fix(mouse): do not fetch clipboard twice when pasting with middle button #33494

Problem:
When doing paste operation mouse code tries to figure out it it is
dealing with a multi-line register by calling yank_register_mline(),
which fetches register data and checks its type. Later the code calls
either do_put() or insert_reg() which fetch register data again. This is
unnoticeable when working with internal neovim registers, but starts
hurting when dealing with clipboards, especially remote one (forwarded X
or socket tunnel or similar).

Solution:
Change yank_register_mline() to also return pointer to the
register structure prepared for pasting, and insert_reg() to accept
such register pointer and use it if it is supplied. do_put() already
has support for accepting a register structure to be used for pasting.

Fixes #33493

(cherry picked from commit 7432781e71)

src/nvim/edit.c

@@ -3244,7 +3244,7 @@ static void ins_reg(void)
 
       do_put(regname, NULL, BACKWARD, 1,
              (literally == Ctrl_P ? PUT_FIXINDENT : 0) | PUT_CURSEND);
-    } else if (insert_reg(regname, literally) == FAIL) {
+    } else if (insert_reg(regname, NULL, literally) == FAIL) {
       vim_beep(kOptBoFlagRegister);
       need_redraw = true;  // remove the '"'
     } else if (stop_insert_mode) {

src/nvim/mouse.c

@@ -503,15 +503,16 @@ bool do_mouse(oparg_T *oap, int c, int dir, int count, bool fixindent)
     // happens for the GUI).
     if ((State & MODE_INSERT)) {
       if (regname == '.') {
-        insert_reg(regname, true);
+        insert_reg(regname, NULL, true);
       } else {
         if (regname == 0 && eval_has_provider("clipboard", false)) {
           regname = '*';
         }
-        if ((State & REPLACE_FLAG) && !yank_register_mline(regname)) {
+        yankreg_T *reg = NULL;
-          insert_reg(regname, true);
+        if ((State & REPLACE_FLAG) && !yank_register_mline(regname, &reg)) {
+          insert_reg(regname, reg, true);
         } else {
-          do_put(regname, NULL, BACKWARD, 1,
+          do_put(regname, reg, BACKWARD, 1,
                  (fixindent ? PUT_FIXINDENT : 0) | PUT_CURSEND);
 
           // Repeat it with CTRL-R CTRL-O r or CTRL-R CTRL-P r
@@ -833,7 +834,8 @@ bool do_mouse(oparg_T *oap, int c, int dir, int count, bool fixindent)
     if (regname == 0 && eval_has_provider("clipboard", false)) {
       regname = '*';
     }
-    if (yank_register_mline(regname)) {
+    yankreg_T *reg = NULL;
+    if (yank_register_mline(regname, &reg)) {
       if (mouse_past_bottom) {
         dir = FORWARD;
       }
@@ -855,7 +857,7 @@ bool do_mouse(oparg_T *oap, int c, int dir, int count, bool fixindent)
     if (restart_edit != 0) {
       where_paste_started = curwin->w_cursor;
     }
-    do_put(regname, NULL, dir, count,
+    do_put(regname, reg, dir, count,
            (fixindent ? PUT_FIXINDENT : 0)| PUT_CURSEND);
   } else if (((mod_mask & MOD_MASK_CTRL) || (mod_mask & MOD_MASK_MULTI_CLICK) == MOD_MASK_2CLICK)
              && bt_quickfix(curbuf)) {

src/nvim/ops.c

@@ -966,16 +966,23 @@ yankreg_T *copy_register(int name)
 }
 
 /// Check if the current yank register has kMTLineWise register type
-bool yank_register_mline(int regname)
+/// For valid, non-blackhole registers also provides pointer to the register
+/// structure prepared for pasting.
+///
+/// @param regname The name of the register used or 0 for the unnamed register
+/// @param reg Pointer to store yankreg_T* for the requested register. Will be
+///        set to NULL for invalid or blackhole registers.
+bool yank_register_mline(int regname, yankreg_T **reg)
 {
+  *reg = NULL;
   if (regname != 0 && !valid_yank_reg(regname, false)) {
     return false;
   }
   if (regname == '_') {  // black hole is always empty
     return false;
   }
-  yankreg_T *reg = get_yank_register(regname, YREG_PASTE);
+  *reg = get_yank_register(regname, YREG_PASTE);
-  return reg->y_type == kMTLineWise;
+  return (*reg)->y_type == kMTLineWise;
 }
 
 /// Start or stop recording into a yank register.
@@ -1322,7 +1329,7 @@ static int put_in_typebuf(char *s, bool esc, bool colon, int silent)
 /// @param literally_arg  insert literally, not as if typed
 ///
 /// @return FAIL for failure, OK otherwise
-int insert_reg(int regname, bool literally_arg)
+int insert_reg(int regname, yankreg_T *reg, bool literally_arg)
 {
   int retval = OK;
   bool allocated;
@@ -1353,7 +1360,9 @@ int insert_reg(int regname, bool literally_arg)
       xfree(arg);
     }
   } else {  // Name or number register.
-    yankreg_T *reg = get_yank_register(regname, YREG_PASTE);
+    if (reg == NULL) {
+      reg = get_yank_register(regname, YREG_PASTE);
+    }
     if (reg->y_array == NULL) {
       retval = FAIL;
     } else {