[tftp] Temporary fix for conveying TFTP block size to callers

pxe_tftp.c assumes that the first seek on its data-transfer interface
represents the block size.  Apart from being an ugly hack, this will
also screw up file size calculation for files smaller than one block.

The proper solution would be to extend the data-transfer interface to
support the reporting of stat()-like data.  This is not going to
happen until the cost of adding interface methods is reduced (a fix I
have planned since June 2008).

In the meantime, abuse the xfer_window() method to return the block
size, since it is not being used for anything else and is vaguely
justifiable.

Astonishingly, having returned the incorrect TFTP blocksize via
PXENV_TFTP_OPEN for almost a year seems not to have affected any of
the test cases run during that time; this bug was found only when
someone tried running the heavily-patched version of pxegrub found in
OpenSolaris.

src/arch/i386/interface/pxe/pxe_tftp.c

 	/* Calculate new buffer position */
 	pxe_tftp.offset += len;
 
-	/* Mildly ugly hack; assume that the first non-zero seek
-	 * indicates the block size.
-	 */
-	if ( pxe_tftp.blksize == 0 )
-		pxe_tftp.blksize = pxe_tftp.offset;
-
 	/* Record maximum offset as the file size */
 	if ( pxe_tftp.max_offset < pxe_tftp.offset )
 		pxe_tftp.max_offset = pxe_tftp.offset;
 
 	/* Wait for OACK to arrive so that we have the block size */
 	while ( ( ( rc = pxe_tftp.rc ) == -EINPROGRESS ) &&
-		( pxe_tftp.blksize == 0 ) ) {
+		( pxe_tftp.max_offset == 0 ) ) {
 		step();
 	}
+	pxe_tftp.blksize = xfer_window ( &pxe_tftp.xfer );
 	tftp_open->PacketSize = pxe_tftp.blksize;
+	DBG ( " blksize=%d", tftp_open->PacketSize );
 
 	/* EINPROGRESS is normal; we don't wait for the whole transfer */
 	if ( rc == -EINPROGRESS )
 		step();
 	}
 	tftp_get_fsize->FileSize = pxe_tftp.max_offset;
+	DBG ( " fsize=%d", tftp_get_fsize->FileSize );
 
 	/* EINPROGRESS is normal; we don't wait for the whole transfer */
 	if ( rc == -EINPROGRESS )

src/net/udp/tftp.c

 	tftp_done ( tftp, rc );
 }
 
+/**
+ * Check flow control window
+ *
+ * @v xfer		Data transfer interface
+ * @ret len		Length of window
+ */
+static size_t tftp_xfer_window ( struct xfer_interface *xfer ) {
+	struct tftp_request *tftp =
+		container_of ( xfer, struct tftp_request, xfer );
+
+	/* We abuse this data-xfer method to convey the blocksize to
+	 * the caller.  This really should be done using some kind of
+	 * stat() method, but we don't yet have the facility to do
+	 * that.
+	 */
+	return tftp->blksize;
+}
+
 /** TFTP data transfer interface operations */
 static struct xfer_interface_operations tftp_xfer_operations = {
 	.close		= tftp_xfer_close,
 	.vredirect	= ignore_xfer_vredirect,
-	.window		= unlimited_xfer_window,
+	.window		= tftp_xfer_window,
 	.alloc_iob	= default_xfer_alloc_iob,
 	.deliver_iob	= xfer_deliver_as_raw,
 	.deliver_raw	= ignore_xfer_deliver_raw,