tag->tagname);
   tv_dict_add_nr(retdict, S_LEN("matchnr"), tag->cur_match + 1);
   tv_dict_add_nr(retdict, S_LEN("bufnr"), tag->cur_fnum);
+  if (tag->user_data) {
+    tv_dict_add_str(retdict, S_LEN("user_data"), (const char *)tag->user_data);
+  }
 
   pos = tv_list_alloc(4);
   tv_dict_add_list(retdict, S_LEN("from"), pos);
@@ -2996,7 +3283,7 @@ static void tagstack_clear(win_T *wp)
 {
   // Free the current tag stack
   for (int i = 0; i < wp->w_tagstacklen; i++) {
-    xfree(wp->w_tagstack[i].tagname);
+    tagstack_clear_entry(&wp->w_tagstack[i]);
   }
   wp->w_tagstacklen = 0;
   wp->w_tagstackidx = 0;
@@ -3007,7 +3294,7 @@ static void tagstack_clear(win_T *wp)
 static void tagstack_shift(win_T *wp)
 {
   taggy_T *tagstack = wp->w_tagstack;
-  xfree(tagstack[0].tagname);
+  tagstack_clear_entry(&tagstack[0]);
   for (int i = 1; i < wp->w_tagstacklen; i++) {
     tagstack[i - 1] = tagstack[i];
   }
@@ -3021,7 +3308,8 @@ static void tagstack_push_item(
     int     cur_fnum,
     int     cur_match,
     pos_T   mark,
-    int     fnum)
+    int     fnum,
+    char_u *user_data)
 {
   taggy_T *tagstack = wp->w_tagstack;
   int idx = wp->w_tagstacklen;  // top of the stack
@@ -3041,6 +3329,7 @@ static void tagstack_push_item(
   }
   tagstack[idx].fmark.mark = mark;
   tagstack[idx].fmark.fnum = fnum;
+  tagstack[idx].user_data = user_data;
 }
 
 // Add a list of items to the tag stack in the specified window
@@ -3076,10 +3365,13 @@ static void tagstack_push_items(win_T *wp, list_T *l)
     if (mark.col > 0) {
       mark.col--;
     }
-    tagstack_push_item(wp, tagname,
-                       (int)tv_dict_get_number(itemdict, "bufnr"),
-                       (int)tv_dict_get_number(itemdict, "matchnr") - 1,
-                       mark, fnum);
+    tagstack_push_item(
+        wp,
+        tagname,
+        (int)tv_dict_get_number(itemdict, "bufnr"),
+        (int)tv_dict_get_number(itemdict, "matchnr") - 1,
+        mark, fnum,
+        (char_u *)tv_dict_get_string(itemdict, "user_data", true));
   }
 }
 
@@ -3103,6 +3395,12 @@ int set_tagstack(win_T *wp, dict_T *d, int action)
   dictitem_T *di;
   list_T *l;
 
+  // not allowed to alter the tag stack entries from inside tagfunc
+  if (tfu_in_use) {
+    EMSG(_(recurmsg));
+    return FAIL;
+  }
+
   if ((di = tv_dict_find(d, "items", -1)) != NULL) {
     if (di->di_tv.v_type != VAR_LIST) {
       return FAIL;
diff --git a/src/nvim/tag.h b/src/nvim/tag.h
index a8fddd05da..9f671043b3 100644
--- a/src/nvim/tag.h
+++ b/src/nvim/tag.h
@@ -20,20 +20,21 @@
 #define DT_LTAG         11      /* tag using location list */
 #define DT_FREE         99      /* free cached matches */
 
-/*
- * flags for find_tags().
- */
-#define TAG_HELP        1       /* only search for help tags */
-#define TAG_NAMES       2       /* only return name of tag */
-#define TAG_REGEXP      4       /* use tag pattern as regexp */
-#define TAG_NOIC        8       /* don't always ignore case */
-#define TAG_CSCOPE      16      /* cscope tag */
-#define TAG_VERBOSE     32      /* message verbosity */
-#define TAG_INS_COMP    64      /* Currently doing insert completion */
-#define TAG_KEEP_LANG   128     /* keep current language */
+//
+// flags for find_tags().
+//
+#define TAG_HELP        1       // only search for help tags
+#define TAG_NAMES       2       // only return name of tag
+#define TAG_REGEXP      4       // use tag pattern as regexp
+#define TAG_NOIC        8       // don't always ignore case
+#define TAG_CSCOPE      16      // cscope tag
+#define TAG_VERBOSE     32      // message verbosity
+#define TAG_INS_COMP    64      // Currently doing insert completion
+#define TAG_KEEP_LANG   128     // keep current language
+#define TAG_NO_TAGFUNC  256     // do not use 'tagfunc'
 
-#define TAG_MANY        300     /* When finding many tags (for completion),
-                                   find up to this many tags */
+#define TAG_MANY        300     // When finding many tags (for completion),
+                                // find up to this many tags
 
 /*
  * Structure used for get_tagfname().
diff --git a/src/nvim/testdir/test_alot.vim b/src/nvim/testdir/test_alot.vim
index 6bf2e8329c..a871924d32 100644
--- a/src/nvim/testdir/test_alot.vim
+++ b/src/nvim/testdir/test_alot.vim
@@ -45,6 +45,7 @@ source test_syn_attr.vim
 source test_tabline.vim
 source test_tabpage.vim
 source test_tagcase.vim
+source test_tagfunc.vim
 source test_tagjump.vim
 source test_taglist.vim
 source test_true_false.vim
diff --git a/src/nvim/testdir/test_tagfunc.vim b/src/nvim/testdir/test_tagfunc.vim
new file mode 100644
index 0000000000..242aa3a235
--- /dev/null
+++ b/src/nvim/testdir/test_tagfunc.vim
@@ -0,0 +1,84 @@
+" Test 'tagfunc'
+
+func TagFunc(pat, flag, info)
+  let g:tagfunc_args = [a:pat, a:flag, a:info]
+  let tags = []
+  for num in range(1,10)
+    let tags += [{
+          \ 'cmd': '2', 'name': 'nothing'.num, 'kind': 'm',
+          \ 'filename': 'Xfile1', 'user_data': 'somedata'.num,
+          \}]
+  endfor
+  return tags
+endfunc
+
+func Test_tagfunc()
+  set tagfunc=TagFunc
+  new Xfile1
+  call setline(1, ['empty', 'one()', 'empty'])
+  write
+
+  call assert_equal({'cmd': '2', 'static': 0,
+        \ 'name': 'nothing2', 'user_data': 'somedata2',
+        \ 'kind': 'm', 'filename': 'Xfile1'}, taglist('.')[1])
+
+  call settagstack(win_getid(), {'items': []})
+
+  tag arbitrary
+  call assert_equal('arbitrary', g:tagfunc_args[0])
+  call assert_equal('', g:tagfunc_args[1])
+  call assert_equal('somedata1', gettagstack().items[0].user_data)
+  5tag arbitrary
+  call assert_equal('arbitrary', g:tagfunc_args[0])
+  call assert_equal('', g:tagfunc_args[1])
+  call assert_equal('somedata5', gettagstack().items[1].user_data)
+  pop
+  tag
+  call assert_equal('arbitrary', g:tagfunc_args[0])
+  call assert_equal('', g:tagfunc_args[1])
+  call assert_equal('somedata5', gettagstack().items[1].user_data)
+
+  let g:tagfunc_args=[]
+  execute "normal! \<c-]>"
+  call assert_equal('one', g:tagfunc_args[0])
+  call assert_equal('c', g:tagfunc_args[1])
+
+  set cpt=t
+  let g:tagfunc_args=[]
+  execute "normal! i\<c-n>\<c-y>"
+  call assert_equal('ci', g:tagfunc_args[1])
+  call assert_equal('nothing1', getline('.')[0:7])
+
+  func BadTagFunc1(...)
+    return 0
+  endfunc
+  func BadTagFunc2(...)
+    return [1]
+  endfunc
+  func BadTagFunc3(...)
+    return [{'name': 'foo'}]
+  endfunc
+
+  for &tagfunc in ['BadTagFunc1', 'BadTagFunc2', 'BadTagFunc3']
+    try
+      tag nothing
+      call assert_false(1, 'tag command should have failed')
+    catch
+      call assert_exception('E987:')
+    endtry
+    exe 'delf' &tagfunc
+  endfor
+
+  func NullTagFunc(...)
+    return v:null
+  endfunc
+  set tags= tfu=NullTagFunc
+  call assert_fails('tag nothing', 'E426')
+  delf NullTagFunc
+
+  bwipe!
+  set tags& tfu& cpt& 
+  call delete('Xfile1')
+endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab
diff --git a/src/nvim/window.c b/src/nvim/window.c
index 4d8eaa9dcc..976f1d8ff0 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -70,8 +70,8 @@ static char *m_onlyone = N_("Already only one window");
 /*
  * all CTRL-W window commands are handled here, called from normal_cmd().
  */
-void 
-do_window (
+void
+do_window(
     int nchar,
     long Prenum,
     int xchar                  /* extra char from ":wincmd gx" or NUL */
@@ -1537,10 +1537,14 @@ static void win_init(win_T *newp, win_T *oldp, int flags)
 
   /* copy tagstack and folds */
   for (i = 0; i < oldp->w_tagstacklen; i++) {
-    newp->w_tagstack[i] = oldp->w_tagstack[i];
-    if (newp->w_tagstack[i].tagname != NULL)
-      newp->w_tagstack[i].tagname =
-        vim_strsave(newp->w_tagstack[i].tagname);
+    taggy_T *tag = &newp->w_tagstack[i];
+    *tag = oldp->w_tagstack[i];
+    if (tag->tagname != NULL) {
+      tag->tagname = vim_strsave(tag->tagname);
+    }
+    if (tag->user_data != NULL) {
+      tag->user_data = vim_strsave(tag->user_data);
+    }
   }
   newp->w_tagstackidx = oldp->w_tagstackidx;
   newp->w_tagstacklen = oldp->w_tagstacklen;