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>

lib/libiscsi.def

@@ -14,6 +14,7 @@ iscsi_get_fd
 iscsi_get_lba_status_sync
 iscsi_get_lba_status_task
 iscsi_get_target_address
+iscsi_get_nops_in_flight
 iscsi_inquiry_sync
 iscsi_inquiry_task
 iscsi_is_logged_in

lib/libiscsi.syms

@@ -12,6 +12,7 @@ iscsi_get_fd
 iscsi_get_lba_status_sync
 iscsi_get_lba_status_task
 iscsi_get_target_address
+iscsi_get_nops_in_flight
 iscsi_inquiry_sync
 iscsi_inquiry_task
 iscsi_is_logged_in

lib/nop.c

@@ -81,6 +81,8 @@ iscsi_nop_out_async(struct iscsi_context *iscsi, iscsi_command_cb cb,
 		return -1;
 	}
 
+	iscsi->nops_in_flight++;
+
 	return 0;
 }
 
@@ -130,6 +132,12 @@ iscsi_process_nop_out_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 {
 	struct iscsi_data data;
 
+	iscsi->nops_in_flight = 0;
+
+	if (pdu->callback == NULL) {
+		return 0;
+	}
+
 	data.data = NULL;
 	data.size = 0;
 
@@ -137,7 +145,13 @@ iscsi_process_nop_out_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 		data.data = in->data;
 		data.size = in->data_pos;
 	}
+	
 	pdu->callback(iscsi, SCSI_STATUS_GOOD, &data, pdu->private_data);
 
 	return 0;
 }
+
+int iscsi_get_nops_in_flight(struct iscsi_context *iscsi)
+{
+	return iscsi->nops_in_flight;
+}