#1271 Fix for expressions with objects with no delta sampling in the DISMAN-EXPRESSION-MIB implementation

[Jahanzeb Farooq]

Net-SNMP Version: 5.7.2
Operating System: Linux hostname 3.2.0-38-generic-pae #61-Ubuntu SMP Tue Feb 19 12:39:51 UTC 2013 i686 i686 i386 GNU/Linux
Distribution: ubuntu 12.04 LTS

Problem Description

I came across the problem while trying out the following simple program using the DISMAN-EXPRESSION-MIB implementation of net-snmp in agent/mibgroup/disman/expr. The program creates a simple expression that uses two objects from IF-MIB (ifInUcastPkts.1 and ifOutUcastPkts.1) and produces sum of their values, as following:

snmpset -uuser Lo localhost DISMAN-EXPRESSION-MIB::expExpressionEntryStatus.'"foo"'.'"bar"' = createAndWait
snmpset -uuser Lo localhost DISMAN-EXPRESSION-MIB::expExpression.'"foo"'.'"bar"' = '$1+$2'
snmpset -uuser Lo localhost DISMAN-EXPRESSION-MIB::expExpressionValueType.'"foo"'.'"bar"' = counter32
snmpset -uuser Lo localhost DISMAN-EXPRESSION-MIB::expExpressionEntryStatus.'"foo"'.'"bar"' = 'active'

snmpset -uuser Lo localhost DISMAN-EXPRESSION-MIB::expObjectEntryStatus.'"foo"'.'"bar"'.1 = createAndWait
snmpset -uuser Lo localhost DISMAN-EXPRESSION-MIB::expObjectID.'"foo"'.'"bar"'.1 = ifInUcastPkts.1
snmpset -uuser Lo localhost DISMAN-EXPRESSION-MIB::expObjectEntryStatus.'"foo"'.'"bar"'.1 = 'active'

snmpset -uuser Lo localhost DISMAN-EXPRESSION-MIB::expObjectEntryStatus.'"foo"'.'"bar"'.2 = createAndWait
snmpset -uuser Lo localhost DISMAN-EXPRESSION-MIB::expObjectID.'"foo"'.'"bar"'.2 = ifOutUcastPkts.1
snmpset -uuser Lo localhost DISMAN-EXPRESSION-MIB::expObjectEntryStatus.'"foo"'.'"bar"'.2 = 'active'

However it wasn't working as querying the expression value (as following) didn't return a value. It seemed like the expression wasn't getting evaluated.

snmpget -uuser Lo localhost DISMAN-EXPRESSION-MIB::expValueCounter32Val.'"foo"'.'"bar"'.0.0.0

While examining the code I found out that the following snippet (agent/mibgroup/disman/expr/expExpression.c, line 317) causes the expression data to be retrieved (by calling expExpression_getData()) only if delta (expExpressionDeltaInterval) is set to a non-zero value. Since this is the only place where expExpression_getData() is called, the expressions with detla value of 0 never get evaluated.

if (entry->expDeltaInterval) {
    entry->alarm = snmp_alarm_register(
                       entry->expDeltaInterval, SA_REPEAT,
                       expExpression_getData, entry );
    expExpression_getData( entry->alarm, (void*)entry );
}

This means even if I have an expression with no regular delta-value sampling (i.e. with all objects with expObjectSampleType 'absoluteValue'), I will still have to set expExpressionDeltaInterval to a non-zero value to make it work. As expected, doing so (as following) fixed the problem and my program worked.

snmpset -uuser Lo localhost DISMAN-EXPRESSION-MIB::expExpressionDeltaInterval.'"foo"'.'"bar"' = 1 

As per my understanding of the DISMAN-EXPRESSION-MIB RFC, this is not how it is supposed to work. expExpressionDeltaInterval is only relevant for objects with expObjectSampleType 'deltaValue' as the following text from RFC states:

expExpressionDeltaInterval OBJECT-TYPE
    SYNTAX      Integer32 (0..86400)
    UNITS       "seconds"
    MAX-ACCESS  read-create
    STATUS      current
    DESCRIPTION
     "Sampling interval for objects in this expression with
     expObjectSampleType 'deltaValue'.

     This object has no effect if the the expression has no
     deltaValue objects.

     A value of 0 indicates no automated sampling.  In this case
     the delta is the difference from the last time the expression
     was evaluated...

Solution

The function expExpression_getData() must also be called for expressions with delta value of zero, i.e. on-demand as the comment inside expExpression_getData() also states.

In the attached patch I call expExpression_getData() every time an expression is evaluated, both if the expression has a zero delta value or non-zero. For expressions with non-zero delta, the call won't have any effect anyway because a check in the beginning of the expExpression_getData() makes it return without doing anything.

[Niels Baggesen]

Thanks for the patch! It has been applied to all active branches.