From: Alexey Svistunov <svalx@svalx.net>
Date:  2017-02-17 16:07:00 +0300
Subject: open device with O_NONBLOCK option
References: https://savannah.gnu.org/patch/?9263, bnc#94449 
Upstream: submitted

When running the 2.6 kernel, "mt -f /dev/nst0 status" blocks if there is
no media in the drive.  The same occurs for other commands.

When running the 2.4.24 kernel, "mt -f /dev/nst0 status" does not block
when there is no tape in the drive.

This behavior change is documented for the 2.6 kernel (see
kernel-source-2.6.3/Documentation/scsi/st.txt for the full doc):

If the open option O_NONBLOCK is used, open succeeds even if the
drive is not ready. If O_NONBLOCK is not used, the driver waits for
the drive to become ready. If this does not happen in ST_BLOCK_SECONDS
seconds, open fails with the errno value EIO. With O_NONBLOCK the
device can be opened for writing even if there is a write protected
tape in the drive (commands trying to write something return error if
attempted).

It appears that the use of O_NONBLOCK is safe with pre-2.6 kernels.
Suggest adding the use of O_NONBLOCK when opening the device.  As it is,
for long-running commands such as "fsf", one cannot tell if the command is
progressing or if it's blocking waiting for media.

Index: src/mt.c
===================================================================
--- src/mt.c.orig
+++ src/mt.c
@@ -333,11 +333,11 @@
 #ifdef MTERASE
     case MTERASE:
 #endif
-      tapedesc = rmtopen (tapedev, O_WRONLY, 0, rsh_command_option);
+      tapedesc = rmtopen (tapedev, O_WRONLY | O_NONBLOCK, 0, rsh_command_option);
       break;
 
     default:
-      tapedesc = rmtopen (tapedev, O_RDONLY, 0, rsh_command_option);
+      tapedesc = rmtopen (tapedev, O_RDONLY | O_NONBLOCK, 0, rsh_command_option);
     }
   
   if (tapedesc == -1)
Index: src/util.c
===================================================================
--- src/util.c.orig
+++ src/util.c
@@ -814,14 +814,14 @@
   copy_in = process_copy_in;
 
   if (copy_function == copy_in)
-    fd = rmtopen (file, O_RDONLY | O_BINARY, MODE_RW, rsh_command_option);
+    fd = rmtopen (file, O_RDONLY | O_BINARY | O_NONBLOCK, MODE_RW, rsh_command_option);
   else
     {
       if (!append_flag)
-	fd = rmtopen (file, O_WRONLY | O_CREAT | O_TRUNC | O_BINARY, MODE_RW,
+	fd = rmtopen (file, O_WRONLY | O_CREAT | O_TRUNC | O_BINARY | O_NONBLOCK, MODE_RW,
 			rsh_command_option);
       else
-	fd = rmtopen (file, O_RDWR | O_BINARY, MODE_RW, rsh_command_option);
+	fd = rmtopen (file, O_RDWR | O_BINARY | O_NONBLOCK, MODE_RW, rsh_command_option);
     }
 
   return fd;