Programming language: Go
License: MIT License
Tags: Testing     Testing Frameworks    
Latest version: v1.1

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go-mutesting is a framework for performing mutation testing on Go source code. Its main purpose is to find source code, which is not covered by any tests.

Quick example

The following command mutates the go-mutesting project with all available mutators.

go-mutesting github.com/zimmski/go-mutesting/...

The execution of this command prints for every mutation if it was successfully tested or not. If not, the source code patch is printed out, so the mutation can be investigated. The following shows an example for a patch of a mutation.

for _, d := range opts.Mutator.DisableMutators {
    pattern := strings.HasSuffix(d, "*")

-   if (pattern && strings.HasPrefix(name, d[:len(d)-2])) || (!pattern && name == d) {
+   if (pattern && strings.HasPrefix(name, d[:len(d)-2])) || false {
        continue MUTATOR

The example shows that the right term (!pattern && name == d) of the || operator is made irrelevant by substituting it with false. Since this change of the source code is not detected by the test suite, meaning the test suite did not fail, we can mark it as untested code.

The next mutation shows code from the removeNode method of a linked list implementation.


    l.first = nil
-   l.last = nil
    l.len = 0

We know that the code originates from a remove method which means that the mutation introduces a leak by ignoring the removal of a reference. This can be tested with go-leaks.

Table of content

What is mutation testing?

The definition of mutation testing is best quoted from Wikipedia:

Mutation testing (or Mutation analysis or Program mutation) is used to design new software tests and evaluate the quality of existing software tests. Mutation testing involves modifying a program in small ways. Each mutated version is called a mutant and tests detect and reject mutants by causing the behavior of the original version to differ from the mutant. This is called killing the mutant. Test suites are measured by the percentage of mutants that they kill. New tests can be designed to kill additional mutants. -- https://en.wikipedia.org/wiki/Mutation_testing

Tests can be created to verify the correctness of the implementation of a given software system, but the creation of tests still poses the question whether the tests are correct and sufficiently cover the requirements that have originated the implementation. -- https://en.wikipedia.org/wiki/Mutation_testing

Although the definition states that the main purpose of mutation testing is finding implementation cases which are not covered by tests, other implementation flaws can be found too. Mutation testing can for example uncover dead and unneeded code.

Mutation testing is also especially interesting for comparing automatically generated test suites with manually written test suites. This was the original intention of go-mutesting which is used to evaluate the generic fuzzing and delta-debugging framework Tavor.

How do I use go-mutesting?

go-mutesting includes a binary which is go-getable.

go get -t -v github.com/zimmski/go-mutesting/...

The binary's help can be invoked by executing the binary without arguments or with the --help argument.

go-mutesting --help

Note: This README describes only a few of the available arguments. It is therefore advisable to examine the output of the --help argument.

The targets of the mutation testing can be defined as arguments to the binary. Every target can be either a Go source file, a directory or a package. Directories and packages can also include the ... wildcard pattern which will search recursively for Go source files. Test source files with the suffix _test are excluded, since this would interfere with the testing process most of the time.

The following example gathers all Go files which are defined by the targets and generate mutations with all available mutators of the binary.

go-mutesting parse.go example/ github.com/zimmski/go-mutesting/mutator/...

Every mutation has to be tested using an exec command. By default the built-in exec command is used, which tests a mutation using the following steps:

  • Replace the original file with the mutation.
  • Execute all tests of the package of the mutated file.
  • Report if the mutation was killed.

Alternatively the --exec argument can be used to invoke an external exec command. The /scripts/exec directory holds basic exec commands for Go projects. The test-mutated-package.sh script implements all steps and almost all features of the built-in exec command. It can be for example used to test the github.com/zimmski/go-mutesting/example package.

go-mutesting --exec "$GOPATH/src/github.com/zimmski/go-mutesting/scripts/exec/test-mutated-package.sh" github.com/zimmski/go-mutesting/example

The execution will print the following output.

Note: This output is from an older version of go-mutesting. Up to date versions of go-mutesting will have different mutations.

PASS "/tmp/go-mutesting-422402775//home/zimmski/go/src/github.com/zimmski/go-mutesting/example/example.go.0" with checksum b705f4c99e6d572de509609eb0a625be
PASS "/tmp/go-mutesting-422402775//home/zimmski/go/src/github.com/zimmski/go-mutesting/example/example.go.1" with checksum eb54efffc5edfc7eba2b276371b29836
PASS "/tmp/go-mutesting-422402775//home/zimmski/go/src/github.com/zimmski/go-mutesting/example/example.go.2" with checksum 011df9567e5fee9bf75cbe5d5dc1c81f
--- /home/zimmski/go/src/github.com/zimmski/go-mutesting/example/example.go
+++ /tmp/go-mutesting-422402775//home/zimmski/go/src/github.com/zimmski/go-mutesting/example/example.go.3
@@ -16,7 +16,7 @@

        if n < 0 {
-               n = 0

FAIL "/tmp/go-mutesting-422402775//home/zimmski/go/src/github.com/zimmski/go-mutesting/example/example.go.3" with checksum 82fc14acf7b561598bfce25bf3a162a2
PASS "/tmp/go-mutesting-422402775//home/zimmski/go/src/github.com/zimmski/go-mutesting/example/example.go.4" with checksum 5720f1bf404abea121feb5a50caf672c
PASS "/tmp/go-mutesting-422402775//home/zimmski/go/src/github.com/zimmski/go-mutesting/example/example.go.5" with checksum d6c1b5e25241453128f9f3bf1b9e7741
--- /home/zimmski/go/src/github.com/zimmski/go-mutesting/example/example.go
+++ /tmp/go-mutesting-422402775//home/zimmski/go/src/github.com/zimmski/go-mutesting/example/example.go.6
@@ -24,7 +24,6 @@
        n += bar()

-       bar()

        return n
FAIL "/tmp/go-mutesting-422402775//home/zimmski/go/src/github.com/zimmski/go-mutesting/example/example.go.6" with checksum 5b1ca0cfedd786d9df136a0e042df23a
PASS "/tmp/go-mutesting-422402775//home/zimmski/go/src/github.com/zimmski/go-mutesting/example/example.go.8" with checksum 6928f4458787c7042c8b4505888300a6
The mutation score is 0.750000 (6 passed, 2 failed, 0 skipped, total is 8)

The output shows that eight mutations have been found and tested. Six of them passed which means that the test suite failed for these mutations and the mutations were therefore killed. However, two mutations did not fail the test suite. Their source code patches are shown in the output which can be used to investigate these mutations.

The summary also shows the mutation score which is a metric on how many mutations are killed by the test suite and therefore states the quality of the test suite. The mutation score is calculated by dividing the number of passed mutations by the number of total mutations, for the example above this would be 6/8=0.75. A score of 1.0 means that all mutations have been killed.

Blacklist false positives

Mutation testing can generate many false positives since mutation algorithms do not fully understand the given source code. early exits are one common example. They can be implemented as optimizations and will almost always trigger a false-positive since the unoptimized code path will be used which will lead to the same result. go-mutesting is meant to be used as an addition to automatic test suites. It is therefore necessary to mark such mutations as false-positives. This is done with the --blacklist argument. The argument defines a file which contains in every line a MD5 checksum of a mutation. These checksums can then be used to ignore mutations.

Note: The blacklist feature is currently badly implemented as a change in the original source code will change all checksums.

The example output of the How do I use go-mutesting? section describes a mutation example.go.6 which has the checksum 5b1ca0cfedd786d9df136a0e042df23a. If we want to mark this mutation as a false-positive, we simple create a file with the following content.


The blacklist file, which is named example.blacklist in this example, can then be used to invoke go-mutesting.

go-mutesting --blacklist example.blacklist github.com/zimmski/go-mutesting/example

The execution will print the following output.

Note: This output is from an older version of go-mutesting. Up to date versions of go-mutesting will have different mutations.

PASS "/tmp/go-mutesting-208240643/example.go.0" with checksum b705f4c99e6d572de509609eb0a625be
PASS "/tmp/go-mutesting-208240643/example.go.1" with checksum eb54efffc5edfc7eba2b276371b29836
PASS "/tmp/go-mutesting-208240643/example.go.2" with checksum 011df9567e5fee9bf75cbe5d5dc1c81f
--- example.go  2014-12-29 23:37:42.813320040 +0100
+++ /tmp/go-mutesting-208240643/example.go.3    2014-12-30 00:49:33.573285038 +0100
@@ -16,7 +16,7 @@

        if n < 0 {
-               n = 0

FAIL "/tmp/go-mutesting-208240643/example.go.3" with checksum 82fc14acf7b561598bfce25bf3a162a2
PASS "/tmp/go-mutesting-208240643/example.go.4" with checksum 5720f1bf404abea121feb5a50caf672c
PASS "/tmp/go-mutesting-208240643/example.go.5" with checksum d6c1b5e25241453128f9f3bf1b9e7741
PASS "/tmp/go-mutesting-208240643/example.go.8" with checksum 6928f4458787c7042c8b4505888300a6
The mutation score is 0.857143 (6 passed, 1 failed, 0 skipped, total is 7)

By comparing this output to the original output we can state that we now have 7 mutations instead of 8.

How do I write my own mutation exec commands?

A mutation exec command is invoked for every mutation which is necessary to test a mutation. Commands should handle at least the following phases.

  1. Setup the source to include the mutation.
  2. Test the source by invoking the test suite and possible other test functionality.
  3. Cleanup all changes and remove all temporary assets.
  4. Report if the mutation was killed.

It is important to note that each invocation should be isolated and therefore stateless. This means that an invocation must not interfere with other invocations.

A set of environment variables, which define exactly one mutation, is passed on to the command.

Name Description
MUTATE_CHANGED Defines the filename to the mutation of the original file.
MUTATE_DEBUG Defines if debugging output should be printed.
MUTATE_ORIGINAL Defines the filename to the original file which was mutated.
MUTATE_PACKAGE Defines the import path of the origianl file.
MUTATE_TIMEOUT Defines a timeout which should be taken into account by the exec command.
MUTATE_VERBOSE Defines if verbose output should be printed.
TEST_RECURSIVE Defines if tests should be run recursively.

A command must exit with an appropriate exit code.

Exit code Description
0 The mutation was killed. Which means that the test led to a failed test after the mutation was applied.
1 The mutation is alive. Which means that this could be a flaw in the test suite or even in the implementation.
2 The mutation was skipped, since there are other problems e.g. compilation errors.
>2 The mutation produced an unknown exit code which might be a flaw in the exec command.

Examples for exec commands can be found in the scripts directory.

Which mutators are implemented?

Branch mutators

Name Description
branch/case Empties case bodies.
branch/if Empties branches of if and else if statements.
branch/else Empties branches of else statements.

Expression mutators

Name Description
expression/comparison Searches for comparison operators, such as > and <=, and replaces them with similar operators to catch off-by-one errors, e.g. > is replaced by >=.
expression/remove Searches for && and \

Statement mutators

Name Description
statement/remove Removes assignment, increment, decrement and expression statements.

How do I write my own mutators?

Each mutator must implement the Mutator interface of the github.com/zimmski/go-mutesting/mutator package. The methods of the interface are described in detail in the source code documentation.

Additionally each mutator has to be registered with the Register function of the github.com/zimmski/go-mutesting/mutator package to make it usable by the binary.

Examples for mutators can be found in the github.com/zimmski/go-mutesting/mutator package and its sub-packages.

Other mutation testing projects and their flaws

go-mutesting is not the first project to implement mutation testing for Go source code. A quick search uncovers the following projects.

All of them have significant flaws in comparison to go-mutesting:

  • Only one type (or even one case) of mutation is implemented.
  • Can only be used for one mutator at a time (manbearpig, Golang-Mutation-testing).
  • Mutation is done by content which can lead to lots of invalid mutations (Golang-Mutation-testing).
  • New mutators are not easily implemented and integrated.
  • Can only be used for one package or file at a time.
  • Other scenarios as go test cannot be applied.
  • Do not properly clean up or handle fatal failures.
  • No automatic tests to ensure that the algorithms are working at all.
  • Uses another language (Golang-Mutation-testing).

Can I make feature requests and report bugs and problems?

Sure, just submit an issue via the project tracker and I will see what I can do. Please note that I do not guarantee to implement anything soon and bugs and problems are more important to me than new features. If you need something implemented or fixed right away you can contact me via mail [email protected] to do contract work for you.