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To:	Cygwin <cygwin AT cygwin DOT com>
Subject:	[PATCH] Strange behavior of cmd.exe when hammered with clear screen operations from Cygwin program.
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Date:	Wed, 27 Jul 2016 07:31:50 -0700
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The patch below seems to solves it for me:

Cygwin should not be faking a scroll of the console by shrinking it
and then restoring its size. Though clever, this is evidently
unreliable; cmd.exe misbehaves if the console handle
is hit with rapid SetConsoleScreenBufferSize calls.

------------

 From: Kaz Kylheku <kaz AT kylheku DOT com>
Date: Wed, 27 Jul 2016 07:12:54 -0700
Subject: Replace bogus resize-window-to-clear-screen logic.

This removes (ab)uses of SetConsoleScreenBufferSize
which sometimes cause cmd.exe to badly misbehave.
It probably doesn't like the closely spaced timing of
shrinking the window down to one line followed by a restore
of the size. Instead we just output newlines to clear
the window.

* winsup/cygwin/fhandler.h (dev_console::scroll_window):
Member function declaration removed.
(dev_console::clear_should_scroll): New member
function declared.

* winsup/cygwin/fhandler_console.cc
(dev_console::scroll_window): Member function removed.
(dev_console::clear_should_scroll): New member function.
Performs only the test that was performed by scroll_window,
not the actual scrolling. The scrolling is now done in
the caller in the fhandler_console class.
(fhandler_console::clear_screen): Call con.clear_should_scroll
instead of con.scroll_window. If this returns false, act
as before. Otherwise, clear the screen by scrolling the
window. This is done not by making SetConsoleScreenBufferSize
calls to shrink and restore the window, but by earnestly
emitting a number of carriage returns equal to the vertical
screen size and then restoring the cursor position.

diff --git a/winsup/cygwin/fhandler.h b/winsup/cygwin/fhandler.h
index 21c71d7..4af584b 100644
--- a/winsup/cygwin/fhandler.h
+++ b/winsup/cygwin/fhandler.h
@@ -1361,7 +1361,7 @@ class dev_console
    int set_cl_x (cltype);
    int set_cl_y (cltype);
    bool fillin (HANDLE);
-  bool __reg3 scroll_window (HANDLE, int, int, int, int);
+  bool __reg3 clear_should_scroll (HANDLE, int, int, int, int);
    void __reg3 scroll_buffer (HANDLE, int, int, int, int, int, int);
    void __reg3 clear_screen (HANDLE, int, int, int, int);
    void __reg3 save_restore (HANDLE, char);
diff --git a/winsup/cygwin/fhandler_console.cc 
b/winsup/cygwin/fhandler_console.cc
index 5fe4480..f4806e0 100644
--- a/winsup/cygwin/fhandler_console.cc
+++ b/winsup/cygwin/fhandler_console.cc
@@ -1209,38 +1209,11 @@ dev_console::set_cl_y (cltype y)
  }

  bool
-dev_console::scroll_window (HANDLE h, int x1, int y1, int x2, int y2)
+dev_console::clear_should_scroll (HANDLE h, int x1, int y1, int x2, int 
y2)
  {
    if (save_buf || x1 != 0 || x2 != dwWinSize.X - 1 || y1 != 
b.srWindow.Top
        || y2 != b.srWindow.Bottom || b.dwSize.Y <= dwWinSize.Y)
      return false;
-
-  SMALL_RECT sr;
-  int toscroll = 2 + dwEnd.Y - b.srWindow.Top;
-  int shrink = 1 + toscroll + b.srWindow.Bottom - b.dwSize.Y;
-  sr.Left = sr.Right = dwEnd.X = 0;
-  /* Can't increment dwEnd yet since we may not have space in
-     the buffer.  */
-  SetConsoleCursorPosition (h, dwEnd);
-  if (shrink > 0)
-    {
-      COORD c = b.dwSize;
-      c.Y = dwEnd.Y - shrink;
-      SetConsoleScreenBufferSize (h, c);
-      SetConsoleScreenBufferSize (h, b.dwSize);
-      dwEnd.Y = 0;
-      fillin (h);
-      toscroll = 2 + dwEnd.Y - b.srWindow.Top;
-    }
-
-  sr.Top = sr.Bottom = toscroll;
-
-  SetConsoleWindowInfo (h, FALSE, &sr);
-
-  dwEnd.Y++;
-  SetConsoleCursorPosition (h, dwEnd);
-
-  fillin (h);
    return true;
  }

@@ -1261,8 +1234,28 @@ fhandler_console::clear_screen (cltype xc1, 
cltype yc1, cltype xc2, cltype yc2)

    /* Detect special case - scroll the screen if we have a buffer in 
order to
       preserve the buffer. */
-  if (!con.scroll_window (h, x1, y1, x2, y2))
-    con.clear_screen (h, x1, y1, x2, y2);
+  if (!con.clear_should_scroll (h, x1, y1, x2, y2))
+    {
+      con.clear_screen (h, x1, y1, x2, y2);
+      return;
+    }
+
+  int xpos, ypos;
+  cursor_get(&xpos, &ypos);
+
+  for (int i = 0; i < con.dwWinSize.Y; i++)
+    {
+      WCHAR buf[] = { L'\n' };
+      DWORD done;
+
+      if (!WriteConsoleW (get_output_handle (), buf, 1, &done, 0))
+        {
+	  __seterrno ();
+	  break;
+	}
+    }
+
+  cursor_set(false, xpos, ypos);
  }

  void __reg3


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