fix(multicursor): 'autocomplete' during insert-cascade raises E565 #41625

Problem:
With mcursors and 'autocomplete', a non-literal key (`<BS>`) during an
active completion attempts to insert-cascade, which attempts a nested
`edit()`, which refuses with E565.

Solution:
Defer the flush while completion is active; the pending keys will be
handled later.

src/nvim/buffer_defs.h

@@ -65,7 +65,7 @@ typedef struct {
 #define BF_NEW          0x10    // file didn't exist when editing started
 #define BF_NEW_W        0x20    // Warned for BF_NEW and file created
 #define BF_READERR      0x40    // got errors while reading the file
-#define BF_DUMMY        0x80    // dummy buffer, only used internally
+#define BF_DUMMY        0x80    // Internal-only dummy buffer.
 #define BF_SYN_SET      0x200   // 'syntax' option was set
 
 // Mask to check for flags that prevent normal writing

src/nvim/ex_docmd.c

@@ -7142,6 +7142,8 @@ void restore_current_state(save_state_T *sst)
   ui_cursor_shape();  // may show different cursor shape
 }
 
+/// True while evaluating an `<expr>` mapping.
+/// Buffer changes are not allowed, except for internal-only dummy buffers.
 bool expr_map_locked(void)
 {
   return expr_map_lock > 0 && !(curbuf->b_flags & BF_DUMMY);

src/nvim/insert.c

@@ -1342,23 +1342,21 @@ static void insert_handle_key_post(InsertState *s)
 
 /// edit(): Start inserting text.
 ///
-/// "cmdchar" can be:
-/// 'i' normal insert command
-/// 'a' normal append command
-/// 'R' replace command
-/// 'r' "r<CR>" command: insert one <CR>.
-///     Note: count can be > 1, for redo, but still only one <CR> is inserted.
-///           <Esc> is not used for redo.
-/// 'g' "gI" command.
-/// 'V' "gR" command for Virtual Replace mode.
-/// 'v' "gr" command for single character Virtual Replace mode.
+/// May nest: ":normal i…" and multicursor insert-cascade enters it while an outer edit() is
+/// suspended. But i_CTRL-O does not nest: edit() returns, and the caller runs the Normal-mode
+/// command.
 ///
-/// This function is not called recursively.  For CTRL-O commands, it returns
-/// and lets the caller handle the Normal-mode command.
-///
-/// @param  cmdchar  command that started the insert
-/// @param  startln  if true, insert at start of line
-/// @param  count    repeat count for the command
+/// @param  cmdchar  Command that started the insert:
+///   - 'i'
+///   - 'a'
+///   - 'R'
+///   - 'r': "r<CR>" command: insert one <CR>. Note: count can be > 1, for redo, but still only one
+///     <CR> is inserted. <Esc> is not used for redo.
+///   - 'g': "gI" command.
+///   - 'V': "gR" command: Virtual Replace mode.
+///   - 'v': "gr" command: single-character Virtual Replace mode.
+/// @param  startln  If true, insert at start of line.
+/// @param  count    Repeat count for the command.
 ///
 /// @return true if a CTRL-O command caused the return (insert mode pending).
 bool edit(int cmdchar, bool startln, int count)
@@ -1381,10 +1379,10 @@ bool edit(int cmdchar, bool startln, int count)
     return false;
   }
 
-  // Don't allow changes in the buffer while editing the cmdline.  The
-  // caller of getcmdline() may get confused.
-  // Don't allow recursive insert mode when busy with completion.
-  // Allow in dummy buffers since they are only used internally
+  // Disallow edit() (insert-mode) when:
+  // - ins-completion is active
+  // - textlock (the caller of getcmdline() may get confused)
+  // - <expr> mapping eval
   if (textlock != 0 || ins_compl_active() || compl_busy || pum_visible()
       || expr_map_locked()) {
     emsg(_(e_textlock));

src/nvim/mcursor.c

@@ -33,6 +33,7 @@
 #include "nvim/input.h"
 #include "nvim/input_cmdatom.h"
 #include "nvim/insert.h"
+#include "nvim/insexpand.h"
 #include "nvim/keycodes.h"
 #include "nvim/log.h"
 #include "nvim/lua/executor.h"
@@ -51,6 +52,7 @@
 #include "nvim/os/input.h"
 #include "nvim/os/time.h"
 #include "nvim/plines.h"
+#include "nvim/popupmenu.h"
 #include "nvim/pos_defs.h"
 #include "nvim/register.h"
 #include "nvim/register_defs.h"
@@ -525,7 +527,7 @@ void mc_clock_edge(bool map_edit)
   }
 }
 
-/// Removes cursors that overlap another cursor, so an edit does not apply N times at one position.
+/// Prunes cursors that overlap others, so an edit does not apply N times at one position.
 static void mc_dedupe(void)
 {
   const bool had_cursors = kv_size(mc_cursors) > 0;
@@ -839,8 +841,11 @@ void mc_ins_cascade(void)
   } else if (ins.size > mc_ins_span.done_len
              && mc_ins_keys_nonliteral(ins.data + mc_ins_span.done_len,
                                        ins.size - mc_ins_span.done_len)) {
-    // Non-literal keys pending (BS, CTRL-U, ...): re-execute instead of previewing.
-    mc_ins_span_flush(&ins, false);
+    // Non-literal keys pending (BS, CTRL-U, …): re-execute instead of previewing.
+    // Not during completion: edit() would raise E565. #41602
+    if (!ins_compl_active() && !pum_visible()) {
+      mc_ins_span_flush(&ins, false);
+    }
   } else {
     MTPair p = extmark_from_id(curbuf, mc_session_ns(), mc_ins_span.region);
     if (p.start.id != 0) {

test/functional/editor/mcursor_spec.lua

@@ -1401,6 +1401,17 @@ describe('multicursor', function()
       feed('cc')
       feed('f<C-n><C-n><C-e><Esc>') -- cycle, then cancel back to the leader
       eq({ 'f', 'f', 'f' }, { get_lines()[4], get_lines()[5], get_lines()[6] })
+
+      -- No candidates, {Visual}Q cursors: <BS> mid-completion. #41602
+      clear_cursors()
+      api.nvim_buf_set_lines(0, 0, -1, true, { '', '', '' })
+      command('let v:errmsg = ""')
+      feed('VggQ')
+      feed('i a<BS>')
+      eq('', api.nvim_get_vvar('errmsg'))
+      feed('<Esc>')
+      eq('', api.nvim_get_vvar('errmsg'))
+      eq({ ' ', ' ', ' ' }, get_lines())
     end)
 
     it('InsertCharPre-driven complete() plugin (cmp-style)', function()