patch 8.2.4007: session does not restore help buffer properly

Problem:    Session does not restore help buffer properly when "options' is
            missing from 'sessionoptions'.
Solution:   Use a ":help" command to create the help window. (closes #9475,
            closes #9458, closes #9472)

src/session.c

@@ -348,16 +348,32 @@ put_view(
     // Edit the file.  Skip this when ":next" already did it.
     if (add_edit && (!did_next || wp->w_arg_idx_invalid))
     {
+	if (bt_help(wp->w_buffer))
+	{
+	    char *curtag = "";
+
+	    // A help buffer needs some options to be set.
+	    // First, create a new empty buffer with "buftype=help".
+	    // Then ":help" will re-use both the buffer and the window and set
+	    // the options, even when "options" is not in 'sessionoptions'.
+	    if (0 < wp->w_tagstackidx
+		    && wp->w_tagstackidx <= wp->w_tagstacklen)
+		curtag = (char *)wp->w_tagstack[wp->w_tagstackidx - 1].tagname;
+
+	    if (put_line(fd, "enew | setl bt=help") == FAIL
+		    || fprintf(fd, "help %s", curtag) < 0
+		    || put_eol(fd) == FAIL)
+		return FAIL;
+	}
 # ifdef FEAT_TERMINAL
-	if (bt_terminal(wp->w_buffer))
+	else if (bt_terminal(wp->w_buffer))
 	{
 	    if (term_write_session(fd, wp, terminal_bufs) == FAIL)
 		return FAIL;
 	}
-	else
 # endif
 	// Load the file.
-	if (wp->w_buffer->b_ffname != NULL
+	else if (wp->w_buffer->b_ffname != NULL
 # ifdef FEAT_QUICKFIX
 		&& !bt_nofilename(wp->w_buffer)
 # endif

src/testdir/test_mksession.vim

@@ -909,6 +909,36 @@ func Test_mksession_foldopt()
   set sessionoptions&
 endfunc
 
+" Test for mksession with "help" but not "options" in 'sessionoptions'
+func Test_mksession_help_noopt()
+  set sessionoptions-=options
+  set sessionoptions+=help
+  help
+  let fname = expand('%')
+  mksession! Xtest_mks.out
+  bwipe
+
+  source Xtest_mks.out
+  call assert_equal('help', &buftype)
+  call assert_equal('help', &filetype)
+  call assert_equal(fname, expand('%'))
+  call assert_false(&modifiable)
+  call assert_true(&readonly)
+
+  helpclose
+  help index
+  let fname = expand('%')
+  mksession! Xtest_mks.out
+  bwipe
+
+  source Xtest_mks.out
+  call assert_equal('help', &buftype)
+  call assert_equal(fname, expand('%'))
+
+  call delete('Xtest_mks.out')
+  set sessionoptions&
+endfunc
+
 " Test for mksession with window position
 func Test_mksession_winpos()
   " Only applicable in GUI Vim

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4007,
 /**/
     4006,
 /**/