patch 8.0.1777: cannot cleanup before loading another colorscheme

Problem:    Cannot cleanup before loading another colorscheme.
Solution:   Add the ColorSchemePre autocommand event.

runtime/colors/README.txt

@@ -42,7 +42,16 @@ this autocmd might be useful:
 Replace "blue_sky" with the name of the colorscheme.
 
 In case you want to tweak a colorscheme after it was loaded, check out the
-ColorScheme autocmd event.
+ColorScheme autocommand event.
+
+To clean up just before loading another colorscheme, use the ColorSchemePre
+autocommand event.  For example:
+	let g:term_ansi_colors = ...
+	augroup MyColorscheme
+	  au!
+	  au ColorSchemePre * unlet g:term_ansi_colors
+	  au ColorSchemePre * au! MyColorscheme
+	augroup END
 
 To customize a colorscheme use another name, e.g.  "~/.vim/colors/mine.vim",
 and use `:runtime` to load the original colorscheme:

src/fileio.c

@@ -7734,6 +7734,7 @@ static struct event_name
     {"CmdwinLeave",	EVENT_CMDWINLEAVE},
     {"CmdUndefined",	EVENT_CMDUNDEFINED},
     {"ColorScheme",	EVENT_COLORSCHEME},
+    {"ColorSchemePre",	EVENT_COLORSCHEMEPRE},
     {"CompleteDone",	EVENT_COMPLETEDONE},
     {"CursorHold",	EVENT_CURSORHOLD},
     {"CursorHoldI",	EVENT_CURSORHOLDI},
@@ -9479,7 +9480,8 @@ apply_autocmds_group(
      */
     if (fname_io == NULL)
     {
-	if (event == EVENT_COLORSCHEME || event == EVENT_OPTIONSET)
+	if (event == EVENT_COLORSCHEME || event == EVENT_COLORSCHEMEPRE
+						   || event == EVENT_OPTIONSET)
 	    autocmd_fname = NULL;
 	else if (fname != NULL && !ends_excmd(*fname))
 	    autocmd_fname = fname;
@@ -9549,6 +9551,7 @@ apply_autocmds_group(
 		|| event == EVENT_SPELLFILEMISSING
 		|| event == EVENT_QUICKFIXCMDPRE
 		|| event == EVENT_COLORSCHEME
+		|| event == EVENT_COLORSCHEMEPRE
 		|| event == EVENT_OPTIONSET
 		|| event == EVENT_QUICKFIXCMDPOST
 		|| event == EVENT_DIRCHANGED)

src/syntax.c

@@ -7224,6 +7224,8 @@ load_colors(char_u *name)
     buf = alloc((unsigned)(STRLEN(name) + 12));
     if (buf != NULL)
     {
+	apply_autocmds(EVENT_COLORSCHEMEPRE, name,
+					       curbuf->b_fname, FALSE, curbuf);
 	sprintf((char *)buf, "colors/%s.vim", name);
 	retval = source_runtime(buf, DIP_START + DIP_OPT);
 	vim_free(buf);

src/testdir/test_gui.vim

@@ -33,13 +33,25 @@ endfunc
 
 func Test_colorscheme()
   let colorscheme_saved = exists('g:colors_name') ? g:colors_name : 'default'
+  let g:color_count = 0
+  augroup TestColors
+    au!
+    au ColorScheme * let g:color_count += 1| let g:after_colors = g:color_count
+    au ColorSchemePre * let g:color_count += 1 |let g:before_colors = g:color_count
+  augroup END
 
   colorscheme torte
   redraw!
   sleep 200m
   call assert_equal('dark', &background)
+  call assert_equal(1, g:before_colors)
+  call assert_equal(2, g:after_colors)
 
   exec 'colorscheme' colorscheme_saved
+  augroup TestColors
+    au!
+  augroup END
+  unlet g:color_count g:after_colors g:before_colors
   redraw!
 endfunc
 

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1777,
 /**/
     1776,
 /**/

src/vim.h

@@ -1277,6 +1277,7 @@ enum auto_event
     EVENT_CMDWINENTER,		/* after entering the cmdline window */
     EVENT_CMDWINLEAVE,		/* before leaving the cmdline window */
     EVENT_COLORSCHEME,		/* after loading a colorscheme */
+    EVENT_COLORSCHEMEPRE,	/* before loading a colorscheme */
     EVENT_COMPLETEDONE,		/* after finishing insert complete */
     EVENT_CURSORHOLD,		/* cursor in same position for a while */
     EVENT_CURSORHOLDI,		/* idem, in Insert mode */