Allow buffers to be pre-expanded on demand.

src/core/buffer.c

 
 	/* Check that block fits within buffer, expand if necessary */
 	if ( data_end > buffer->len ) {
-		if ( ! buffer->expand ) {
-			DBGC ( buffer, "BUFFER %p not expandable\n", buffer );
-			return -ENOBUFS;
-		}
-		if ( ( rc = buffer->expand ( buffer, data_end ) ) != 0 ) {
+		if ( ( rc = expand_buffer ( buffer, data_end ) ) != 0 ) {
 			DBGC ( buffer, "BUFFER %p could not expand :%s\n",
 			       buffer, strerror ( rc ) );
 			return rc;

src/include/gpxe/buffer.h

 #define _GPXE_BUFFER_H
 
 #include <stdint.h>
+#include <errno.h>
 #include <gpxe/uaccess.h>
 
 /** @file
 extern int fill_buffer ( struct buffer *buffer, const void *data,
 			 size_t offset, size_t len );
 
+/** Expand data buffer
+ *
+ * @v buffer		Data buffer
+ * @v new_len		New length
+ * @ret rc		Return status code
+ *
+ * Expand the data buffer to accommodate more data.  Some buffers may
+ * not support being expanded.
+ */
+static inline int expand_buffer ( struct buffer *buffer, size_t new_len ) {
+
+	if ( new_len <= buffer->len )
+		return 0;
+
+	if ( ! buffer->expand )
+		return -ENOBUFS;
+
+	return buffer->expand ( buffer, new_len );
+}
+
 #endif /* _GPXE_BUFFER_H */