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diff --git a/Documentation/technical/unit-tests.txt b/Documentation/technical/unit-tests.txt new file mode 100644 index 0000000..206037f --- /dev/null +++ b/Documentation/technical/unit-tests.txt @@ -0,0 +1,240 @@ += Unit Testing + +In our current testing environment, we spend a significant amount of effort +crafting end-to-end tests for error conditions that could easily be captured by +unit tests (or we simply forgo some hard-to-setup and rare error conditions). +Unit tests additionally provide stability to the codebase and can simplify +debugging through isolation. Writing unit tests in pure C, rather than with our +current shell/test-tool helper setup, simplifies test setup, simplifies passing +data around (no shell-isms required), and reduces testing runtime by not +spawning a separate process for every test invocation. + +We believe that a large body of unit tests, living alongside the existing test +suite, will improve code quality for the Git project. + +== Definitions + +For the purposes of this document, we'll use *test framework* to refer to +projects that support writing test cases and running tests within the context +of a single executable. *Test harness* will refer to projects that manage +running multiple executables (each of which may contain multiple test cases) and +aggregating their results. + +In reality, these terms are not strictly defined, and many of the projects +discussed below contain features from both categories. + +For now, we will evaluate projects solely on their framework features. Since we +are relying on having TAP output (see below), we can assume that any framework +can be made to work with a harness that we can choose later. + + +== Summary + +We believe the best way forward is to implement a custom TAP framework for the +Git project. We use a version of the framework originally proposed in +https://lore.kernel.org/git/c902a166-98ce-afba-93f2-ea6027557176@gmail.com/[1]. + +See the <<framework-selection,Framework Selection>> section below for the +rationale behind this decision. + + +== Choosing a test harness + +During upstream discussion, it was occasionally noted that `prove` provides many +convenient features, such as scheduling slower tests first, or re-running +previously failed tests. + +While we already support the use of `prove` as a test harness for the shell +tests, it is not strictly required. The t/Makefile allows running shell tests +directly (though with interleaved output if parallelism is enabled). Git +developers who wish to use `prove` as a more advanced harness can do so by +setting DEFAULT_TEST_TARGET=prove in their config.mak. + +We will follow a similar approach for unit tests: by default the test +executables will be run directly from the t/Makefile, but `prove` can be +configured with DEFAULT_UNIT_TEST_TARGET=prove. + + +[[framework-selection]] +== Framework selection + +There are a variety of features we can use to rank the candidate frameworks, and +those features have different priorities: + +* Critical features: we probably won't consider a framework without these +** Can we legally / easily use the project? +*** <<license,License>> +*** <<vendorable-or-ubiquitous,Vendorable or ubiquitous>> +*** <<maintainable-extensible,Maintainable / extensible>> +*** <<major-platform-support,Major platform support>> +** Does the project support our bare-minimum needs? +*** <<tap-support,TAP support>> +*** <<diagnostic-output,Diagnostic output>> +*** <<runtime-skippable-tests,Runtime-skippable tests>> +* Nice-to-have features: +** <<parallel-execution,Parallel execution>> +** <<mock-support,Mock support>> +** <<signal-error-handling,Signal & error-handling>> +* Tie-breaker stats +** <<project-kloc,Project KLOC>> +** <<adoption,Adoption>> + +[[license]] +=== License + +We must be able to legally use the framework in connection with Git. As Git is +licensed only under GPLv2, we must eliminate any LGPLv3, GPLv3, or Apache 2.0 +projects. + +[[vendorable-or-ubiquitous]] +=== Vendorable or ubiquitous + +We want to avoid forcing Git developers to install new tools just to run unit +tests. Any prospective frameworks and harnesses must either be vendorable +(meaning, we can copy their source directly into Git's repository), or so +ubiquitous that it is reasonable to expect that most developers will have the +tools installed already. + +[[maintainable-extensible]] +=== Maintainable / extensible + +It is unlikely that any pre-existing project perfectly fits our needs, so any +project we select will need to be actively maintained and open to accepting +changes. Alternatively, assuming we are vendoring the source into our repo, it +must be simple enough that Git developers can feel comfortable making changes as +needed to our version. + +In the comparison table below, "True" means that the framework seems to have +active developers, that it is simple enough that Git developers can make changes +to it, and that the project seems open to accepting external contributions (or +that it is vendorable). "Partial" means that at least one of the above +conditions holds. + +[[major-platform-support]] +=== Major platform support + +At a bare minimum, unit-testing must work on Linux, MacOS, and Windows. + +In the comparison table below, "True" means that it works on all three major +platforms with no issues. "Partial" means that there may be annoyances on one or +more platforms, but it is still usable in principle. + +[[tap-support]] +=== TAP support + +The https://testanything.org/[Test Anything Protocol] is a text-based interface +that allows tests to communicate with a test harness. It is already used by +Git's integration test suite. Supporting TAP output is a mandatory feature for +any prospective test framework. + +In the comparison table below, "True" means this is natively supported. +"Partial" means TAP output must be generated by post-processing the native +output. + +Frameworks that do not have at least Partial support will not be evaluated +further. + +[[diagnostic-output]] +=== Diagnostic output + +When a test case fails, the framework must generate enough diagnostic output to +help developers find the appropriate test case in source code in order to debug +the failure. + +[[runtime-skippable-tests]] +=== Runtime-skippable tests + +Test authors may wish to skip certain test cases based on runtime circumstances, +so the framework should support this. + +[[parallel-execution]] +=== Parallel execution + +Ideally, we will build up a significant collection of unit test cases, most +likely split across multiple executables. It will be necessary to run these +tests in parallel to enable fast develop-test-debug cycles. + +In the comparison table below, "True" means that individual test cases within a +single test executable can be run in parallel. We assume that executable-level +parallelism can be handled by the test harness. + +[[mock-support]] +=== Mock support + +Unit test authors may wish to test code that interacts with objects that may be +inconvenient to handle in a test (e.g. interacting with a network service). +Mocking allows test authors to provide a fake implementation of these objects +for more convenient tests. + +[[signal-error-handling]] +=== Signal & error handling + +The test framework should fail gracefully when test cases are themselves buggy +or when they are interrupted by signals during runtime. + +[[project-kloc]] +=== Project KLOC + +The size of the project, in thousands of lines of code as measured by +https://dwheeler.com/sloccount/[sloccount] (rounded up to the next multiple of +1,000). As a tie-breaker, we probably prefer a project with fewer LOC. + +[[adoption]] +=== Adoption + +As a tie-breaker, we prefer a more widely-used project. We use the number of +GitHub / GitLab stars to estimate this. + + +=== Comparison + +:true: [lime-background]#True# +:false: [red-background]#False# +:partial: [yellow-background]#Partial# + +:gpl: [lime-background]#GPL v2# +:isc: [lime-background]#ISC# +:mit: [lime-background]#MIT# +:expat: [lime-background]#Expat# +:lgpl: [lime-background]#LGPL v2.1# + +:custom-impl: https://lore.kernel.org/git/c902a166-98ce-afba-93f2-ea6027557176@gmail.com/[Custom Git impl.] +:greatest: https://github.com/silentbicycle/greatest[Greatest] +:criterion: https://github.com/Snaipe/Criterion[Criterion] +:c-tap: https://github.com/rra/c-tap-harness/[C TAP] +:check: https://libcheck.github.io/check/[Check] + +[format="csv",options="header",width="33%",subs="specialcharacters,attributes,quotes,macros"] +|===== +Framework,"<<license,License>>","<<vendorable-or-ubiquitous,Vendorable or ubiquitous>>","<<maintainable-extensible,Maintainable / extensible>>","<<major-platform-support,Major platform support>>","<<tap-support,TAP support>>","<<diagnostic-output,Diagnostic output>>","<<runtime--skippable-tests,Runtime- skippable tests>>","<<parallel-execution,Parallel execution>>","<<mock-support,Mock support>>","<<signal-error-handling,Signal & error handling>>","<<project-kloc,Project KLOC>>","<<adoption,Adoption>>" +{custom-impl},{gpl},{true},{true},{true},{true},{true},{true},{false},{false},{false},1,0 +{greatest},{isc},{true},{partial},{true},{partial},{true},{true},{false},{false},{false},3,1400 +{criterion},{mit},{false},{partial},{true},{true},{true},{true},{true},{false},{true},19,1800 +{c-tap},{expat},{true},{partial},{partial},{true},{false},{true},{false},{false},{false},4,33 +{check},{lgpl},{false},{partial},{true},{true},{true},{false},{false},{false},{true},17,973 +|===== + +=== Additional framework candidates + +Several suggested frameworks have been eliminated from consideration: + +* Incompatible licenses: +** https://github.com/zorgnax/libtap[libtap] (LGPL v3) +** https://cmocka.org/[cmocka] (Apache 2.0) +* Missing source: https://www.kindahl.net/mytap/doc/index.html[MyTap] +* No TAP support: +** https://nemequ.github.io/munit/[µnit] +** https://github.com/google/cmockery[cmockery] +** https://github.com/lpabon/cmockery2[cmockery2] +** https://github.com/ThrowTheSwitch/Unity[Unity] +** https://github.com/siu/minunit[minunit] +** https://cunit.sourceforge.net/[CUnit] + + +== Milestones + +* Add useful tests of library-like code +* Integrate with + https://lore.kernel.org/git/20230502211454.1673000-1-calvinwan@google.com/[stdlib + work] +* Run alongside regular `make test` target |