Add a SLT_BackendStart log tag, and log address and port on backend req

This log record is a counterpart to the SLT_ReqStart for backends, and
shows the socket endpoint we are talking to at the start of backend
requests. Today this information is only logged on opening a backend
connection (SLT_BackendOpen), and is thus not available on the backend
log transaction when on a reused backend connection.

We need this information to complete the backend view varnishncsa
extension.

bin/varnishd/http1/cache_http1_fetch.c

@@ -81,6 +81,8 @@ V1F_SendReq(struct worker *wrk, struct busyobj *bo, uint64_t *ctr,
 	ssize_t i;
 	struct http_conn *htc;
 	int do_chunked = 0;
+	char abuf[VTCP_ADDRBUFSIZE];
+	char pbuf[VTCP_PORTBUFSIZE];
 
 	CHECK_OBJ_NOTNULL(wrk, WORKER_MAGIC);
 	CHECK_OBJ_NOTNULL(bo, BUSYOBJ_MAGIC);
@@ -96,6 +98,9 @@ V1F_SendReq(struct worker *wrk, struct busyobj *bo, uint64_t *ctr,
 		do_chunked = 1;
 	}
 
+	VTCP_hisname(htc->fd, abuf, sizeof abuf, pbuf, sizeof pbuf);
+	VSLb(bo->vsl, SLT_BackendStart, "%s %s", abuf, pbuf);
+
 	(void)VTCP_blocking(htc->fd);	/* XXX: we should timeout instead */
 	V1L_Reserve(wrk, wrk->aws, &htc->fd, bo->vsl, bo->t_prev);
 	*ctr += HTTP1_Write(wrk, hp, HTTP1_Req);

include/tbl/vsl_tags.h

@@ -522,5 +522,16 @@ SLTM(Witness, 0, "Lock order witness records",
 	"Diagnostic recording of locking order.\n"
 )
 
+SLTM(BackendStart, 0, "Backend request start",
+	"Start of backend processing. Logs the backend IP address and port"
+	" number.\n\n"
+	"The format is::\n\n"
+	"\t%s %s\n"
+	"\t|  |\n"
+	"\t|  +- Backend Port number\n"
+	"\t+---- Backend IP4/6 address\n"
+	"\n"
+)
+
 #undef NODEF_NOTICE
 /*lint -restore */