flexnbd: Add a proxy mode

This lets us proxy connections between NBD clients and servers, resiliently.

src/proxy.h

@@ -0,0 +1,71 @@
+#ifndef PROXY_H
+#define PROXY_H
+
+#include <sys/types.h>
+#include <unistd.h>
+
+#include "flexnbd.h"
+#include "parse.h"
+#include "nbdtypes.h"
+#include "self_pipe.h"
+
+struct proxier {
+	/* The flexnbd wrapper this proxier is attached to */
+	struct flexnbd*   flexnbd;
+
+	/** address/port to bind to */
+	union mysockaddr  listen_on;
+
+	/** address/port to connect to */
+	union mysockaddr  connect_to;
+
+	/** address to bind to when making outgoing connections */
+	union mysockaddr  connect_from;
+
+	/* The socket we listen() on and accept() against */
+	int               listen_fd;
+
+	/* The socket returned by accept() that we receive requests from and send
+	 * responses to
+	 */
+	int               downstream_fd;
+
+	/* The socket returned by connect() that we send requests to and receive
+	 * responses from
+	 */
+	int               upstream_fd;
+
+	/* This is the size we advertise to the downstream server */
+	off64_t           upstream_size;
+
+	/* Scratch space for the current NBD request from downstream */
+	unsigned char*     req_buf;
+
+	/* Number of bytes currently sat in req_buf */
+	size_t             req_buf_size;
+
+	/* We transform the raw request header into here */
+	struct nbd_request req_hdr;
+
+	/* Scratch space for the current NBD reply from upstream */
+	unsigned char*     rsp_buf;
+
+	/* Number of bytes currently sat in rsp_buf */
+	size_t             rsp_buf_size;
+
+    /* We transform the raw reply header into here */
+	struct nbd_reply   rsp_hdr;
+};
+
+struct proxier* proxy_create(
+	struct flexnbd * flexnbd,
+	char* s_downstream_address,
+	char* s_downstream_port,
+	char* s_upstream_address,
+	char* s_upstream_port,
+	char* s_upstream_bind );
+int do_proxy( struct proxier* proxy );
+void proxy_destroy( struct proxier* proxy );
+
+#endif
+