Retire the MAIN.sess_drop VSC

When de16dba2 landed its documentation was mixed up with the other
sess_dropped counter. When sess_dropped appeared in ac393c4b the
sess_drop counter was still function, it died short after as a side
effect of ac2e067b.

bin/varnishd/VSC_main.vsc

@@ -27,12 +27,6 @@
 
 	Count of sessions successfully accepted
 
-.. varnish_vsc:: sess_drop
-	:group: wrk
-	:oneliner:	Sessions dropped
-
-	Count of sessions silently dropped due to lack of worker thread.
-
 .. varnish_vsc:: sess_fail
 	:group: wrk
 	:oneliner:	Session accept failures

bin/varnishd/cache/cache_acceptor.c

@@ -373,19 +373,6 @@ vca_make_session(struct worker *wrk, void *arg)
 	/* Turn accepted socket into a session */
 	AN(wrk->aws->r);
 	sp = SES_New(wrk->pool);
-	if (sp == NULL) {
-		/*
-		 * We consider this a DoS situation and silently close the
-		 * connection with minimum effort and fuzz, rather than try
-		 * to send an intelligent message back.
-		 */
-		vca_pace_bad();
-		VTCP_nonblocking(wa->acceptsock);
-		closefd(&wa->acceptsock);
-		wrk->stats->sess_drop++;
-		WS_Release(wrk->aws, 0);
-		return;
-	}
 	CHECK_OBJ_NOTNULL(sp, SESS_MAGIC);
 	wrk->stats->s_sess++;
 

bin/varnishd/cache/cache_session.c

@@ -365,7 +365,8 @@ SES_New(struct pool *pp)
 
 	CHECK_OBJ_NOTNULL(pp, POOL_MAGIC);
 	sp = MPL_Get(pp->mpl_sess, &sz);
-	sp->magic = SESS_MAGIC;
+	AN(sp);
+	INIT_OBJ(sp, SESS_MAGIC);
 	sp->pool = pp;
 	sp->refcnt = 1;
 	memset(sp->sattr, 0xff, sizeof sp->sattr);

bin/varnishtest/tests/t02011.vtc

@@ -72,7 +72,6 @@ client c1 {
 varnish v1 -cliok "param.set thread_pool_min 3"
 delay 1
 varnish v1 -vsl_catchup
-varnish v1 -expect sess_drop == 0
 varnish v1 -expect sess_dropped == 0
 varnish v1 -expect req_dropped == 1
 varnish v1 -expect MEMPOOL.req0.live == 0

doc/changes.rst

@@ -30,7 +30,7 @@ release process.
 NEXT (2020-03-15)
 ================================
 
-(nothing yet)
+* The ``MAIN.sess_drop`` counter is gone.
 
 ================================
 Varnish Cache 6.3.0 (2019-09-15)