Fix snmpd crash (#3312)

* Enable debug image build for kvm image.

* Fix a bug in cleaning up requests referring the netsmp_session being closed to avoid a crash, during netsnmp_session close/abort due to timeout.
RCA:
The netsnmp_agent_session requests use netsmp_subtree objects that matches the associated variable name.
The netsnmp_subtrees created through a netsnmp_session are tied to that session.
When a subagent connection is closed/dropped due to timeout, all associated netsnmp_subtree objects are fred.
Hence during a netsnmp_session close, all the delegated netsnmp_agent_sessions are scanned for requets that could be using netsnmp_subtree objects associated with this netsnmp_sesssion or its subsession. For each of the found request, they are explicitly marked to fail and a call is made to complete them.

But due to the bug in scanning, it leaves behind requests and hence later when the requests get processed, they refer the *now* freed netsnmp_subtree. As often these requests gets completed pretty soon, they escape crashing. But if it so happens that the freed memory happened to complete a memory unit, hence returned to kernel or it got reallocated & changed enough to crash, the snmpd crashes.

* Revert the changes

* Revert

src/snmpd/patch-5.7.3+dfsg/0006-Release-all-requests-that-use-this-session.patch

@@ -0,0 +1,26 @@
+From 84846206c7ee230bd7b6274af98513952c4a7a7f Mon Sep 17 00:00:00 2001
+From: Renuka Manavalan <remanava@microsoft.com>
+Date: Wed, 7 Aug 2019 21:48:33 +0000
+Subject: [PATCH] Release all requests that use this session.
+
+---
+ agent/snmp_agent.c | 3 ++-
+ 1 file changed, 2 insertions(+), 1 deletion(-)
+
+diff --git a/agent/snmp_agent.c b/agent/snmp_agent.c
+index b96d650..ee3b0da 100644
+--- a/agent/snmp_agent.c
++++ b/agent/snmp_agent.c
+@@ -1542,7 +1542,8 @@ netsnmp_remove_delegated_requests_for_session(netsnmp_session *sess)
+          * check each request
+          */
+         netsnmp_request_info *request;
+-        for(request = asp->requests; request; request = request->next) {
++        int i;
++        for(i = 0, request = asp->requests; i < asp->vbcount; ++i, ++request) {
+             /*
+              * check session
+              */
+-- 
+2.17.1
+

src/snmpd/patch-5.7.3+dfsg/series

@@ -3,3 +3,4 @@
 0003-CHANGES-BUG-2743-snmpd-crashes-when-receiving-a-GetN.patch
 0004-Disable-SNMPv1.patch
 0005-Port-OpenSSL-1.1.0-with-support-for-1.0.2.patch
+0006-Release-all-requests-that-use-this-session.patch