Add v:exiting

Contains the exit value nvim will use.

Before exiting, it is v:null. That way jobs or autocmds (in VimLeavePre or
VimLeave) can check if Neovim is about to quit and with what exit value.

Closes #4666.

runtime/doc/eval.txt

@@ -1383,6 +1383,9 @@ v:dying		Normally zero.	When a deadly signal is caught it's set to
 <		Note: if another deadly signal is caught when v:dying is one,
 		VimLeave autocommands will not be executed.
 
+					*v:exiting* *exiting-variable*
+v:exiting	The exit value Nvim will use. Before exiting, it is |v:null|.
+
 					*v:errmsg* *errmsg-variable*
 v:errmsg	Last given error message.  It's allowed to set this variable.
 		Example: >

src/nvim/eval.c

@@ -394,6 +394,7 @@ static struct vimvar {
   VV(VV_TYPE_DICT,      "t_dict",           VAR_NUMBER, VV_RO),
   VV(VV_TYPE_FLOAT,     "t_float",          VAR_NUMBER, VV_RO),
   VV(VV_TYPE_BOOL,      "t_bool",           VAR_NUMBER, VV_RO),
+  VV(VV_EXITING,        "exiting",          VAR_NUMBER, VV_RO),
 };
 #undef VV
 
@@ -581,6 +582,7 @@ void eval_init(void)
   set_vim_var_special(VV_FALSE, kSpecialVarFalse);
   set_vim_var_special(VV_TRUE, kSpecialVarTrue);
   set_vim_var_special(VV_NULL, kSpecialVarNull);
+  set_vim_var_special(VV_EXITING, kSpecialVarNull);
 
   set_reg_var(0);  // default for v:register is not 0 but '"'
 }
@@ -17763,6 +17765,8 @@ void set_vcount(long count, long count1, int set_prevcount)
 /// @param[in]  val  Value to set to.
 void set_vim_var_nr(const VimVarIndex idx, const varnumber_T val)
 {
+  clear_tv(&vimvars[idx].vv_tv);
+  vimvars[idx].vv_type = VAR_NUMBER;
   vimvars[idx].vv_nr = val;
 }
 
@@ -17772,6 +17776,8 @@ void set_vim_var_nr(const VimVarIndex idx, const varnumber_T val)
 /// @param[in]  val  Value to set to.
 void set_vim_var_special(const VimVarIndex idx, const SpecialVarValue val)
 {
+  clear_tv(&vimvars[idx].vv_tv);
+  vimvars[idx].vv_type = VAR_SPECIAL;
   vimvars[idx].vv_special = val;
 }
 

src/nvim/eval.h

@@ -134,6 +134,7 @@ typedef enum {
     VV_TYPE_DICT,
     VV_TYPE_FLOAT,
     VV_TYPE_BOOL,
+    VV_EXITING,
 } VimVarIndex;
 
 /// All recognized msgpack types

src/nvim/main.c

@@ -557,6 +557,8 @@ void getout(int exitval)
   if (exmode_active)
     exitval += ex_exitval;
 
+  set_vim_var_nr(VV_EXITING, exitval);
+
   /* Position the cursor on the last screen line, below all the text */
   ui_cursor_goto((int)Rows - 1, 0);