cygwin: block devices: fix file offset after short writes

When reading/writing block devices, Cygwin emulates Linux, providing
a byte-exact file position, albeit the underlying device drivers don't.

Unfortunately this only worked correctly for reading.  The raw_write
method failed to revalidate the buffer after the read-modify-write
cycle in case len is not a multiple of the sector length.  This in
turn resulted in lseek reporting a wrong file pointer.

Also, fix a condition for invalidating the buffer after writing from
a remaining read buffer.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/fhandler_floppy.cc

@@ -566,11 +566,11 @@ fhandler_dev_floppy::raw_write (const void *ptr, size_t len)
 	  /* Align pointers, lengths, etc. */
 	  cplen = MIN (cplen, written);
 	  devbufstart += cplen;
+	  if (devbufstart >= devbufend)
+	    devbufstart = devbufend = 0;
 	  p += cplen;
 	  len -= cplen;
 	  bytes_written += cplen;
-	  if (len)
-	    devbufstart = devbufend = 0;
 	}
       /* As long as there's still something left in the input buffer ... */
       while (len)
@@ -607,6 +607,14 @@ fhandler_dev_floppy::raw_write (const void *ptr, size_t len)
 	  p += cplen;
 	  len -= cplen;
 	  bytes_written += cplen;
+	  /* If we overwrote, revalidate devbuf.  It still contains the
+	     content from the above read/modify/write.  Revalidating makes
+	     sure lseek reports the correct position. */
+	  if (cplen < bytes_per_sector)
+	    {
+	      devbufstart = cplen;
+	      devbufend = bytes_per_sector;
+	    }
 	}
       return bytes_written;
     }

winsup/cygwin/release/2.10.0

@@ -31,3 +31,6 @@ Bug Fixes
 
 - Remove a call to fflush from ftell{o}, which may result in wrong offsets.
   Addresses: https://cygwin.com/ml/cygwin/2017-12/msg00151.html
+
+- Fix file pointer computation after short writes on block devices.
+  Addresses: https://cygwin.com/ml/cygwin/2017-12/msg00151.html