vim-patch:9.1.1301: completion: cannot configure completion functions with 'complete'

Problem:  completion: cannot configure completion functions with
          'complete'
Solution: add support for setting completion functions using the f and o
          flag for 'complete' (Girish Palya)

This change adds two new values to the `'complete'` (`'cpt'`) option:
- `f` – invokes the function specified by the `'completefunc'` option
- `f{func}` – invokes a specific function `{func}` (can be a string or `Funcref`)

These new flags extend keyword completion behavior (e.g., via `<C-N>` /
`<C-P>`) by allowing function-based sources to participate in standard keyword
completion.

**Key behaviors:**

- Multiple `f{func}` values can be specified, and all will be called in order.
- Functions should follow the interface defined in `:help complete-functions`.
- When using `f{func}`, escaping is required for spaces (with `\`) and commas
  (with `\\`) in `Funcref` names.
- If a function sets `'refresh'` to `'always'`, it will be re-invoked on every
  change to the input text. Otherwise, Vim will attempt to reuse and filter
  existing matches as the input changes, which matches the default behavior of
  other completion sources.
- Matches are inserted at the keyword boundary for consistency with other completion methods.
- If finding matches is time-consuming, `complete_check()` can be used to
  maintain responsiveness.
- Completion matches are gathered in the sequence defined by the `'cpt'`
  option, preserving source priority.

This feature increases flexibility of standard completion mechanism and may
reduce the need for external completion plugins for many users.

**Examples:**

Complete matches from [LSP](https://github.com/yegappan/lsp) client. Notice the use of `refresh: always` and `function()`.

```vim
set cpt+=ffunction("g:LspCompletor"\\,\ [5]). # maxitems = 5

def! g:LspCompletor(maxitems: number, findstart: number, base: string): any
    if findstart == 1
        return g:LspOmniFunc(findstart, base)
    endif
    return {words: g:LspOmniFunc(findstart, base)->slice(0, maxitems), refresh: 'always'}
enddef
autocmd VimEnter * g:LspOptionsSet({ autoComplete: false, omniComplete: true })
```

Complete matches from `:iabbrev`.

```vim
set cpt+=fAbbrevCompletor

def! g:AbbrevCompletor(findstart: number, base: string): any
    if findstart > 0
        var prefix = getline('.')->strpart(0, col('.') - 1)->matchstr('\S\+$')
        if prefix->empty()
            return -2
        endif
        return col('.') - prefix->len() - 1
    endif
    var lines = execute('ia', 'silent!')
    if lines =~? gettext('No abbreviation found')
        return v:none  # Suppresses warning message
    endif
    var items = []
    for line in lines->split("\n")
        var m = line->matchlist('\v^i\s+\zs(\S+)\s+(.*)$')
        if m->len() > 2 && m[1]->stridx(base) == 0
            items->add({ word: m[1], info: m[2], dup: 1 })
        endif
    endfor
    return items->empty() ? v:none :
        items->sort((v1, v2) => v1.word < v2.word ? -1 : v1.word ==# v2.word ? 0 : 1)
enddef
```

**Auto-completion:**

Vim's standard completion frequently checks for user input while searching for
new matches. It is responsive irrespective of file size. This makes it
well-suited for smooth auto-completion. You can try with above examples:

```vim
set cot=menuone,popup,noselect inf

autocmd TextChangedI * InsComplete()

def InsComplete()
    if getcharstr(1) == '' && getline('.')->strpart(0, col('.') - 1) =~ '\k$'
        SkipTextChangedIEvent()
        feedkeys("\<c-n>", "n")
    endif
enddef

inoremap <silent> <c-e> <c-r>=<SID>SkipTextChangedIEvent()<cr><c-e>

def SkipTextChangedIEvent(): string
    # Suppress next event caused by <c-e> (or <c-n> when no matches found)
    set eventignore+=TextChangedI
    timer_start(1, (_) => {
        set eventignore-=TextChangedI
    })
    return ''
enddef
```

closes: vim/vim#17065

cbe53191d0

Temporarily remove bufname completion with #if 0 to make merging easier.

Co-authored-by: Girish Palya <girishji@gmail.com>
Co-authored-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: glepnir <glephunter@gmail.com>

src/nvim/insexpand.c

@@ -174,6 +174,7 @@ struct compl_S {
   bool cp_in_match_array;        ///< collected by compl_match_array
   int cp_user_abbr_hlattr;       ///< highlight attribute for abbr
   int cp_user_kind_hlattr;       ///< highlight attribute for kind
+  int cp_cpt_value_idx;          ///< index of this match's source in 'cpt' option
 };
 
 /// state information used for getting the next set of insert completion
@@ -190,6 +191,7 @@ typedef struct {
   bool found_all;         ///< found all matches of a certain type.
   char *dict;             ///< dictionary file to search
   int dict_f;             ///< "dict" is an exact file name or not
+  Callback *func_cb;      ///< callback of function in 'cpt' option
 } ins_compl_next_state_T;
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
@@ -213,7 +215,7 @@ static const char e_compldel[] = N_("E840: Completion function deleted text");
 // "compl_first_match" points to the start of the list.
 // "compl_curr_match" points to the currently selected entry.
 // "compl_shown_match" is different from compl_curr_match during
-// ins_compl_get_exp().
+// ins_compl_get_exp(), when new matches are added to the list.
 // "compl_old_match" points to previous "compl_curr_match".
 
 static compl_T *compl_first_match = NULL;
@@ -259,7 +261,8 @@ static bool compl_started = false;
 static int ctrl_x_mode = CTRL_X_NORMAL;
 
 static int compl_matches = 0;           ///< number of completion matches
-static String compl_pattern = STRING_INIT;
+static String compl_pattern = STRING_INIT;      ///< search pattern for matching items
+static String cpt_compl_pattern = STRING_INIT;  ///< pattern returned by func in 'cpt'
 static Direction compl_direction = FORWARD;
 static Direction compl_shows_dir = FORWARD;
 static int compl_pending = 0;           ///< > 1 for postponed CTRL-N
@@ -302,6 +305,13 @@ static int compl_selected_item = -1;
 
 static int *compl_fuzzy_scores;
 
+/// array indicating which 'cpt' functions have 'refresh:always' set
+static bool *cpt_func_refresh_always;
+/// total number of completion sources specified in the 'cpt' option
+static int cpt_value_count;
+/// index of the current completion source being expanded
+static int cpt_value_idx;
+
 // "compl_match_array" points the currently displayed list of entries in the
 // popup menu.  It is NULL when there is no popup menu.
 static pumitem_T *compl_match_array = NULL;
@@ -975,7 +985,6 @@ static int ins_compl_add(char *const str, int len, char *const fname, char *cons
   match = xcalloc(1, sizeof(compl_T));
   match->cp_number = flags & CP_ORIGINAL_TEXT ? 0 : -1;
   match->cp_str = cbuf_to_string(str, (size_t)len);
-  match->cp_score = score;
 
   // match-fname is:
   // - compl_curr_match->cp_fname if it is a string equal to fname.
@@ -995,6 +1004,8 @@ static int ins_compl_add(char *const str, int len, char *const fname, char *cons
   match->cp_flags = flags;
   match->cp_user_abbr_hlattr = user_hl ? user_hl[0] : -1;
   match->cp_user_kind_hlattr = user_hl ? user_hl[1] : -1;
+  match->cp_score = score;
+  match->cp_cpt_value_idx = cpt_value_idx;
 
   if (cptext != NULL) {
     for (int i = 0; i < CPT_COUNT; i++) {
@@ -1887,6 +1898,19 @@ char *find_line_end(char *ptr)
   return s;
 }
 
+/// Free a completion item in the list
+static void ins_compl_item_free(compl_T *match)
+{
+  API_CLEAR_STRING(match->cp_str);
+  // several entries may use the same fname, free it just once.
+  if (match->cp_flags & CP_FREE_FNAME) {
+    xfree(match->cp_fname);
+  }
+  free_cptext(match->cp_text);
+  tv_clear(&match->cp_user_data);
+  xfree(match);
+}
+
 /// Free the list of completions
 static void ins_compl_free(void)
 {
@@ -1904,14 +1928,7 @@ static void ins_compl_free(void)
   do {
     compl_T *match = compl_curr_match;
     compl_curr_match = compl_curr_match->cp_next;
-    API_CLEAR_STRING(match->cp_str);
-    // several entries may use the same fname, free it just once.
-    if (match->cp_flags & CP_FREE_FNAME) {
-      xfree(match->cp_fname);
-    }
-    free_cptext(match->cp_text);
-    tv_clear(&match->cp_user_data);
-    xfree(match);
+    ins_compl_item_free(match);
   } while (compl_curr_match != NULL && !is_first_match(compl_curr_match));
   compl_first_match = compl_curr_match = NULL;
   compl_shown_match = NULL;
@@ -1935,6 +1952,7 @@ void ins_compl_clear(void)
   kv_destroy(compl_orig_extmarks);
   API_CLEAR_STRING(compl_orig_text);
   compl_enter_selects = false;
+  cpt_compl_src_clear();
   // clear v:completed_item
   set_vim_var_dict(VV_COMPLETED_ITEM, tv_dict_alloc_lock(VAR_FIXED));
 }
@@ -1952,8 +1970,8 @@ bool ins_compl_win_active(win_T *wp)
   return ins_compl_active() && wp == compl_curr_win && wp->w_buffer == compl_curr_buf;
 }
 
-/// Selected one of the matches.  When false the match was edited or using the
-/// longest common string.
+/// Selected one of the matches.  When false, the match was edited or
+/// using the longest common string.
 bool ins_compl_used_match(void)
 {
   return compl_used_match;
@@ -2087,6 +2105,9 @@ static void ins_compl_new_leader(void)
 
   if (compl_started) {
     ins_compl_set_original_text(compl_leader.data, compl_leader.size);
+    if (is_cpt_func_refresh_always()) {
+      cpt_compl_refresh();
+    }
   } else {
     spell_bad_len = 0;  // need to redetect bad word
     // Matches were cleared, need to search for them now.
@@ -2170,6 +2191,7 @@ static void ins_compl_restart(void)
   compl_matches = 0;
   compl_cont_status = 0;
   compl_cont_mode = 0;
+  cpt_compl_src_clear();
 }
 
 /// Set the first match, the original text.
@@ -2791,8 +2813,9 @@ static Callback *get_insert_callback(int type)
 /// Execute user defined complete function 'completefunc', 'omnifunc' or
 /// 'thesaurusfunc', and get matches in "matches".
 ///
-/// @param type  either CTRL_X_OMNI or CTRL_X_FUNCTION or CTRL_X_THESAURUS
-static void expand_by_function(int type, char *base)
+/// @param type  one of CTRL_X_OMNI or CTRL_X_FUNCTION or CTRL_X_THESAURUS
+/// @param cb    set if triggered by a function in 'cpt' option, otherwise NULL
+static void expand_by_function(int type, char *base, Callback *cb)
 {
   list_T *matchlist = NULL;
   dict_T *matchdict = NULL;
@@ -2800,9 +2823,14 @@ static void expand_by_function(int type, char *base)
   const int save_State = State;
 
   assert(curbuf != NULL);
-  char *funcname = get_complete_funcname(type);
-  if (*funcname == NUL) {
-    return;
+
+  const bool is_cpt_function = (cb != NULL);
+  if (!is_cpt_function) {
+    char *funcname = get_complete_funcname(type);
+    if (*funcname == NUL) {
+      return;
+    }
+    cb = get_insert_callback(type);
   }
 
   // Call 'completefunc' to obtain the list of matches.
@@ -2819,8 +2847,6 @@ static void expand_by_function(int type, char *base)
   // Insert mode in another buffer.
   textlock++;
 
-  Callback *cb = get_insert_callback(type);
-
   // Call a function, which returns a list or dict.
   if (callback_call(cb, 2, args, &rettv)) {
     switch (rettv.v_type) {
@@ -3492,12 +3518,25 @@ static int process_next_cpt_value(ins_compl_next_state_T *st, int *compl_type_ar
         st->dict = st->e_cpt;
         st->dict_f = DICT_FIRST;
       }
+    } else if (*st->e_cpt == 'f' || *st->e_cpt == 'o') {
+      compl_type = CTRL_X_FUNCTION;
+      if (*st->e_cpt == 'o') {
+        st->func_cb = &curbuf->b_ofu_cb;
+      } else {
+        st->func_cb = (*++st->e_cpt != ',' && *st->e_cpt != NUL)
+                      ? get_cpt_func_callback(st->e_cpt) : &curbuf->b_cfu_cb;
+      }
+      if (!st->func_cb) {
+        compl_type = -1;
+      }
     } else if (*st->e_cpt == 'i') {
       compl_type = CTRL_X_PATH_PATTERNS;
     } else if (*st->e_cpt == 'd') {
       compl_type = CTRL_X_PATH_DEFINES;
+#if 0
     } else if (*st->e_cpt == 'f') {
       compl_type = CTRL_X_BUFNAMES;
+#endif
     } else if (*st->e_cpt == ']' || *st->e_cpt == 't') {
       compl_type = CTRL_X_TAGS;
       if (!shortmess(SHM_COMPLETIONSCAN)) {
@@ -3539,7 +3578,7 @@ static void get_next_include_file_completion(int compl_type)
 static void get_next_dict_tsr_completion(int compl_type, char *dict, int dict_f)
 {
   if (thesaurus_func_complete(compl_type)) {
-    expand_by_function(compl_type, compl_pattern.data);
+    expand_by_function(compl_type, compl_pattern.data, NULL);
   } else {
     ins_compl_dictionaries(dict != NULL
                            ? dict
@@ -4102,6 +4141,30 @@ static void get_register_completion(void)
   }
 }
 
+/// Return the callback function associated with "funcname".
+static Callback *get_cpt_func_callback(char *funcname)
+{
+  static Callback cb;
+  char buf[LSIZE];
+
+  size_t slen = copy_option_part(&funcname, buf, LSIZE, ",");
+  if (slen > 0 && option_set_callback_func(buf, &cb)) {
+    return &cb;
+  }
+  return NULL;
+}
+
+/// Retrieve new completion matches by invoking callback "cb".
+static void expand_cpt_function(Callback *cb)
+{
+  // Re-insert the text removed by ins_compl_delete().
+  ins_compl_insert_bytes(compl_orig_text.data + get_compl_len(), -1);
+  // Get matches
+  get_cpt_func_completion_matches(cb);
+  // Undo insertion
+  ins_compl_delete(false);
+}
+
 /// get the next set of completion matches for "type".
 /// @return  true if a new match is found, otherwise false.
 static bool get_next_completion_match(int type, ins_compl_next_state_T *st, pos_T *ini)
@@ -4136,8 +4199,14 @@ static bool get_next_completion_match(int type, ins_compl_next_state_T *st, pos_
     break;
 
   case CTRL_X_FUNCTION:
+    if (ctrl_x_mode_normal()) {  // Invoked by a func in 'cpt' option
+      expand_cpt_function(st->func_cb);
+    } else {
+      expand_by_function(type, compl_pattern.data, NULL);
+    }
+    break;
   case CTRL_X_OMNI:
-    expand_by_function(type, compl_pattern.data);
+    expand_by_function(type, compl_pattern.data, NULL);
     break;
 
   case CTRL_X_SPELL:
@@ -4210,6 +4279,10 @@ static int ins_compl_get_exp(pos_T *ini)
     st.e_cpt_copy = xstrdup((compl_cont_status & CONT_LOCAL) ? "." : curbuf->b_p_cpt);
     st.e_cpt = st.e_cpt_copy;
     st.last_match_pos = st.first_match_pos = *ini;
+
+    if (ctrl_x_mode_normal() || ctrl_x_mode_line_or_eval()) {
+      cpt_compl_src_init(st.e_cpt);
+    }
   } else if (st.ins_buf != curbuf && !buf_valid(st.ins_buf)) {
     st.ins_buf = curbuf;  // In case the buffer was wiped out.
   }
@@ -4219,7 +4292,7 @@ static int ins_compl_get_exp(pos_T *ini)
   st.cur_match_pos = compl_dir_forward() ? &st.last_match_pos : &st.first_match_pos;
 
   // For ^N/^P loop over all the flags/windows/buffers in 'complete'
-  while (true) {
+  for (cpt_value_idx = 0;;) {
     found_new_match = FAIL;
     st.set_match_pos = false;
 
@@ -4233,6 +4306,7 @@ static int ins_compl_get_exp(pos_T *ini)
         break;
       }
       if (status == INS_COMPL_CPT_CONT) {
+        cpt_value_idx++;
         continue;
       }
     }
@@ -4246,6 +4320,10 @@ static int ins_compl_get_exp(pos_T *ini)
     // get the next set of completion matches
     found_new_match = get_next_completion_match(type, &st, ini);
 
+    if (type > 0) {
+      cpt_value_idx++;
+    }
+
     // break the loop for specialized modes (use 'complete' just for the
     // generic ctrl_x_mode == CTRL_X_NORMAL) or when we've found a new match
     if ((ctrl_x_mode_not_default() && !ctrl_x_mode_line_or_eval())
@@ -4273,6 +4351,7 @@ static int ins_compl_get_exp(pos_T *ini)
       compl_started = false;
     }
   }
+  cpt_value_idx = -1;
   compl_started = true;
 
   if ((ctrl_x_mode_normal() || ctrl_x_mode_line_or_eval())
@@ -5025,17 +5104,25 @@ static int get_cmdline_compl_info(char *line, colnr_T curs_col)
 /// 'completefunc' and 'thesaurusfunc')
 /// Sets the global variables: compl_col, compl_length and compl_pattern.
 /// Uses the global variable: spell_bad_len
-static int get_userdefined_compl_info(colnr_T curs_col)
+///
+/// @param cb        set if triggered by a function in the 'cpt' option, otherwise NULL
+/// @param startcol  when not NULL, contains the column returned by function.
+static int get_userdefined_compl_info(colnr_T curs_col, Callback *cb, int *startcol)
 {
   // Call user defined function 'completefunc' with "a:findstart"
   // set to 1 to obtain the length of text to use for completion.
   const int save_State = State;
 
-  // Call 'completefunc' or 'omnifunc' and get pattern length as a string
-  char *funcname = get_complete_funcname(ctrl_x_mode);
-  if (*funcname == NUL) {
-    semsg(_(e_notset), ctrl_x_mode_function() ? "completefunc" : "omnifunc");
-    return FAIL;
+  const bool is_cpt_function = (cb != NULL);
+  if (!is_cpt_function) {
+    // Call 'completefunc' or 'omnifunc' or 'thesaurusfunc' and get pattern
+    // length as a string
+    char *funcname = get_complete_funcname(ctrl_x_mode);
+    if (*funcname == NUL) {
+      semsg(_(e_notset), ctrl_x_mode_function() ? "completefunc" : "omnifunc");
+      return FAIL;
+    }
+    cb = get_insert_callback(ctrl_x_mode);
   }
 
   typval_T args[3];
@@ -5047,7 +5134,6 @@ static int get_userdefined_compl_info(colnr_T curs_col)
 
   pos_T pos = curwin->w_cursor;
   textlock++;
-  Callback *cb = get_insert_callback(ctrl_x_mode);
   colnr_T col = (colnr_T)callback_call_retnr(cb, 2, args);
   textlock--;
 
@@ -5060,6 +5146,10 @@ static int get_userdefined_compl_info(colnr_T curs_col)
     return FAIL;
   }
 
+  if (startcol != NULL) {
+    *startcol = col;
+  }
+
   // Return value -2 means the user complete function wants to cancel the
   // complete without an error, do the same if the function did not execute
   // successfully.
@@ -5069,6 +5159,9 @@ static int get_userdefined_compl_info(colnr_T curs_col)
 
   // Return value -3 does the same as -2 and leaves CTRL-X mode.
   if (col == -3) {
+    if (is_cpt_function) {
+      return FAIL;
+    }
     ctrl_x_mode = CTRL_X_NORMAL;
     edit_submode = NULL;
     if (!shortmess(SHM_COMPLETIONMENU)) {
@@ -5081,20 +5174,22 @@ static int get_userdefined_compl_info(colnr_T curs_col)
   // completion.
   compl_opt_refresh_always = false;
 
-  if (col < 0) {
+  if (col < 0 || col > curs_col) {
     col = curs_col;
   }
-  compl_col = col;
-  if (compl_col > curs_col) {
-    compl_col = curs_col;
-  }
 
   // Setup variables for completion.  Need to obtain "line" again,
   // it may have become invalid.
   char *line = ml_get(curwin->w_cursor.lnum);
-  compl_length = curs_col - compl_col;
-  compl_pattern = cbuf_to_string(line + compl_col, (size_t)compl_length);
+  int len = curs_col - col;
+  String *compl_pat = is_cpt_function ? &cpt_compl_pattern : &compl_pattern;
+  compl_pat->data = xstrnsave(line + col, (size_t)len);
+  compl_pat->size = (size_t)compl_length;
 
+  if (!is_cpt_function) {
+    compl_col = col;
+    compl_length = len;
+  }
   return OK;
 }
 
@@ -5146,7 +5241,7 @@ static int compl_get_info(char *line, int startcol, colnr_T curs_col, bool *line
     return get_cmdline_compl_info(line, curs_col);
   } else if (ctrl_x_mode_function() || ctrl_x_mode_omni()
              || thesaurus_func_complete(ctrl_x_mode)) {
-    if (get_userdefined_compl_info(curs_col) == FAIL) {
+    if (get_userdefined_compl_info(curs_col, NULL, NULL) != OK) {
       return FAIL;
     }
     *line_invalid = true;  // "line" may have become invalid
@@ -5576,3 +5671,175 @@ static void spell_back_to_badword(void)
     start_arrow(&tpos);
   }
 }
+
+/// Reset the info associated with completion sources.
+static void cpt_compl_src_clear(void)
+{
+  XFREE_CLEAR(cpt_func_refresh_always);
+  cpt_value_idx = -1;
+  cpt_value_count = 0;
+}
+
+/// Initialize the info associated with completion sources.
+static void cpt_compl_src_init(char *cpt_str)
+{
+  int count = 0;
+  char *p = cpt_str;
+
+  while (*p) {
+    while (*p == ',' || *p == ' ') {  // Skip delimiters
+      p++;
+    }
+    if (*p) {  // If not end of string, count this segment
+      count++;
+      copy_option_part(&p, IObuff, IOSIZE, ",");  // Advance p
+    }
+  }
+  cpt_compl_src_clear();
+  cpt_value_count = count;
+  if (count > 0) {
+    cpt_func_refresh_always = xcalloc((size_t)count, sizeof(bool));
+  }
+}
+
+/// Return true if any of the completion sources have 'refresh' set to 'always'.
+static bool is_cpt_func_refresh_always(void)
+{
+  for (int i = 0; i < cpt_value_count; i++) {
+    if (cpt_func_refresh_always[i]) {
+      return true;
+    }
+  }
+  return false;
+}
+
+/// Make the completion list non-cyclic.
+static void ins_compl_make_linear(void)
+{
+  if (compl_first_match == NULL || compl_first_match->cp_prev == NULL) {
+    return;
+  }
+  compl_T *m = compl_first_match->cp_prev;
+  m->cp_next = NULL;
+  compl_first_match->cp_prev = NULL;
+}
+
+/// Remove the matches linked to the current completion source (as indicated by
+/// cpt_value_idx) from the completion list.
+static compl_T *remove_old_matches(void)
+{
+  compl_T *sublist_start = NULL, *sublist_end = NULL, *insert_at = NULL;
+  compl_T *current, *next;
+  bool compl_shown_removed = false;
+  int forward = compl_dir_forward();
+
+  // Identify the sublist of old matches that needs removal
+  for (current = compl_first_match; current != NULL; current = current->cp_next) {
+    if (current->cp_cpt_value_idx < cpt_value_idx && (forward || (!forward && !insert_at))) {
+      insert_at = current;
+    }
+
+    if (current->cp_cpt_value_idx == cpt_value_idx) {
+      if (!sublist_start) {
+        sublist_start = current;
+      }
+      sublist_end = current;
+      if (!compl_shown_removed && compl_shown_match == current) {
+        compl_shown_removed = true;
+      }
+    }
+
+    if ((forward && current->cp_cpt_value_idx > cpt_value_idx) || (!forward && insert_at)) {
+      break;
+    }
+  }
+
+  // Re-assign compl_shown_match if necessary
+  if (compl_shown_removed) {
+    if (forward) {
+      compl_shown_match = compl_first_match;
+    } else {    // Last node will have the prefix that is being completed
+      for (current = compl_first_match; current->cp_next != NULL; current = current->cp_next) {}
+      compl_shown_match = current;
+    }
+  }
+
+  if (!sublist_start) {  // No nodes to remove
+    return insert_at;
+  }
+
+  // Update links to remove sublist
+  if (sublist_start->cp_prev) {
+    sublist_start->cp_prev->cp_next = sublist_end->cp_next;
+  } else {
+    compl_first_match = sublist_end->cp_next;
+  }
+
+  if (sublist_end->cp_next) {
+    sublist_end->cp_next->cp_prev = sublist_start->cp_prev;
+  }
+
+  // Free all nodes in the sublist
+  sublist_end->cp_next = NULL;
+  for (current = sublist_start; current != NULL; current = next) {
+    next = current->cp_next;
+    ins_compl_item_free(current);
+  }
+
+  return insert_at;
+}
+
+/// Retrieve completion matches using the callback function "cb" and store the
+/// 'refresh:always' flag.
+static void get_cpt_func_completion_matches(Callback *cb)
+{
+  API_CLEAR_STRING(cpt_compl_pattern);
+  int startcol;
+  int ret = get_userdefined_compl_info(curwin->w_cursor.col, cb, &startcol);
+  if (ret == FAIL && startcol == -3) {
+    cpt_func_refresh_always[cpt_value_idx] = false;
+  } else if (ret == OK) {
+    expand_by_function(0, cpt_compl_pattern.data, cb);
+    cpt_func_refresh_always[cpt_value_idx] = compl_opt_refresh_always;
+    compl_opt_refresh_always = false;
+  }
+}
+
+/// Retrieve completion matches from functions in the 'cpt' option where the
+/// 'refresh:always' flag is set.
+static void cpt_compl_refresh(void)
+{
+  Callback *cb;
+
+  // Make the completion list linear (non-cyclic)
+  ins_compl_make_linear();
+  // Make a copy of 'cpt' in case the buffer gets wiped out
+  char *cpt = xstrdup(curbuf->b_p_cpt);
+
+  cpt_value_idx = 0;
+  for (char *p = cpt; *p; cpt_value_idx++) {
+    while (*p == ',' || *p == ' ') {  // Skip delimiters
+      p++;
+    }
+
+    if (cpt_func_refresh_always[cpt_value_idx]) {
+      if (*p == 'o') {
+        cb = &curbuf->b_ofu_cb;
+      } else if (*p == 'f') {
+        cb = (*(p + 1) != ',' && *(p + 1) != NUL)
+             ? get_cpt_func_callback(p + 1) : &curbuf->b_cfu_cb;
+      }
+      if (cb) {
+        compl_curr_match = remove_old_matches();
+        get_cpt_func_completion_matches(cb);
+      }
+    }
+
+    copy_option_part(&p, IObuff, IOSIZE, ",");  // Advance p
+  }
+  cpt_value_idx = -1;
+
+  xfree(cpt);
+  // Make the list cyclic
+  compl_matches = ins_compl_make_cyclic();
+}

src/nvim/options.lua

@@ -1412,7 +1412,7 @@ local options = {
       abbreviation = 'cpt',
       cb = 'did_set_complete',
       defaults = '.,w,b,u,t',
-      values = { '.', 'w', 'b', 'u', 'k', 'kspell', 's', 'i', 'd', ']', 't', 'U', 'f' },
+      values = { '.', 'w', 'b', 'u', 'k', 'kspell', 's', 'i', 'd', ']', 't', 'U', 'f', 'o' },
       deny_duplicates = true,
       desc = [=[
         This option specifies how keyword completion |ins-completion| works
@@ -1438,6 +1438,28 @@ local options = {
         ]	tag completion
         t	same as "]"
         f	scan the buffer names (as opposed to buffer contents)
+        f{func}	call the function {func}. Multiple "f" flags may be specified.
+        	Refer to |complete-functions| for details on how the function
+        	is invoked and what it should return.  The value can be the
+        	name of a function or a |Funcref|.  For |Funcref| values,
+        	spaces must be escaped with a backslash ('\'), and commas with
+        	double backslashes ('\\') (see |option-backslash|).
+        	If the Dict returned by the {func} includes {"refresh": "always"},
+        	the function will be invoked again whenever the leading text
+        	changes.
+        	Completion matches are always inserted at the keyword
+        	boundary, regardless of the column returned by {func} when
+        	a:findstart is 1.  This ensures compatibility with other
+        	completion sources.
+        	To make further modifications to the inserted text, {func}
+        	can make use of |CompleteDonePre|.
+        	If generating matches is potentially slow, |complete_check()|
+        	should be used to avoid blocking and preserve editor
+        	responsiveness.
+        f	equivalent to using "f{func}", where the function is taken from
+        	the 'completefunc' option.
+        o	equivalent to using "f{func}", where the function is taken from
+        	the 'omnifunc' option.
 
         Unloaded buffers are not loaded, thus their autocmds |:autocmd| are
         not executed, this may lead to unexpected completions from some files

src/nvim/optionstr.c

@@ -45,6 +45,7 @@
 #include "nvim/spellfile.h"
 #include "nvim/spellsuggest.h"
 #include "nvim/strings.h"
+#include "nvim/tag.h"
 #include "nvim/terminal.h"
 #include "nvim/types_defs.h"
 #include "nvim/vim_defs.h"
@@ -54,10 +55,12 @@
 # include "optionstr.c.generated.h"
 #endif
 
-static const char e_unclosed_expression_sequence[]
-  = N_("E540: Unclosed expression sequence");
+static const char e_illegal_character_after_chr[]
+  = N_("E535: Illegal character after <%c>");
 static const char e_comma_required[]
   = N_("E536: Comma required");
+static const char e_unclosed_expression_sequence[]
+  = N_("E540: Unclosed expression sequence");
 static const char e_unbalanced_groups[]
   = N_("E542: Unbalanced groups");
 static const char e_backupext_and_patchmode_are_equal[]
@@ -836,40 +839,45 @@ const char *did_set_commentstring(optset_T *args)
   return NULL;
 }
 
-/// The 'complete' option is changed.
+/// Check if value for 'complete' is valid when 'complete' option is changed.
 const char *did_set_complete(optset_T *args)
 {
   char **varp = (char **)args->os_varp;
+  char buffer[LSIZE];
 
-  // check if it is a valid value for 'complete' -- Acevedo
-  for (char *s = *varp; *s;) {
-    while (*s == ',' || *s == ' ') {
-      s++;
-    }
-    if (!*s) {
-      break;
-    }
-    if (vim_strchr(".wbuksid]tUf", (uint8_t)(*s)) == NULL) {
-      return illegal_char(args->os_errbuf, args->os_errbuflen, (uint8_t)(*s));
-    }
-    if (*++s != NUL && *s != ',' && *s != ' ') {
-      if (s[-1] == 'k' || s[-1] == 's') {
-        // skip optional filename after 'k' and 's'
-        while (*s && *s != ',' && *s != ' ') {
-          if (*s == '\\' && s[1] != NUL) {
-            s++;
-          }
-          s++;
-        }
+  for (char *p = *varp; *p;) {
+    memset(buffer, 0, LSIZE);
+    char *buf_ptr = buffer;
+    int escape = 0;
+
+    // Extract substring while handling escaped commas
+    while (*p && (*p != ',' || escape) && buf_ptr < (buffer + LSIZE - 1)) {
+      if (*p == '\\' && *(p + 1) == ',') {
+        escape = 1;  // Mark escape mode
+        p++;         // Skip '\'
       } else {
-        if (args->os_errbuf != NULL) {
-          vim_snprintf(args->os_errbuf, args->os_errbuflen,
-                       _("E535: Illegal character after <%c>"),
-                       *--s);
-          return args->os_errbuf;
-        }
-        return "";
+        escape = 0;
+        *buf_ptr++ = *p;
       }
+      p++;
+    }
+    *buf_ptr = NUL;
+
+    if (vim_strchr(".wbuksid]tUfo", (uint8_t)(*buffer)) == NULL) {
+      return illegal_char(args->os_errbuf, args->os_errbuflen, (uint8_t)(*buffer));
+    }
+
+    if (!vim_strchr("ksf", (uint8_t)(*buffer)) && *(buffer + 1) != NUL) {
+      if (args->os_errbuf != NULL) {
+        vim_snprintf(args->os_errbuf, args->os_errbuflen,
+                     _(e_illegal_character_after_chr), (uint8_t)(*buffer));
+        return args->os_errbuf;
+      }
+    }
+
+    // Skip comma and spaces
+    while (*p == ',' || *p == ' ') {
+      p++;
     }
   }
   return NULL;