vim-patch:8.1.1372: when evaluating 'statusline' the current window is unknown

Problem:    When evaluating 'statusline' the current window is unknown.
            (Daniel Hahler)
Solution:   Set "g:actual_curwin" for %{} items.  Set "g:statusline_winid"
            when evaluationg %!. (closes vim/vim#4406, closes vim/vim#3299)
https://github.com/vim/vim/commit/1c6fd1e100fd0457375642ec50d483bcc0f61bb2

4 files changed, 68 insertions, 28 deletions

diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 8d4f76d3dd..e1beea0fed 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -3947,6 +3947,8 @@ A jump table for the options with a short description can be found at |Q_op|.
 	When on allow some options that are an expression to be set in the
 	modeline.  Check the option for whether it is affected by
 	'modelineexpr'.  Also see |modeline|.
+	This option cannot be set from a |modeline| or in the |sandbox|, for
+	security reasons.
 
 						*'modelines'* *'mls'*
 'modelines' 'mls'	number	(default 5)
@@ -5807,7 +5809,9 @@ A jump table for the options with a short description can be found at |Q_op|.
 	When the option starts with "%!" then it is used as an expression,
 	evaluated and the result is used as the option value.  Example: >
 		:set statusline=%!MyStatusLine()
-<	The result can contain %{} items that will be evaluated too.
+<	The *g:statusline_winid* variable will be set to the |window-ID| of the
+	window that the status line belongs to.
+	The result can contain %{} items that will be evaluated too.
 	Note that the "%!" expression is evaluated in the context of the
 	current window and buffer, while %{} items are evaluated in the
 	context of the window that the statusline belongs to.
@@ -5936,13 +5940,15 @@ A jump table for the options with a short description can be found at |Q_op|.
 	become empty.  This will make a group like the following disappear
 	completely from the statusline when none of the flags are set. >
 		:set statusline=...%(\ [%M%R%H]%)...
-<							*g:actual_curbuf*
-	Beware that an expression is evaluated each and every time the status
-	line is displayed.  The current buffer and current window will be set
-	temporarily to that of the window (and buffer) whose statusline is
-	currently being drawn.  The expression will evaluate in this context.
-	The variable "g:actual_curbuf" is set to the `bufnr()` number of the
-	real current buffer.
+<	Beware that an expression is evaluated each and every time the status
+	line is displayed.
+					*g:actual_curbuf* *g:actual_curwin*
+	The current buffer and current window will be set temporarily to that
+	of the window (and buffer) whose statusline is currently being drawn.
+	The expression will evaluate in this context.  The variable
+	"g:actual_curbuf" is set to the `bufnr()` number of the real current
+	buffer and "g:actual_curwin" to the |window-ID| of the real current
+	window.  These values are strings.
 
 	The 'statusline' option will be evaluated in the |sandbox| if set from
 	a modeline, see |sandbox-option|.
diff --git a/src/nvim/buffer.c b/src/nvim/buffer.c
index 29712e9f52..86067aceac 100644
--- a/src/nvim/buffer.c
+++ b/src/nvim/buffer.c
@@ -3464,7 +3464,8 @@ int build_stl_str_hl(
     } type;
   } items[STL_MAX_ITEM];
 #define TMPLEN 70
-  char_u tmp[TMPLEN];
+  char_u buf_tmp[TMPLEN];
+  char_u win_tmp[TMPLEN];
   char_u      *usefmt = fmt;
   const int save_must_redraw = must_redraw;
   const int save_redr_type = curwin->w_redr_type;
@@ -3472,10 +3473,18 @@ int build_stl_str_hl(
   // When the format starts with "%!" then evaluate it as an expression and
   // use the result as the actual format string.
   if (fmt[0] == '%' && fmt[1] == '!') {
+    typval_T tv = {
+      .v_type = VAR_NUMBER,
+      .vval.v_number = wp->handle,
+    };
+    set_var(S_LEN("g:statusline_winid"), &tv, false);
+
     usefmt = eval_to_string_safe(fmt + 2, NULL, use_sandbox);
     if (usefmt == NULL) {
       usefmt = fmt;
     }
+
+    do_unlet(S_LEN("g:statusline_winid"), true);
   }
 
   if (fillchar == 0) {
@@ -3904,8 +3913,10 @@ int build_stl_str_hl(
       // { Evaluate the expression
 
       // Store the current buffer number as a string variable
-      vim_snprintf((char *)tmp, sizeof(tmp), "%d", curbuf->b_fnum);
-      set_internal_string_var((char_u *)"g:actual_curbuf", tmp);
+      vim_snprintf((char *)buf_tmp, sizeof(buf_tmp), "%d", curbuf->b_fnum);
+      set_internal_string_var((char_u *)"g:actual_curbuf", buf_tmp);
+      vim_snprintf((char *)win_tmp, sizeof(win_tmp), "%d", curwin->handle);
+      set_internal_string_var((char_u *)"g:actual_curwin", win_tmp);
 
       buf_T *const save_curbuf = curbuf;
       win_T *const save_curwin = curwin;
@@ -3926,6 +3937,7 @@ int build_stl_str_hl(
 
       // Remove the variable we just stored
       do_unlet(S_LEN("g:actual_curbuf"), true);
+      do_unlet(S_LEN("g:actual_curwin"), true);
 
       // }
 
@@ -3984,8 +3996,8 @@ int build_stl_str_hl(
       // Store the position percentage in our temporary buffer.
       // Note: We cannot store the value in `num` because
       //       `get_rel_pos` can return a named position. Ex: "Top"
-      get_rel_pos(wp, tmp, TMPLEN);
-      str = tmp;
+      get_rel_pos(wp, buf_tmp, TMPLEN);
+      str = buf_tmp;
       break;
 
     case STL_ARGLISTSTAT:
@@ -3995,19 +4007,19 @@ int build_stl_str_hl(
       //       at the end of the null-terminated string.
       //       Setting the first byte to null means it will place the argument
       //       number string at the beginning of the buffer.
-      tmp[0] = 0;
+      buf_tmp[0] = 0;
 
       // Note: The call will only return true if it actually
-      //       appended data to the `tmp` buffer.
-      if (append_arg_number(wp, tmp, (int)sizeof(tmp), false)) {
-        str = tmp;
+      //       appended data to the `buf_tmp` buffer.
+      if (append_arg_number(wp, buf_tmp, (int)sizeof(buf_tmp), false)) {
+        str = buf_tmp;
       }
       break;
 
     case STL_KEYMAP:
       fillable = false;
-      if (get_keymap_str(wp, (char_u *)"<%s>", tmp, TMPLEN)) {
-        str = tmp;
+      if (get_keymap_str(wp, (char_u *)"<%s>", buf_tmp, TMPLEN)) {
+        str = buf_tmp;
       }
       break;
     case STL_PAGENUM:
@@ -4064,9 +4076,9 @@ int build_stl_str_hl(
       // (including the brackets and null terminating character)
       if (*wp->w_buffer->b_p_ft != NUL
           && STRLEN(wp->w_buffer->b_p_ft) < TMPLEN - 3) {
-        vim_snprintf((char *)tmp, sizeof(tmp), "[%s]",
-            wp->w_buffer->b_p_ft);
-        str = tmp;
+        vim_snprintf((char *)buf_tmp, sizeof(buf_tmp), "[%s]",
+                     wp->w_buffer->b_p_ft);
+        str = buf_tmp;
       }
       break;
 
@@ -4078,13 +4090,13 @@ int build_stl_str_hl(
       // (including the comma and null terminating character)
       if (*wp->w_buffer->b_p_ft != NUL
           && STRLEN(wp->w_buffer->b_p_ft) < TMPLEN - 2) {
-        vim_snprintf((char *)tmp, sizeof(tmp), ",%s",
-            wp->w_buffer->b_p_ft);
+        vim_snprintf((char *)buf_tmp, sizeof(buf_tmp), ",%s",
+                     wp->w_buffer->b_p_ft);
         // Uppercase the file extension
-        for (char_u *t = tmp; *t != 0; t++) {
+        for (char_u *t = buf_tmp; *t != 0; t++) {
           *t = (char_u)TOUPPER_LOC(*t);
         }
-        str = tmp;
+        str = buf_tmp;
       }
       break;
     }
diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index b7e827e86b..27c2ed1ea5 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -2818,7 +2818,7 @@ static int do_unlet_var(lval_T *const lp, char_u *const name_end, int forceit)
 /// @param[in]  fonceit  If true, do not complain if variable doesn’t exist.
 ///
 /// @return OK if it existed, FAIL otherwise.
-int do_unlet(const char *const name, const size_t name_len, const int forceit)
+int do_unlet(const char *const name, const size_t name_len, const bool forceit)
   FUNC_ATTR_NONNULL_ALL
 {
   const char *varname;
diff --git a/src/nvim/testdir/test_statusline.vim b/src/nvim/testdir/test_statusline.vim
index 66b6e6c05c..8c81ec3431 100644
--- a/src/nvim/testdir/test_statusline.vim
+++ b/src/nvim/testdir/test_statusline.vim
@@ -7,6 +7,7 @@
 "   %X
 
 source view_util.vim
+source term_util.vim
 
 func s:get_statusline()
   return ScreenLines(&lines - 1, &columns)[0]
@@ -29,7 +30,9 @@ endfunc
 
 " Function used to display syntax group.
 func SyntaxItem()
-  return synIDattr(synID(line("."),col("."),1),"name")
+  call assert_equal(s:expected_curbuf, g:actual_curbuf)
+  call assert_equal(s:expected_curwin, g:actual_curwin)
+  return synIDattr(synID(line("."), col("."),1), "name")
 endfunc
 
 func Test_caught_error_in_statusline()
@@ -218,6 +221,8 @@ func Test_statusline()
 
   "%{: Evaluate expression between '%{' and '}' and substitute result.
   syntax on
+  let s:expected_curbuf = string(bufnr(''))
+  let s:expected_curwin = string(win_getid())
   set statusline=%{SyntaxItem()}
   call assert_match('^vimNumber\s*$', s:get_statusline())
   s/^/"/
@@ -332,6 +337,23 @@ func Test_statusline()
   set statusline=%!2*3+1
   call assert_match('7\s*$', s:get_statusline())
 
+  func GetNested()
+    call assert_equal(string(win_getid()), g:actual_curwin)
+    call assert_equal(string(bufnr('')), g:actual_curbuf)
+    return 'nested'
+  endfunc
+  func GetStatusLine()
+    call assert_equal(win_getid(), g:statusline_winid)
+    return 'the %{GetNested()} line'
+  endfunc
+  set statusline=%!GetStatusLine()
+  call assert_match('the nested line', s:get_statusline())
+  call assert_false(exists('g:actual_curwin'))
+  call assert_false(exists('g:actual_curbuf'))
+  call assert_false(exists('g:statusline_winid'))
+  delfunc GetNested
+  delfunc GetStatusLine
+
   " Check statusline in current and non-current window
   " with the 'fillchars' option.
   set fillchars=stl:^,stlnc:=,vert:\|,fold:-,diff:-