TCP: immediately trigger the service thread to write PDU

IF qe queue a new PDU to an empty outqueue then the mt service thread
will still be stuck in poll() until it timesout or the socket becomes
readable.

Fix this by sending SIGUSR1 to the service thread when we queue a PDU
to an empty queue. This will break out of poll() and we can immediately
go and write to the socket.

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>

lib/multithreading.c

@@ -139,6 +139,9 @@ int iscsi_mt_sem_wait(libiscsi_sem_t* sem)
 
 #elif defined(HAVE_PTHREAD) /* WIN32 */
 
+#include <errno.h>
+#include <stdio.h>
+#include <signal.h>
 #include <unistd.h>
 #include <sys/syscall.h>
 
@@ -156,13 +159,20 @@ iscsi_tid_t iscsi_mt_get_tid(void)
 #endif
 }
 
+static void on_sigusr1(int _unused)
+{
+}
+
 static void *iscsi_mt_service_thread(void *arg)
 {
         struct iscsi_context *iscsi = (struct iscsi_context *)arg;
 	struct pollfd pfd;
 	int revents;
 	int ret;
-
+        
+        /* set signal to break poll when we need to send more data */
+        signal(SIGUSR1, on_sigusr1);
+        
         iscsi->multithreading_enabled = 1;
 
         /* TODO: add timeout scanning */
@@ -172,12 +182,21 @@ static void *iscsi_mt_service_thread(void *arg)
 		pfd.revents = 0;
         
 		ret = poll(&pfd, 1, iscsi->poll_timeout);
-		if (ret < 0) {
+                if (ret < 0 && errno == EINTR) {
+                        /*
+                         * Got a signal. Assume it is because we need to start writing new PDUs
+                         * to the socket.
+                         */
+                        revents = POLLOUT;
+                        goto call_service;
+                }
+                if (ret < 0) {
 			iscsi_set_error(iscsi, "Poll failed");
 			revents = -1;
 		} else {
 			revents = pfd.revents;
 		}
+        call_service:                
 		if (iscsi_service(iscsi, revents) < 0) {
 			if (revents != -1)
 				iscsi_set_error(iscsi, "iscsi_service failed");