NOP count in flight of client generated NOP-Outs

This patch adds the abilitiy to libiscsi to count the number
of consecutive outstanding NOPs.

With this ability its fairly easy to implement a keepalive
check with NOPs in your application.

Periodically (e.g. every 5 secs) create a NOP-Out with:

iscsi_nop_out_async(iscsi, NULL, NULL, 0, NULL);

At that time check the number of consecutive missing NOP-Ins
with

iscsi_get_nops_in_flight(iscsi) > N.

Where N is the number of missing NOP-Ins you will allow.
Please note that it is legitime for the Target to ignore
a NOP if the load is very high as those packet are mark
as IMMEDIATE.

Signed-off-by: Peter Lieven <pl@kamp.de>

include/iscsi-private.h

@@ -95,6 +95,7 @@ struct iscsi_context {
 	int is_loggedin;
 	int is_reconnecting;
 	int bind_interfaces_cnt;
+	int nops_in_flight;
 
 	int chap_a;
 	int chap_i;
@@ -122,7 +123,7 @@ struct iscsi_context {
 	int no_auto_reconnect;
 	int reconnect_deferred;
 	int reconnect_max_retries;
-	
+
 	int log_level;
 	iscsi_log_fn log_fn;
 

include/iscsi.h

@@ -35,6 +35,7 @@ struct sockaddr;
 
 /* FEATURES */
 #define LIBISCSI_FEATURE_IOVECTOR (1)
+#define LIBISCSI_FEATURE_NOP_COUNTER (1)
 
 #define MAX_STRING_SIZE (255)
 
@@ -474,10 +475,17 @@ struct iscsi_discovery_address {
  *                            structure containing the data returned from
  *                            the server.
  *    ISCSI_STATUS_CANCELLED: Discovery was aborted. Command_data is NULL.
+ * 
+ * The callback may be NULL if you only want to let libiscsi count the in-flight
+ * NOPs.
  */
 EXTERN int iscsi_nop_out_async(struct iscsi_context *iscsi, iscsi_command_cb cb,
 			unsigned char *data, int len, void *private_data);
 
+
+/* read out the number of consecutive nop outs that did not receive an answer */
+EXTERN int iscsi_get_nops_in_flight(struct iscsi_context *iscsi);
+
 struct scsi_task;
 
 enum iscsi_task_mgmt_funcs {