Separate the "is data ready" function of xfer_seek() into an

xfer_window() function, which can return a scalar rather than a
boolean.

src/core/downloader.c

@@ -228,6 +228,7 @@ static struct xfer_interface_operations downloader_xfer_operations = {
 	.close		= downloader_xfer_close,
 	.vredirect	= xfer_vopen,
 	.seek		= downloader_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.deliver_iob	= xfer_deliver_as_raw,
 	.deliver_raw	= downloader_xfer_deliver_raw,
 };

src/core/hw.c

@@ -37,6 +37,7 @@ static struct xfer_interface_operations hw_xfer_operations = {
 	.close		= hw_xfer_close,
 	.vredirect	= ignore_xfer_vredirect,
 	.seek		= ignore_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.deliver_iob	= xfer_deliver_as_raw,
 	.deliver_raw	= ignore_xfer_deliver_raw,
 };
@@ -45,7 +46,7 @@ static void hw_step ( struct process *process ) {
 	struct hw *hw = container_of ( process, struct hw, process );
 	int rc;
 
-	if ( xfer_ready ( &hw->xfer ) == 0 ) {
+	if ( xfer_window ( &hw->xfer ) ) {
 		rc = xfer_deliver_raw ( &hw->xfer, hw_msg, sizeof ( hw_msg ) );
 		hw_finished ( hw, rc );
 	}

src/core/posix_io.c

@@ -160,6 +160,7 @@ static struct xfer_interface_operations posix_file_xfer_operations = {
 	.close		= posix_file_xfer_close,
 	.vredirect	= xfer_vopen,
 	.seek		= posix_file_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.alloc_iob	= default_xfer_alloc_iob,
 	.deliver_iob	= posix_file_xfer_deliver_iob,
 	.deliver_raw	= xfer_deliver_as_iob,

src/core/resolv.c

@@ -308,25 +308,12 @@ struct named_socket {
 	int have_local;
 };
 
-/**
- * Handle seek() event
- *
- * @v xfer		Data transfer interface
- * @v offset		Offset to new position
- * @v whence		Basis for new position
- * @ret rc		Return status code
- */
-static int resolv_xfer_seek ( struct xfer_interface *xfer __unused,
-			      off_t offset __unused, int whence __unused ) {
-	/* Never ready to accept data */
-	return -EAGAIN;
-}
-
 /** Named socket opener data transfer interface operations */
 static struct xfer_interface_operations named_xfer_ops = {
 	.close		= ignore_xfer_close,
 	.vredirect	= ignore_xfer_vredirect,
-	.seek		= resolv_xfer_seek,
+	.seek		= ignore_xfer_seek,
+	.window		= no_xfer_window,
 	.alloc_iob	= default_xfer_alloc_iob,
 	.deliver_iob	= xfer_deliver_as_raw,
 	.deliver_raw	= ignore_xfer_deliver_raw,

src/core/xfer.c

@@ -110,6 +110,22 @@ int xfer_seek ( struct xfer_interface *xfer, off_t offset, int whence ) {
 	return rc;
 }
 
+/**
+ * Check flow control window
+ *
+ * @v xfer		Data transfer interface
+ * @ret len		Length of window
+ */
+size_t xfer_window ( struct xfer_interface *xfer ) {
+	struct xfer_interface *dest = xfer_get_dest ( xfer );
+	size_t len;
+
+	len = dest->op->window ( dest );
+
+	xfer_put ( dest );
+	return len;
+}
+
 /**
  * Test to see if interface is ready to accept data
  *
@@ -297,6 +313,32 @@ int ignore_xfer_seek ( struct xfer_interface *xfer __unused,
 	return 0;
 }
 
+/**
+ * Unlimited flow control window
+ *
+ * @v xfer		Data transfer interface
+ * @ret len		Length of window
+ *
+ * This handler indicates that the interface is always ready to accept
+ * data.
+ */
+size_t unlimited_xfer_window ( struct xfer_interface *xfer __unused ) {
+	return ~( ( size_t ) 0 );
+}
+
+/**
+ * No flow control window
+ *
+ * @v xfer		Data transfer interface
+ * @ret len		Length of window
+ *
+ * This handler indicates that the interface is never ready to accept
+ * data.
+ */
+size_t no_xfer_window ( struct xfer_interface *xfer __unused ) {
+	return 0;
+}
+
 /**
  * Allocate I/O buffer
  *
@@ -374,6 +416,7 @@ struct xfer_interface_operations null_xfer_ops = {
 	.close		= ignore_xfer_close,
 	.vredirect	= ignore_xfer_vredirect,
 	.seek		= ignore_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.alloc_iob	= default_xfer_alloc_iob,
 	.deliver_iob	= xfer_deliver_as_raw,
 	.deliver_raw	= ignore_xfer_deliver_raw,