terminal: Handle loss of focus in event loop.

While in a terminal and insert mode, if an event caused loss of focus,
nvim would stay in the terminal event loop causing an inconsistent view
of internal state and/or segfault.

Remove the "term" argument from terminal_enter() as it only makes sense
to call it with curbuf->terminal. Terminate the loop when switched to a
different buffer.

fixes #2301

src/nvim/edit.c

@@ -246,7 +246,7 @@ edit (
 )
 {
   if (curbuf->terminal) {
-    terminal_enter(curbuf->terminal, true);
+    terminal_enter(true);
     return false;
   }
 

src/nvim/terminal.c

@@ -353,8 +353,11 @@ void terminal_resize(Terminal *term, uint16_t width, uint16_t height)
   invalidate_terminal(term, -1, -1);
 }
 
-void terminal_enter(Terminal *term, bool process_deferred)
+void terminal_enter(bool process_deferred)
 {
+  Terminal *term = curbuf->terminal;
+  assert(term && "should only be called when curbuf has a terminal");
+
   checkpcmark();
   setpcmark();
   int save_state = State;
@@ -373,7 +376,7 @@ void terminal_enter(Terminal *term, bool process_deferred)
   int c;
   bool close = false;
 
-  for (;;) {
+  while (term->buf == curbuf) {
     if (process_deferred) {
       event_enable_deferred();
     }
@@ -431,7 +434,7 @@ end:
   invalidate_terminal(term, term->cursor.row, term->cursor.row + 1);
   mapped_ctrl_c = save_mapped_ctrl_c;
   unshowmode(true);
-  redraw(false);
+  redraw(term->buf != curbuf);
   ui_busy_stop();
   if (close) {
     term->opts.close_cb(term->opts.data);
@@ -1018,6 +1021,11 @@ static void refresh_screen(Terminal *term)
 
 static void redraw(bool restore_cursor)
 {
+  Terminal *term = curbuf->terminal;
+  if (!term) {
+    restore_cursor = true;
+  }
+
   int save_row, save_col;
   if (restore_cursor) {
     // save the current row/col to restore after updating screen when not
@@ -1040,7 +1048,6 @@ static void redraw(bool restore_cursor)
 
   showruler(false);
 
-  Terminal *term = curbuf->terminal;
   if (term && is_focused(term)) {
     curwin->w_wrow = term->cursor.row;
     curwin->w_wcol = term->cursor.col + win_col_off(curwin);

test/functional/terminal/buffer_spec.lua

@@ -2,7 +2,9 @@ local helpers = require('test.functional.helpers')
 local Screen = require('test.functional.ui.screen')
 local thelpers = require('test.functional.terminal.helpers')
 local feed, clear, nvim = helpers.feed, helpers.clear, helpers.nvim
-local wait, execute, eq = helpers.wait, helpers.execute, helpers.eq
+local wait = helpers.wait
+local eval, execute, source = helpers.eval, helpers.execute, helpers.source
+local eq, neq = helpers.eq, helpers.neq
 
 
 describe('terminal buffer', function()
@@ -155,5 +157,41 @@ describe('terminal buffer', function()
       :bnext                                            |
     ]])
   end)
+
+  it('handles loss of focus gracefully', function()
+    -- Temporarily change the statusline to avoid printing the file name, which
+    -- varies be where the test is run.
+    nvim('set_option', 'statusline', '==========')
+    execute('set laststatus=0')
+
+    -- Save the buffer number of the terminal for later testing.
+    local tbuf = eval('bufnr("%")')
+
+    source([[
+    function! SplitWindow()
+      new
+    endfunction
+
+    startinsert
+    call jobstart(['sh', '-c', 'exit'], {'on_exit': function("SplitWindow")})
+    ]])
+
+    -- We should be in a new buffer now.
+    screen:expect([[
+      ^                                                  |
+      ~                                                 |
+      ==========                                        |
+      rows: 2, cols: 50                                 |
+      {2: }                                                 |
+      {1:==========                                        }|
+                                                        |
+    ]])
+
+    neq(tbuf, eval('bufnr("%")'))
+    execute('quit')  -- Should exit the new window, not the terminal.
+    eq(tbuf, eval('bufnr("%")'))
+
+    execute('set laststatus=1')  -- Restore laststatus to the default.
+  end)
 end)