1. Welcome to CS 119 Reliable Software: Testing and Monitoring
First half of lectures will cover testing
Taught by me
Last half of class will focus on monitoring
Taught by Klaus Havelund
“In runtime verification a software component, an observer, monitors the execution of a program and checks its conformity with a requirement specification.” - Klaus

2. Today Some general background Basic definitions
Topics of the class
Testing project
Basic definitions
Black box testing (FSM) algorithms
Why is testing difficult, in theory and practice?

3. Before we start What do I know about testing, anyway?
I’ve written programs and tested them
So have most of you, I would bet
Split my time at JPL between model checking & testing research
E.g., testing the file systems that will be used in the Mars Science Laboratory – JPL’s next big Mars mission

4. they turn the file system off during EDL (Entry, Descent, and Landing), which helps me sleep at night

5. Topics in Testing We’ll Cover
Black box (Finite State Machine) testing
Design for testability
Coverage measures
Random testing
Constraint-based testing
Debugging and test case minimization
Using model checkers for testing
Coverage revisited (“small model property”)

6. Read All About It No textbook for this class, only papers
Books I like that have something important to say about testing (though none of these are about testing):
The Practice of Programming, Kernighan and Pike
Programming Pearls, Bentley
Why Programs Fail: A Guide to Systematic Debugging, Zeller
Code Complete, McConnell
The Mythical Man-Month, Brooks

7. Read All About It Book about testing:
Introduction to Software Testing, Ammons and Offutt
I like it myself
Recommended by colleagues who’ve taught classes on testing (and are first-rate testing researchers)
Book is thorough and cleverly organized, provokes some real thought about how to test programs
I might just follow this book if doing a whole class on testing
As it is, we’ll take more of a “hit the highlights” approach –
More concentrated on automated techniques: random testing, constraint-based testing, and model checking
Won’t stop me from using some of their slides for areas they cover well

8. Testing Project
Start with source code for YAFFS (Yet Another Flash File System): an open source flash file system
Plus a set of (buggy) variations of the YAFFS code
A (minimal) automated testing framework
Project:
Write a better automated tester
Must be efficient: spend no more than 45 minutes to test a version of YAFFS
Write a test report on the YAFFS versions

9. Testing Project
You will also turn in two new buggy variations of YAFFS
Make sure your tester finds the bug
Produce a test case
Make sure the test case succeeds for original YAFFS!
You get to debug programs in lots of classes and in real life – in this class you get to bug a program intentionally, for once in your life
I will apply all of your testers to these bugs plus another (top secret) set of mutations of YAFFS that I generate
Let me know any time you think you find a bug in the original YAFFS!

10. Testing Project Grading criteria Design/implementation of the tester
Effectiveness of the tester
Quality of the test report
Can I figure out how you tested YAFFS?
Can I figure out what wasn’t tested?
Can I figure out how reliable you think YAFFS is, and how buggy the various versions are?
How “interesting” and hard-to-find your new bugs are

11. Testing Project Expectations Hopes You can program in C
“he who learns to play the harp learns to play by playing it” - Aristotle, Metaphysics, Book IX
Expectations
You can program in C
You can use makefiles / build system
Hopes
Maybe we’ll find some previously unknown bugs in YAFFS – that would be cool
I hope you’ll help me make sure MSL figures out if there was life on Mars
Get started with YAFFS at

12. Basic Definitions: Testing
What is software testing?
Running a program
In order to find faults
a.k.a. defects
a.k.a. errors
a.k.a. flaws
a.k.a. faults
a.k.a. BUGS

13. Bugs
Hopper’s “bug” (moth stuck in a relay on an early machine)
“an analyzing process must equally have been performed in order to furnish the Analytical Engine with the necessary operative data; and that herein may also lie a possible source of error. Granted that the actual mechanism is unerring in its processes, the cards may give it wrong orders. ” – Ada, Countess Lovelace (notes on Babbage’s Analytical Engine)
“It has been just so in all of my inventions. The first step is an intuition, and comes with a burst, then difficulties arise—this thing gives out and [it is] then that 'Bugs'—as such little faults and difficulties are called—show themselves and months of intense watching, study and labor are requisite. . .” – Thomas Edison

14. Testing What isn’t software testing?
Purely static analysis: examining a program’s source code or binary in order to find bugs, but not executing the program
Good stuff, and very important, but it’s not testing
Fuzzy borderline: if we only symbolically execute the program
For this class, we’ll stick to testing where the program actually runs (but maybe in a virtual machine)

15. Why Testing?
Ideally: we prove code correct, using formal mathematical techniques (with a computer, not chalk)
Extremely difficult: for some trivial programs (100 lines) and many small (5K lines) programs
Simply not practical to prove correctness in most cases – often not even for safety or mission critical code

16. Why Testing?
Nearly ideally: use symbolic or abstract model checking to prove the system correct
Automatically extracts a mathematical abstraction from a system
Proves properties over all possible executions
In practice, can work well for very simple properties (“this program never crashes in this particular way”), but can’t handle complex properties (“this is a working file system”)
Doesn’t work well for programs with complex data structures (like a file system)

17. As a last resort…
… we can actually run the program, to see if it works
This is software testing
Always necessary, even when you can prove correctness – because the proof is seldom directly tied to the actual code that runs
“Beware of bugs in the above code; I have only proved it correct, not tried it” – Knuth

18. Why Does Testing Matter?
Ariane 5: exception-handling bug : forced self destruct on maiden flight (64-bit to 16-bit conversion: about 370 million $ lost)
NIST report, “The Economic Impacts of Inadequate Infrastructure for Software Testing” (2002)
Inadequate software testing costs the US alone between $22 and $59 billion annually
Better approaches could cut this amount in half
Major failures: Ariane 5 explosion, Mars Polar Lander, Intel’s Pentium FDIV bug
Insufficient testing of safety-critical software can cost lives: THERAC-25 radiation machine: 3 dead
We want our programs to be reliable
Testing is how, in most cases, we find out if they are
Mars Polar Lander crash site?
THERAC-25 design

19. Testing and Monitoring
In this first half of the class, we’ll look at which executions of a program to run
I’ll call this problem “the” testing problem
Second problem: how do we know if an execution reveals a bug?
Key question when monitoring deployed programs to handle faults or send in bug reports from the field
I’ll (mostly) take this for granted: we have a reference model or assertions to check
Klaus

20. Example: File System Testing
File system is a library, called by other components of the flight software
Accepts a fixed set of operations that manipulate files:
Operation Result
mkdir (“/eng”, …) SUCCESS
mkdir (“/data”, …) SUCCESS
creat (“/data/image01”, …) SUCCESS
creat (“/eng/fsw/code”, …) ENOENT
mkdir (“/data/telemetry”, …) SUCCESS
unlink (“/data/image01”) SUCCESS
File system /
/eng
/data
image01
/telemetry

21. Example: File System Testing
Easy to detect many errors: we have access to many working file systems, and can just compare results
Choose operation F
Perform F on Tested FS
Perform F on Reference
(if applicable)
Compare return values
Compare error codes
Compare file systems
Check invariants
(in this unusual case, the problem Klaus will discuss is not much of a problem)
(inject a fault?)

22. Example: File System Testing
How hard would it be to just try “all” the possibilities?
Consider only core 7 operations (mkdir, rmdir, creat, open, close, read, write)
Most of these take either a file name or a numeric argument, or both
Even for a “reasonable” (but not provably safe) limitation of the parameters, there are executions of length 10 to try
Not a realistic possibility (unless we have years to test)

23. The Testing Problem
This is the topic of the first half of the class: what “questions” do we pose to the software, i.e.,
How do we select a small set of executions out of a very large set of executions?
Fundamental problem of software testing research and practice
An open (and essentially unsolvable, in the general case) problem

24. The Testing Problem / Terms
This is not a class in the management or even the basic practices of testing
Hard, important problem
But not the focus of this class
This class is going to focus on state-of-the-art automated approaches
Using tools
To catch the bugs that you don’t catch with basic practices
I will briefly cover some basic terms of testing and testing management today, then we’ll mostly dive into “How To Test It” at a more technical level

25. Terms: Verification and Validation
These two terms appear a lot, often in vague or sloppy ways, in the literature
Verification is checking that a program matches a specification
Validation is making sure it meets the original requirements – satisfies customers, operates ok onboard the spacecraft, etc.
Verification: “you built it right”
Validation: “you built the right thing”
(our focus, for the most part)

26. Terms: Unit, Integration, System Testing
Stages of testing
Unit testing is the first phase, done by developers of modules
Integration testing combines unit tested modules and tests how they interact
System testing tests a whole program to make sure it meets requirements
“Design testing” is testing prototypes or very abstract models before implementation – seldom mentioned, but when possible it can save your bacon
Exhaustive model checking may be possible at this stage

27. Terms: Functional Testing
Functional testing is a related term
Tests a program from a “user’s” perspective – does it do what it should?
Opposed to unit testing, which often proceeds from the perspective of other parts of the program
Module spec/interface, not user interaction
Sort of a fuzzy line – consider a file system – how different is the use by a program and use of UNIX commands at a prompt by a user?
Building inspector does “unit testing”; you, walking through the house to see if its livable, perform “functional testing”
Kick the tires vs. take it for a spin?

28. Terms: Regression Testing
Changes can break code, reintroduce old bugs
Things that used to work may stop working (e.g., because of another “fix”) – software regresses
Usually a set of cases that have failed (& then succeeded) in the past
Finding small regressions is an ongoing research area – analyze dependencies
“. . . as a consequence of the introduction of new bugs, program maintenance requires far more system testing Theoretically, after each fix one must run the entire batch of test cases previously run against the system, to ensure that it has not been damaged in an obscure way. In practice, such regression testing must indeed approximate this theoretical idea, and it is very costly." - Brooks, The Mythical Man-Month

29. Terms: The Oracle Problem (Klaus)
(oracle: a magical source of truth, often cryptic, given by the gods)
The oracle problem
How to know if a test fails
If the oracle says every execution is good, why bother running the program?
Some obvious, easily automated approaches:
The program probably shouldn’t crash
Assertions shouldn’t be violated
Automatable, but more difficult to apply:
Differential testing (McKeeman, etc.) – when you have another program, likely correct, that does the same thing, just compare outputs over same inputs
Last resort, not automatable:
Hand inspection of executions

30. Terms: Test (Case) vs. Test Suite
Test (case): one execution of the program, that may expose a bug
Test suite: a set of executions of a program, grouped together
A test suite is made of test cases
Tester: a program that generates tests
Line gets blurry when testing functions, not programs – especially with persistent state

31. Terms: Black Box Testing
Treats a program or system as a
That is, testing that does not look at source code or internal structure of the system
Send a program a stream of inputs, observe the outputs, decide if the system passed or failed the test
Abstracts away the internals – a useful perspective for integration and system testing
Sometimes you don’t have access to source code, and can make little use of object code
True black box? Access only over a network

32. Terms: White Box Testing
Opens up the box!
(also known as glass box, clear box, or structural testing)
Use source code (or other structure beyond the input/output spec.) to design test cases
Brings us to the idea of coverage

33. Terms: Coverage Coverage measures or metrics
Abstraction of “what a test suite tests” in a structural sense
Best explained by giving examples
Common measures:
Statement coverage
A.k.a line coverage or basic block coverage
Which statements execute in a test suite
Decision coverage
Which boolean expressions in control structures evaluated to both true and false during suite execution
Path coverage
Which paths through a program’s control flow graph are taken in the test suite

34. Terms: Coverage Measures
In general, used to measure the quality of a test suite
Even in cases where the suite was designed for some other purpose (such as testing lots of different use scenarios)
Not always a very good measure of suite quality, but “better than nothing”
We “open the box” in white box testing partly in order to look at (and design tests to achieve) coverage
We’ll cover coverage in much more detail

35. Terms: Mutation Testing
A mutation of a program is a version of the program with one or more random changes
Mutation testing is another way to measure the quality of a test suite
Amman and Offutt call it syntax-based coverage
Idea: generate a large number of mutants
Run the test suite on these
If few mutants are detected, the test suite may not be very good
Difficulties
Cost of testing many versions of a program
How to generate mutants (operators)
In principle, can subsume many other forms of coverage

36. Black Box (Finite State Machine) Testing

37. Black box(FSM) testing
Let’s step back from software testing
Let’s look at a simpler model
Finite state machines
Software is a finite state machine
What? Software is a Turing machine, right?
Only with an infinite tape.
That is, only if your software has access to infinite memory.
Lego “Turing machine”

38. Black box (FSM) testing
With static memory allocation or with limited dynamic allocation nothing is infinite
Even if you add in disk or network storage
We don’t have infinite electrons, much less memory
So software systems are finite state machines, in reality
Don’t you feel better now?
No more late nights worrying about the halting problem!
there are only ~1079 of these little guys, y’know?

39. Black box (FSM) testing
Theoretical issues aside, why do we care about testing finite state machines?
Abstraction: designs can often be best understood as finite-state machines
String processing/searching
Protocols – communication, cache coherence, etc.
Control component of any discrete system
Automatic abstraction:
Tools that take systems and produce (coarse) finite state abstractions

40. Black box (FSM) testing
Useful for modeling aspects of many designs
FD = open (“/foo”)
close(FD)
read(FD, buf, nbytes)
write(FD, buf, nbytes)

41. Very Simple FSM Model c b a 1 1 FSM is a tuple, <S, , T, I>
S is a set of states
 is the input alphabet
T is the transition relation
T: S x  x S
I  S is the initial state
Further assume:
Machine is deterministic
T is a (partial) function S x   S
Given an input from , machine either
Outputs 0 (if no transition)
Or outputs 1 and takes the transition to s’ d a a b a c c b a 1 1

42. Conformance Testing How do we test finite state machines?
Let’s say we have
Known FSM A
Know all states and transitions
Unknown FSM B (same alphabet)
Can only perform experiments
How do we tell if A = B?
Known as the conformance testing or equivalence testing problem
As stated, we cannot solve the problem
Why? d a a b a c

43. Combination Lock Machine
How many states does B have?
If we don’t know, we can never be sure it is the same machine as A
B is a combination lock: looks like A unless we input exact sequence “b u g” – in which case it deadlocks
Machine B b u
Machine A g
a, c-z
a-f, h-z
a-t, v-z
a-z

44. Combination Lock Machine
Machine B
Even if we know upper limit n on B’s size, for alphabet of size ||
It takes n|| tests to check equivalence to this particular A
This pathological case imposes some limits on conformance testing in general b u
Machine A g
a, c-z
a-f, h-z
a-t, v-z
a-z

45. Conformance Testing (VC Algorithm)
Algorithm due to Vasilevskii and Chow for conformance testing
Assumptions
A is minimized, has m states
B has no more than n states
A, B both have a reliable reset
We can start from initial state at will
Worst-case complexity: O(n2 m ||n-m+1)
I’ll cover this quickly and informally, skipping over the sub-algorithms

46. Conformance Testing (VC Algorithm)
First, we find a path to each state of A
Typically, we compute a spanning tree
For example, by a depth first search (DFS)
Call this set P
Read the paths off of the tree:
<no input> a
a a
a a d
a b a d a a a b a b c d

47. Conformance Testing (VC Algorithm)
Next, compute a characterizing (or distinguishing) set for A
Set W of input sequences such that
 s, s’  S
s  s’
Exists w  W
Output for w from s not equal to output from s’
i.e., we can use W to tell what state we’re in

48. Conformance Testing (VC Algorithm)
Next, compute a characterizing (or distinguishing) set for A
For example, W for A might be:
{aa, b}
aa: 11, 10, 01, 00, 10
Distinguishes all but these two states
Which are distinguished by b (1 vs. 0)
Can we find another (better?) set? a d a a b c

49. Conformance Testing (VC Algorithm)
Now we can compute Z:
W U
  W U
2  W U …
m-n  W
To test B for conformance with A
Run the tests produced by taking cross-product of P and Z on both A and B

50. Conformance Testing (VC Algorithm)
P: {<>, a, aa, aad, ab}
W: {aa, b}
Let’s say we know B has no more than 6 states
The complete testing sequence (with reset before each test on each machine) is:
{aa, b, a aa, a b, aa aa, aa b, aad aa, aad b, ab aa, ab b, a aa, b aa, c aa, d aa, a b, b b, c b, d b, a a aa, a b aa, a c aa, a d aa, a a b, a b b, a c b, a d b, aa a aa, aa b aa, aa c aa, aa d aa, aa a b, aa b b, aa c b, aa d b, aad a aa, aad b aa, aad c aa, aad d aa, aad a b, aad b b, aad c b, aad d b, ab a aa, ab b aa, ab c aa, ab d aa, ab a b, ab b b, ab c b, ab d b} d a a b c

51. Conformance Testing (VC Algorithm)
As this small example shows, exhaustive tests can be very expensive
In general, we cannot computationally afford to perform complete testing
We will always face the risk of missing errors
Even when we reduce our problem to the simplest model
The complexity of testing full equivalence to a reference model is simply too high
Exhaustion is exhausting

52. From FSM Testing to the Big Picture
Testing (almost always) is an attempt to
Cover some measure of a structure
Nodes of a graph (e.g., VC’s spanning tree)
Inputs that give different outputs (e.g., VC’s distinguishing set)
All possible inputs (e.g., m-n)
Logical expression evaluations
Predicates over program variables
Pairs of where a variable is defined and where it is used (data flow)
Usually, we can’t even guarantee that coverage directly correlates to more bugs found

53. Basic Definitions: Testing
What is software testing?
Running a program
In order to find faults
a.k.a. defects
a.k.a. errors
a.k.a. flaws
a.k.a. faults
a.k.a. BUGS
But also, in order to
Increase our confidence that the program has high quality and low risk
Because we can never be sure we caught all bugs
How does a set of executions increase confidence?
Sometimes, by algorithmic argument (VC)
Sometimes by less formal arguments (coverage in general)

54. FSM Testing: Further Reading
Yannakakis and Lee, “Principles and Methods of Testing Finite State Machines: A Survey”
Chow, “Testing Software Design Modeled by Finite-State Machines”
Links on the class website

55. “Assignment 0” Download the YAFFS tarball from the class website
cd direct
make
look at yaffscfg2k.c
run ./directtest2k
write a very simple test (e.g., open a file, write something, and read it back) and add it to the main
get it to compile and run, and the test case (and results) to me at