patch 8.2.4090: after restoring a session buffer order can be quite different

Problem:    After restoring a session buffer order can be quite different.
Solution:   Create buffers first. (Evgeni Chasnovski, closes #9520)

src/session.c

@@ -690,6 +690,28 @@ makeopens(
     if (put_line(fd, "set shortmess=aoO") == FAIL)
 	goto fail;
 
+    // Put all buffers into the buffer list.
+    // Do it very early to preserve buffer order after loading session (which
+    // can be disrupted by prior `edit` or `tabedit` calls).
+    FOR_ALL_BUFFERS(buf)
+    {
+	if (!(only_save_windows && buf->b_nwindows == 0)
+		&& !(buf->b_help && !(ssop_flags & SSOP_HELP))
+#ifdef FEAT_TERMINAL
+		// Skip terminal buffers: finished ones are not useful, others
+		// will be resurrected and result in a new buffer.
+		&& !bt_terminal(buf)
+#endif
+		&& buf->b_fname != NULL
+		&& buf->b_p_bl)
+	{
+	    if (fprintf(fd, "badd +%ld ", buf->b_wininfo == NULL ? 1L
+					   : buf->b_wininfo->wi_fpos.lnum) < 0
+		    || ses_fname(fd, buf, &ssop_flags, TRUE) == FAIL)
+		goto fail;
+	}
+    }
+
     // the global argument list
     if (ses_arglist(fd, "argglobal", &global_alist.al_ga,
 			    !(ssop_flags & SSOP_CURDIR), &ssop_flags) == FAIL)
@@ -741,7 +763,11 @@ makeopens(
 	// Similar to ses_win_rec() below, populate the tab pages first so
 	// later local options won't be copied to the new tabs.
 	FOR_ALL_TABPAGES(tp)
-	    if (tp->tp_next != NULL && put_line(fd, "tabnew") == FAIL)
+	    // Use `bufhidden=wipe` to remove empty "placeholder" buffers once
+	    // they are not needed. This prevents creating extra buffers (see
+	    // cause of patch 8.1.0829)
+	    if (tp->tp_next != NULL
+		  && put_line(fd, "tabnew +setlocal\\ bufhidden=wipe") == FAIL)
 		goto fail;
 	if (first_tabpage->tp_next != NULL && put_line(fd, "tabrewind") == FAIL)
 	    goto fail;
@@ -919,29 +945,6 @@ makeopens(
     if (restore_stal && put_line(fd, "set stal=1") == FAIL)
 	goto fail;
 
-    // Now put the remaining buffers into the buffer list.
-    // This is near the end, so that when 'hidden' is set we don't create extra
-    // buffers.  If the buffer was already created with another command the
-    // ":badd" will have no effect.
-    FOR_ALL_BUFFERS(buf)
-    {
-	if (!(only_save_windows && buf->b_nwindows == 0)
-		&& !(buf->b_help && !(ssop_flags & SSOP_HELP))
-#ifdef FEAT_TERMINAL
-		// Skip terminal buffers: finished ones are not useful, others
-		// will be resurrected and result in a new buffer.
-		&& !bt_terminal(buf)
-#endif
-		&& buf->b_fname != NULL
-		&& buf->b_p_bl)
-	{
-	    if (fprintf(fd, "badd +%ld ", buf->b_wininfo == NULL ? 1L
-					   : buf->b_wininfo->wi_fpos.lnum) < 0
-		    || ses_fname(fd, buf, &ssop_flags, TRUE) == FAIL)
-		goto fail;
-	}
-    }
-
     // Wipe out an empty unnamed buffer we started in.
     if (put_line(fd, "if exists('s:wipebuf') && len(win_findbuf(s:wipebuf)) == 0")
 								       == FAIL)

src/testdir/test_mksession.vim

@@ -362,6 +362,31 @@ func Test_mksession_buffer_count()
   set hidden&
 endfunc
 
+func Test_mksession_buffer_order()
+  %bwipe!
+  e Xfoo | e Xbar | e Xbaz | e Xqux
+  bufdo write
+  mksession! Xtest_mks.out
+
+  " Verify that loading the session preserves order of buffers
+  %bwipe!
+  source Xtest_mks.out
+
+  let s:buf_info = getbufinfo()
+  call assert_true(s:buf_info[0]['name'] =~# 'Xfoo$')
+  call assert_true(s:buf_info[1]['name'] =~# 'Xbar$')
+  call assert_true(s:buf_info[2]['name'] =~# 'Xbaz$')
+  call assert_true(s:buf_info[3]['name'] =~# 'Xqux$')
+
+  " Clean up.
+  call delete('Xfoo')
+  call delete('Xbar')
+  call delete('Xbaz')
+  call delete('Xqux')
+  call delete('Xtest_mks.out')
+  %bwipe!
+endfunc
+
 if has('extra_search')
 
 func Test_mksession_hlsearch()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4090,
 /**/
     4089,
 /**/