patch 8.1.0334: 'autowrite' takes effect when buffer is not to be written

Problem:    'autowrite' takes effect when buffer is not to be written.
Solution:   Don't write buffers that are not supposed to be written. (Even Q
            Jones, closes #3391)  Add tests for 'autowrite'.

src/ex_cmds2.c

@@ -2041,7 +2041,7 @@ autowrite(buf_T *buf, int forceit)
 }
 
 /*
- * flush all buffers, except the ones that are readonly
+ * Flush all buffers, except the ones that are readonly or are never written.
  */
     void
 autowrite_all(void)
@@ -2051,7 +2051,7 @@ autowrite_all(void)
     if (!(p_aw || p_awa) || !p_write)
 	return;
     FOR_ALL_BUFFERS(buf)
-	if (bufIsChanged(buf) && !buf->b_p_ro)
+	if (bufIsChanged(buf) && !buf->b_p_ro && !bt_dontwrite(buf))
 	{
 	    bufref_T	bufref;
 

src/testdir/test_writefile.vim

@@ -112,3 +112,41 @@ func Test_writefile_sync_dev_stdout()
     throw 'Skipped: /dev/stdout is not writable'
   endif
 endfunc
+
+func Test_writefile_autowrite()
+  set autowrite
+  new
+  next Xa Xb Xc
+  call setline(1, 'aaa')
+  next
+  call assert_equal(['aaa'], readfile('Xa'))
+  call setline(1, 'bbb')
+  call assert_fails('edit XX')
+  call assert_false(filereadable('Xb'))
+
+  set autowriteall
+  edit XX
+  call assert_equal(['bbb'], readfile('Xb'))
+
+  bwipe!
+  call delete('Xa')
+  call delete('Xb')
+  set noautowrite
+endfunc
+
+func Test_writefile_autowrite_nowrite()
+  set autowrite
+  new
+  next Xa Xb Xc
+  set buftype=nowrite
+  call setline(1, 'aaa')
+  let buf = bufnr('%')
+  " buffer contents silently lost
+  edit XX
+  call assert_false(filereadable('Xa'))
+  rewind
+  call assert_equal('', getline(1))
+
+  bwipe!
+  set noautowrite
+endfunc

src/version.c

@@ -794,6 +794,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    334,
 /**/
     333,
 /**/