Testing a Media Proxy with … QuickCheck

Thomas Arts John Hughes Chalmers/ITU Joakim Johansson Ulf Wiger Ericsson

QuickCheck: Properties not Test Cases • Write

general properties

test cases of code instead of prop_reverse() -> ?FORALL(Xs,list(int()), ?FORALL(Ys,list(int()), lists:reverse(Xs++Ys)== lists:reverse(Xs)++lists:reverse(Ys))).

• Test in many, many randomly generated cases

QuickCheck Testing

3> eqc:quickcheck(test:prop_reverse()).

...........Failed! After 12 tests.

[-3,2] A random counter [-3,1] Shrinking..........(10 times) example: Xs and Ys [0] [1] false

NEW

: Automatic simplification of failing cases • Simplification is extremely important— separates the

signal

from the

noise!

Why Did it Fail?

prop_reverse() -> ?FORALL(Xs,list(int()), ?FORALL(Ys,list(int()), lists:reverse(

Xs

++

Ys

)== lists:reverse(

Xs

)++lists:reverse(

Ys

))).

• QuickCheck says: Xs=[0], Ys=[1] • reverse([0,1])==reverse([0])++reverse([1]) ?

• Xs, Ys the wrong way round

QuickCheck in a Nutshell

• Features – Properties, not test cases – Controlled random generation – Automated simplification • Result – Compact, reusable test code – Specification

checked against the code

– Shortcut from property to bug

Property First Development?

• Code code code… quickcheck A Bug!

• Fix it… quickcheck A Bug!

• Fix it… quickcheck… quickcheck quickcheck quickcheck • Code some more…

Ericsson’s Media Proxy

Media Gateway Controller IP network, servers, etc etc Media stream

How hard can it be?

• All we need to do is open and close ”media pinholes” when the controller says… • Megaco H.248 protocol – ITU standard… 212 pages!

– Ericsson Interwork Description… 183 pages!

– Control software… 150KLOC • 20-30K Megaco

Our Strategy

QuickCheck Sends random command sequences Results make sense?

Completely random commands are all just rejected —non-sensical!

Poor test data

Generating Sensible Messages

Message Generators •

Example:

A

media descriptor

contains a list of streams – ASN.1 from the standard (simplified):

MediaDescriptor { streams { ::= SEQUENCE CHOICE oneStream StreamParms, multiStream SEQUENCE OF StreamDescriptor } }

QuickCheck Generator mediadescriptor(Streams) when Streams=/=[]->

{mediaDescriptor, #'MediaDescriptor'{ streams =

case Streams of [{Id,Mode}] ->

oneof(

[

{oneStream,

Records generated by ASN.1 compiler streamParms(Mode)

}, {multiStream,[

stream(Id,Mode)

]}

]

)

; _ ->

{multiStream,

[stream(I,M) || {I,M}<-Streams]

}

end

}}.

stream(I,Mode) ->

#'StreamDescriptor'{ streamID =

I

, streamParms = Message construction

Logic from the IWD

QuickCheck

streamParms(Mode)

}.

Conditions in the IWD

Add Request Desc.

Desc.

Media Stream Desc.

Local control Properties, Package mode M Comment M M O O Multiple Stream descriptors can be included.

LocalControl will be included in all cases except when no media (m-line) is defined in the remote SDP.

The default value of the mode property is “Inactive”, the property is not mandatory if the wanted value is “Inactive”.

StreamParms { ::= SEQUENCE localControlDescriptor LocalControlDescriptor OPTIONAL, localDescriptor remoteDescriptor …, LocalRemoteDescriptor OPTIONAL, LocalRemoteDescriptor OPTIONAL, statisticsDescriptor StatisticsDescriptor OPTIONAL }

Remot e SDP in here

Two Cases: With and Without Remote Media streamParms(Mode) ->

?LET(RemoteMediaDefined, bool(),

if RemoteMediaDefined ->

#'StreamParms'{ localControlDescriptor =

localControl(Mode),

localDescriptor = remoteDescriptor =

Included in this case localDescriptor(RemoteMediaDefined), remoteDescriptor(RemoteMediaDefined)

}

; true -> …… end

)

.

Passed on to ensure an m-line is generated

Generating Sensible Commands

Megaco Commands

Context Context

Megaco Commands

Context Context Termination Context Add

Add

Megaco Commands

Context Context Termination Termination Context

Megaco Commands

Context Termination Context Termination Stream Stream Context Modify Uses termination ID

Megaco Commands

Context Subtract Termination Context Termination Stream Stream Context

Megaco Commands

Context Context Termination Stream Stream Context Subtract

Megaco Commands

Context Context

Sensible Command Sequences?

• Track the

state

of each test case – What calls in progress, which terminations in each call… – Simple to model in Erlang • State machine model –

Preconditions

say which commands are appropriate – –

Postconditions

check command results

Next state

function specifies how commands behave • QuickCheck library – generates valid sequences, tests, and shrinks them – (developed ”just in time”)

Faults

• Proxy was already well tested • 6 days of work, mostly writing generators • 5 faults found: – One in Megaco message encoding/decoding – Four command sequences crashed

Error-provoking Sequences

• Add Mod • Add Sub • Add Add Mod Streams must have two ends Here today, gone tomorrow… …with differing numbers of streams

The Best Error!

Shrinking reduced 160 commands to seven!

Add Add Sub Add Sub Add Sub No one in their right minds would test this!

Due to data corruption here

”Most Likely Bug” Effect

• Once a bug is found, almost

every

QuickCheck run finds the same one!

– (or at least, shrinks to it) • Add

bug preconditions

to the model to avoid it and find the next one – Formulate a ”bug hypothesis” – Test the hypothesis!

– Document it as a precondition

Real bugs or not?

• Most of these sequences are not sent by

Ericsson’s

Media Gateway Controller – Controller and Proxy were tested together!

• But the Interwork Description doesn’t say so!

•

One

of the IWD/the code was buggy!

Using QuickCheck Earlier?

• Testing an older version – Using the

same

generators and model • 9 errors found in 6 hours! – (2 TRs for this version) • Most of the work: writing bug preconditions

QuickCheck is Moreish…

• Follow-up projects under way • Testing non-Erlang systems (the big one!)

Conclusions

• QuickCheck + Erlang – Simple declarative models of the SUT – Concise, maintainable test code • Testing: a great application for FP – Performance irrelevant – No need to commit to Erlang in the product •

A winning combination?