When working with Surefire, it is necessary to understand a few things:
The plugin is built as part of a multi-module plugin. You need to check out the complete module and build from there:
git clone https://git-wip-us.apache.org/repos/asf/maven-surefire.git
When reporting an issue, it is immensely useful to create a small sample project that demonstrates the problem. Surefire already contains a large number of such projects, and they can be found at surefire-integration-tests/src/test/resources/. Typically you can check out one of the pre-existing projects and run it like this:
cd surefire-integration-tests/src/test/resources/failsafe-buildfail mvn -Dsurefire.version=2.12 verify
Sometimes it's appropriate to attach a remote debugger to the Surefire fork to try to determine what is really going on. If you checkout and build trunk, you'd usually do something like this:
mvn -Dmaven.surefire.debug=true install
Load the source in your IDE, set a breakpoint at the start of ForkedBooter#main and attach a debugger to port 5005.
The forked Surefire process uses standard input and output to communicate back to the source. Sometimes when tracking troubles it can be helpful to look at just the output of the fork.
This can be done by running:
mvn -e -X install | grep Forking
If you copy the command part of the output, you should be able to re-run the command by just pasting it on the command line (you might have to do only the bits after &&).
You can now paste this command on the command line and capture the output of the fork. This may help you determine if the problem is in the forked end or the receiving end.
When investigating a particular class, you probably want to grep the output for the class name. Additionally the booter code (first field) can be seen at https://git-wip-us.apache.org/repos/asf?p=maven-surefire.git;a=blob;f=surefire-api/src/main/java/org/apache/maven/surefire/booter/ForkingRunListener.java
The second field in the output (after the booter code) is the logical channel number, where different threads in the fork should come in different channels.
All patches to Surefire must contain test coverage, either as an integration test or a unit test. All new features (changed/added plugin options) must be covered by an end-to-end integration test.
There are numerous other integration tests that all operate on small sample projects in surefire-integration-tests/src/test/resources.
Example integration tests are Surefire141PluggableProvidersIT and the corresponding surefire-integration-tests/src/test/resources/surefire-141-pluggableproviders.
Some methods/classes reveal more about the basic working of a piece of code than others. The following classes/methods are a "reading list" for getting quickly acquainted with the code:
AbstractSurefireMojo#executeAllProviders ForkStarter#fork ForkedBooter#main
The surefire booter is capable of booting all the way back to jdk1.5. Specifically this means surefire-api, surefire-booter, common-junit3 and surefire-junit3 are source/target 1.5. The plugin and several providers are 1.5.
Classes in the SUT (Subject Under Test) override any classes within the Surefire providers. This means providers using any third party dependencies (other than the test framework itself) should shade these classes to a different package.
The surefire-providers module contains common-junitXX modules. These modules depend on the XX version of JUnit and can access the JUnit APIs at the correct JUnit version level. Unit tests can also be written that will run with the correct JUnit version. At build time, all of the relevant parts of these "common" modules are just shaded into the provider jar files.
"Shadefire" is the first module to be run in the Surefire build. This creates as shaded version of the JUnit provider, and this provider is thereafter used to build Surefire itself (as of Surefire 2.8). This is because the SUT overrides the provider, and the Shadefire provider has been relocated to avoid this overriding when Surefire is building itself.