Cygwin: pty: fix bug in reduced input transfer

When launching an MSYS2 process via a non-MSYS2 process, the response
for, say, a CSI [ 6n (Device Status Report) sent from that MSYS2 process
was sometimes not sent (or sent only partially) to the process but to
the terminal instead.

This regression was introduced in f206417 (Cygwin: pty: Reduce
unecessary input transfer., 2021-02-11).

While it is not yet completely clear why, this patch fixes this. The
explanation is left for the commit message that Takashi will craft for
the Cygwin project, but we will take this patch early into the MSYS2/Git
for Windows projects.

This addresses git-for-windows/git#3579

winsup/cygwin/fhandler_tty.cc

@@ -1247,10 +1247,13 @@ fhandler_pty_slave::mask_switch_to_pcon_in (bool mask, bool xfer)
   else if (InterlockedDecrement (&num_reader) == 0)
     CloseHandle (slave_reading);
 
+  bool need_xfer =
+    get_ttyp ()->switch_to_pcon_in && !get_ttyp ()->pcon_activated;
+
   /* In GDB, transfer input based on setpgid() does not work because
      GDB may not set terminal process group properly. Therefore,
      transfer input here if isHybrid is set. */
-  if (isHybrid && !!masked != mask && xfer
+  if ((isHybrid || need_xfer) && !!masked != mask && xfer
       && GetStdHandle (STD_INPUT_HANDLE) == get_handle ())
     {
       if (mask && get_ttyp ()->pcon_input_state_eq (tty::to_nat))
@@ -1544,7 +1547,7 @@ fhandler_pty_slave::read (void *ptr, size_t& len)
   if (ptr0)
     { /* Not tcflush() */
       bool saw_eol = totalread > 0 && strchr ("\r\n", ptr0[totalread -1]);
-      mask_switch_to_pcon_in (false, saw_eol);
+      mask_switch_to_pcon_in (false, saw_eol || len == 0);
     }
 }
 
@@ -2226,6 +2229,15 @@ fhandler_pty_master::write (const void *ptr, size_t len)
       return len;
     }
 
+  if (to_be_read_from_pcon () && !get_ttyp ()->pcon_activated
+      && get_ttyp ()->pcon_input_state == tty::to_cyg)
+    {
+      WaitForSingleObject (input_mutex, INFINITE);
+      fhandler_pty_slave::transfer_input (tty::to_nat, from_master,
+					  get_ttyp (), input_available_event);
+      ReleaseMutex (input_mutex);
+    }
+
   line_edit_status status = line_edit (p, len, ti, &ret);
   if (status > line_edit_signalled && status != line_edit_pipe_full)
     ret = -1;