Documentation/technical/unit-tests.txt

   1 = Unit Testing
   2
   3 In our current testing environment, we spend a significant amount of effort
   4 crafting end-to-end tests for error conditions that could easily be captured by
   5 unit tests (or we simply forgo some hard-to-setup and rare error conditions).
   6 Unit tests additionally provide stability to the codebase and can simplify
   7 debugging through isolation. Writing unit tests in pure C, rather than with our
   8 current shell/test-tool helper setup, simplifies test setup, simplifies passing
   9 data around (no shell-isms required), and reduces testing runtime by not
  10 spawning a separate process for every test invocation.
  11
  12 We believe that a large body of unit tests, living alongside the existing test
  13 suite, will improve code quality for the Git project.
  14
  15 == Definitions
  16
  17 For the purposes of this document, we'll use *test framework* to refer to
  18 projects that support writing test cases and running tests within the context
  19 of a single executable. *Test harness* will refer to projects that manage
  20 running multiple executables (each of which may contain multiple test cases) and
  21 aggregating their results.
  22
  23 In reality, these terms are not strictly defined, and many of the projects
  24 discussed below contain features from both categories.
  25
  26 For now, we will evaluate projects solely on their framework features. Since we
  27 are relying on having TAP output (see below), we can assume that any framework
  28 can be made to work with a harness that we can choose later.
  29
  30
  31 == Summary
  32
  33 We believe the best way forward is to implement a custom TAP framework for the
  34 Git project. We use a version of the framework originally proposed in
  35 https://lore.kernel.org/git/c902a166-98ce-afba-93f2-ea6027557176@gmail.com/[1].
  36
  37 See the <<framework-selection,Framework Selection>> section below for the
  38 rationale behind this decision.
  39
  40
  41 == Choosing a test harness
  42
  43 During upstream discussion, it was occasionally noted that `prove` provides many
  44 convenient features, such as scheduling slower tests first, or re-running
  45 previously failed tests.
  46
  47 While we already support the use of `prove` as a test harness for the shell
  48 tests, it is not strictly required. The t/Makefile allows running shell tests
  49 directly (though with interleaved output if parallelism is enabled). Git
  50 developers who wish to use `prove` as a more advanced harness can do so by
  51 setting DEFAULT_TEST_TARGET=prove in their config.mak.
  52
  53 We will follow a similar approach for unit tests: by default the test
  54 executables will be run directly from the t/Makefile, but `prove` can be
  55 configured with DEFAULT_UNIT_TEST_TARGET=prove.
  56
  57
  58 [[framework-selection]]
  59 == Framework selection
  60
  61 There are a variety of features we can use to rank the candidate frameworks, and
  62 those features have different priorities:
  63
  64 * Critical features: we probably won't consider a framework without these
  65 ** Can we legally / easily use the project?
  66 *** <<license,License>>
  67 *** <<vendorable-or-ubiquitous,Vendorable or ubiquitous>>
  68 *** <<maintainable-extensible,Maintainable / extensible>>
  69 *** <<major-platform-support,Major platform support>>
  70 ** Does the project support our bare-minimum needs?
  71 *** <<tap-support,TAP support>>
  72 *** <<diagnostic-output,Diagnostic output>>
  73 *** <<runtime-skippable-tests,Runtime-skippable tests>>
  74 * Nice-to-have features:
  75 ** <<parallel-execution,Parallel execution>>
  76 ** <<mock-support,Mock support>>
  77 ** <<signal-error-handling,Signal & error-handling>>
  78 * Tie-breaker stats
  79 ** <<project-kloc,Project KLOC>>
  80 ** <<adoption,Adoption>>
  81
  82 [[license]]
  83 === License
  84
  85 We must be able to legally use the framework in connection with Git. As Git is
  86 licensed only under GPLv2, we must eliminate any LGPLv3, GPLv3, or Apache 2.0
  87 projects.
  88
  89 [[vendorable-or-ubiquitous]]
  90 === Vendorable or ubiquitous
  91
  92 We want to avoid forcing Git developers to install new tools just to run unit
  93 tests. Any prospective frameworks and harnesses must either be vendorable
  94 (meaning, we can copy their source directly into Git's repository), or so
  95 ubiquitous that it is reasonable to expect that most developers will have the
  96 tools installed already.
  97
  98 [[maintainable-extensible]]
  99 === Maintainable / extensible
 100
 101 It is unlikely that any pre-existing project perfectly fits our needs, so any
 102 project we select will need to be actively maintained and open to accepting
 103 changes. Alternatively, assuming we are vendoring the source into our repo, it
 104 must be simple enough that Git developers can feel comfortable making changes as
 105 needed to our version.
 106
 107 In the comparison table below, "True" means that the framework seems to have
 108 active developers, that it is simple enough that Git developers can make changes
 109 to it, and that the project seems open to accepting external contributions (or
 110 that it is vendorable). "Partial" means that at least one of the above
 111 conditions holds.
 112
 113 [[major-platform-support]]
 114 === Major platform support
 115
 116 At a bare minimum, unit-testing must work on Linux, MacOS, and Windows.
 117
 118 In the comparison table below, "True" means that it works on all three major
 119 platforms with no issues. "Partial" means that there may be annoyances on one or
 120 more platforms, but it is still usable in principle.
 121
 122 [[tap-support]]
 123 === TAP support
 124
 125 The https://testanything.org/[Test Anything Protocol] is a text-based interface
 126 that allows tests to communicate with a test harness. It is already used by
 127 Git's integration test suite. Supporting TAP output is a mandatory feature for
 128 any prospective test framework.
 129
 130 In the comparison table below, "True" means this is natively supported.
 131 "Partial" means TAP output must be generated by post-processing the native
 132 output.
 133
 134 Frameworks that do not have at least Partial support will not be evaluated
 135 further.
 136
 137 [[diagnostic-output]]
 138 === Diagnostic output
 139
 140 When a test case fails, the framework must generate enough diagnostic output to
 141 help developers find the appropriate test case in source code in order to debug
 142 the failure.
 143
 144 [[runtime-skippable-tests]]
 145 === Runtime-skippable tests
 146
 147 Test authors may wish to skip certain test cases based on runtime circumstances,
 148 so the framework should support this.
 149
 150 [[parallel-execution]]
 151 === Parallel execution
 152
 153 Ideally, we will build up a significant collection of unit test cases, most
 154 likely split across multiple executables. It will be necessary to run these
 155 tests in parallel to enable fast develop-test-debug cycles.
 156
 157 In the comparison table below, "True" means that individual test cases within a
 158 single test executable can be run in parallel. We assume that executable-level
 159 parallelism can be handled by the test harness.
 160
 161 [[mock-support]]
 162 === Mock support
 163
 164 Unit test authors may wish to test code that interacts with objects that may be
 165 inconvenient to handle in a test (e.g. interacting with a network service).
 166 Mocking allows test authors to provide a fake implementation of these objects
 167 for more convenient tests.
 168
 169 [[signal-error-handling]]
 170 === Signal & error handling
 171
 172 The test framework should fail gracefully when test cases are themselves buggy
 173 or when they are interrupted by signals during runtime.
 174
 175 [[project-kloc]]
 176 === Project KLOC
 177
 178 The size of the project, in thousands of lines of code as measured by
 179 https://dwheeler.com/sloccount/[sloccount] (rounded up to the next multiple of
 180 1,000). As a tie-breaker, we probably prefer a project with fewer LOC.
 181
 182 [[adoption]]
 183 === Adoption
 184
 185 As a tie-breaker, we prefer a more widely-used project. We use the number of
 186 GitHub / GitLab stars to estimate this.
 187
 188
 189 === Comparison
 190
 191 :true: [lime-background]#True#
 192 :false: [red-background]#False#
 193 :partial: [yellow-background]#Partial#
 194
 195 :gpl: [lime-background]#GPL v2#
 196 :isc: [lime-background]#ISC#
 197 :mit: [lime-background]#MIT#
 198 :expat: [lime-background]#Expat#
 199 :lgpl: [lime-background]#LGPL v2.1#
 200
 201 :custom-impl: https://lore.kernel.org/git/c902a166-98ce-afba-93f2-ea6027557176@gmail.com/[Custom Git impl.]
 202 :greatest: https://github.com/silentbicycle/greatest[Greatest]
 203 :criterion: https://github.com/Snaipe/Criterion[Criterion]
 204 :c-tap: https://github.com/rra/c-tap-harness/[C TAP]
 205 :check: https://libcheck.github.io/check/[Check]
 206
 207 [format="csv",options="header",width="33%",subs="specialcharacters,attributes,quotes,macros"]
 208 |=====
 209 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>>"
 210 {custom-impl},{gpl},{true},{true},{true},{true},{true},{true},{false},{false},{false},1,0
 211 {greatest},{isc},{true},{partial},{true},{partial},{true},{true},{false},{false},{false},3,1400
 212 {criterion},{mit},{false},{partial},{true},{true},{true},{true},{true},{false},{true},19,1800
 213 {c-tap},{expat},{true},{partial},{partial},{true},{false},{true},{false},{false},{false},4,33
 214 {check},{lgpl},{false},{partial},{true},{true},{true},{false},{false},{false},{true},17,973
 215 |=====
 216
 217 === Additional framework candidates
 218
 219 Several suggested frameworks have been eliminated from consideration:
 220
 221 * Incompatible licenses:
 222 ** https://github.com/zorgnax/libtap[libtap] (LGPL v3)
 223 ** https://cmocka.org/[cmocka] (Apache 2.0)
 224 * Missing source: https://www.kindahl.net/mytap/doc/index.html[MyTap]
 225 * No TAP support:
 226 ** https://nemequ.github.io/munit/[µnit]
 227 ** https://github.com/google/cmockery[cmockery]
 228 ** https://github.com/lpabon/cmockery2[cmockery2]
 229 ** https://github.com/ThrowTheSwitch/Unity[Unity]
 230 ** https://github.com/siu/minunit[minunit]
 231 ** https://cunit.sourceforge.net/[CUnit]
 232
 233
 234 == Milestones
 235
 236 * Add useful tests of library-like code
 237 * Integrate with
 238   https://lore.kernel.org/git/20230502211454.1673000-1-calvinwan@google.com/[stdlib
 239   work]
 240 * Run alongside regular `make test` target