sess_fail_* detailled accept error counters

We already emitted Debug log records for accept failures, but for
all practical purposes this is useless for forensics.

These counters will point directly to the root cause for the most
common issues with sess_fail (accept failures). sess_fail is preserved
as an overall counter as it will most likely be already monitored for
many installations.

Ref	varnishcache#2622

bin/varnishd/VSC_main.vsc

@@ -30,9 +30,46 @@
 .. varnish_vsc:: sess_fail
 	:oneliner:	Session accept failures
 
-	Count of failures to accept TCP connection.  Either the client
-	changed its mind, or the kernel ran out of some resource like file
-	descriptors.
+	Count of failures to accept TCP connection.
+
+	This counter is the sum of the sess_fail_* counters, which
+	give more detailled information.
+
+.. varnish_vsc:: sess_fail_econnaborted
+	:oneliner:	Session accept failures: connection aborted
+
+	Detailled reason for sess_fail: Connection aborted by the
+	client, usually harmless.
+
+.. varnish_vsc:: sess_fail_eintr
+	:oneliner:	Session accept failures: interrupted system call
+
+	Detailled reason for sess_fail: The accept() call was
+	interrupted, usually harmless
+
+.. varnish_vsc:: sess_fail_emfile
+	:oneliner:	Session accept failures: too many open files
+
+	Detailled reason for sess_fail: No file descriptor was
+	available. Consider raising RLIMIT_NOFILE (see ulimit -n).
+
+.. varnish_vsc:: sess_fail_ebadf
+	:oneliner:	Session accept failures: bad file descriptor
+
+	Detailled reason for sess_fail: The listen socket file
+	descriptor was invalid. Should never happen.
+
+.. varnish_vsc:: sess_fail_enomem
+	:oneliner:	Session accept failures: not enough memory
+
+	Detailled reason for sess_fail: Most likely insufficient
+	socket buffer memory. Should never happen
+
+.. varnish_vsc:: sess_fail_other
+	:oneliner:	Session accept failures: other
+
+	Detailled reason for sess_fail: neither of the above, see
+	Debug log (varnishlog -g raw -I Debug:^Accept).
 
 .. varnish_vsc:: client_req_400
 	:oneliner:	Client requests received, subject to 400 errors

bin/varnishd/cache/cache_acceptor.c

@@ -489,23 +489,32 @@ vca_accept_task(struct worker *wrk, void *arg)
 		if (i < 0) {
 			switch (errno) {
 			case ECONNABORTED:
+				wrk->stats->sess_fail_econnaborted++;
+				break;
+			case EINTR:
+				wrk->stats->sess_fail_eintr++;
 				break;
 			case EMFILE:
-				VSL(SLT_Debug, ls->sock, "Too many open files");
+				wrk->stats->sess_fail_emfile++;
 				vca_pace_bad();
 				break;
 			case EBADF:
-				VSL(SLT_Debug, ls->sock, "Accept failed: %s",
-				    strerror(errno));
+				wrk->stats->sess_fail_ebadf++;
+				vca_pace_bad();
+				break;
+			case ENOBUFS:
+			case ENOMEM:
+				wrk->stats->sess_fail_enomem++;
 				vca_pace_bad();
 				break;
 			default:
-				VSL(SLT_Debug, ls->sock, "Accept failed: %s",
-				    strerror(errno));
+				wrk->stats->sess_fail_other++;
 				vca_pace_bad();
 				break;
 			}
 			wrk->stats->sess_fail++;
+			VSL(SLT_Debug, ls->sock, "Accept failed: %s",
+			    strerror(errno));
 			(void)Pool_TrySumstat(wrk);
 			continue;
 		}