[Extensive records of build metrics](https://hydra.nixos.org/job/nix/master/coverage#tabs-charts), such as test coverage over time, are also available online.
> An example of some files, demonstrating much of what is described below
>
> ```
> src
> ├── libexpr
> │ ├── value/context.hh
> │ ├── value/context.cc
> │ │
> │ …
> └── tests
> │ ├── value/context.hh
> │ ├── value/context.cc
> │ │
> │ …
> │
> ├── unit-test-data
> │ ├── libstore
> │ │ ├── worker-protocol/content-address.bin
> │ │ …
> │ …
> …
> ```
The unit tests for each Nix library (`libnixexpr`, `libnixstore`, etc..) live inside a directory `src/${library_shortname}/tests` within the directory for the library (`src/${library_shortname}`).
The data is in `unit-test-data`, with one subdir per library, with the same name as where the code goes.
For example, `libnixstore` code is in `src/libstore`, and its test data is in `unit-test-data/libstore`.
The path to the `unit-test-data` directory is passed to the unit test executable with the environment variable `_NIX_TEST_UNIT_DATA`.
> **Note**
> Due to the way googletest works, downstream unit test executables will actually include and re-run upstream library tests.
> Therefore it is important that the same value for `_NIX_TEST_UNIT_DATA` be used with the tests for each library.
> That is why we have the test data nested within a single `unit-test-data` directory.
You can run the whole testsuite with `make check`, or the tests for a specific component with `make libfoo-tests_RUN`.
Finer-grained filtering is also possible using the [--gtest_filter](https://google.github.io/googletest/advanced.html#running-a-subset-of-the-tests) command-line option, or the `GTEST_FILTER` environment variable.
The test script will then be traced with `set -x` and the output displayed as it happens, regardless of whether the test succeeds or fails.
### Debugging failing functional tests
When a functional test fails, it usually does so somewhere in the middle of the script.
To figure out what's wrong, it is convenient to run the test regularly up to the failing `nix` command, and then run that command with a debugger like GDB.
For example, if the script looks like:
```bash
foo
nix blah blub
bar
```
edit it like so:
```diff
foo
-nix blah blub
+gdb --args nix blah blub
bar
```
Then, running the test with `./mk/debug-test.sh` will drop you into GDB once the script reaches that point:
```shell-session
$ ./mk/debug-test.sh tests/${testName}.sh
...
+ gdb blash blub
GNU gdb (GDB) 12.1
...
(gdb)
```
One can debug the Nix invocation in all the usual ways.
For example, enter `run` to start the Nix invocation.
Occasionally, Nix utilizes a technique called [Characterization Testing](https://en.wikipedia.org/wiki/Characterization_test) as part of the functional tests.
This technique is to include the exact output/behavior of a former version of Nix in a test in order to check that Nix continues to produce the same behavior going forward.
For example, this technique is used for the language tests, to check both the printed final value if evaluation was successful, and any errors and warnings encountered.
It is frequently useful to regenerate the expected output.
An interesting situation to document is the case when these tests are "overfitted".
The language tests are, again, an example of this.
The expected successful output of evaluation is supposed to be highly stable – we do not intend to make breaking changes to (the stable parts of) the Nix language.
However, the errors and warnings during evaluation (successful or not) are not stable in this way.
We are free to change how they are displayed at any time.
It may be surprising that we would test non-normative behavior like diagnostic outputs.
Diagnostic outputs are indeed not a stable interface, but they still are important to users.
By recording the expected output, the test suite guards against accidental changes, and ensure the *result* (not just the code that implements it) of the diagnostic code paths are under code review.
Regressions are caught, and improvements always show up in code review.
To ensure that characterization testing doesn't make it harder to intentionally change these interfaces, there always must be an easy way to regenerate the expected output, as we do with `_NIX_TEST_ACCEPT=1`.
The integration tests are defined in the Nix flake under the `hydraJobs.tests` attribute.
These tests include everything that needs to interact with external services or run Nix in a non-trivial distributed setup.
Because these tests are expensive and require more than what the standard github-actions setup provides, they only run on the master branch (on <https://hydra.nixos.org/jobset/nix/master>).
You can run them manually with `nix build .#hydraJobs.tests.{testName}` or `nix-build -A hydraJobs.tests.{testName}`
## Installer tests
After a one-time setup, the Nix repository's GitHub Actions continuous integration (CI) workflow can test the installer each time you push to a branch.
Creating a Cachix cache for your installer tests and adding its authorization token to GitHub enables [two installer-specific jobs in the CI workflow](https://github.com/NixOS/nix/blob/88a45d6149c0e304f6eb2efcc2d7a4d0d569f8af/.github/workflows/ci.yml#L50-L91):
- The `installer` job generates installers for the platforms below and uploads them to your Cachix cache:
-`x86_64-linux`
-`armv6l-linux`
-`armv7l-linux`
-`x86_64-darwin`
- The `installer_test` job (which runs on `ubuntu-latest` and `macos-latest`) will try to install Nix with the cached installer and run a trivial Nix command.
### One-time setup
1. Have a GitHub account with a fork of the [Nix repository](https://github.com/NixOS/nix).
2. At cachix.org:
- Create or log in to an account.
- Create a Cachix cache using the format `<github-username>-nix-install-tests`.
- Navigate to the new cache > Settings > Auth Tokens.
- Generate a new Cachix auth token and copy the generated value.
3. At github.com:
- Navigate to your Nix fork > Settings > Secrets > Actions > New repository secret.
- Name the secret `CACHIX_AUTH_TOKEN`.
- Paste the copied value of the Cachix cache auth token.
## Working on documentation
### Using the CI-generated installer for manual testing
After the CI run completes, you can check the output to extract the installer URL:
1. Click into the detailed view of the CI run.
2. Click into any `installer_test` run (the URL you're here to extract will be the same in all of them).
3. Click into the `Run cachix/install-nix-action@v...` step and click the detail triangle next to the first log line (it will also be `Run cachix/install-nix-action@v...`)
4. Copy the value of `install_url`
5. To generate an install command, plug this `install_url` and your GitHub username into this template:
```console
curl -L <install_url> | sh -s -- --tarball-url-prefix https://<github-username>-nix-install-tests.cachix.org/serve
```
<!-- #### Manually generating test installers
There's obviously a manual way to do this, and it's still the only way for
platforms that lack GA runners.
I did do this back in Fall 2020 (before the GA approach encouraged here). I'll
sketch what I recall in case it encourages someone to fill in detail, but: I
didn't know what I was doing at the time and had to fumble/ask around a lot--
so I don't want to uphold any of it as "right". It may have been dumb or
the _hard_ way from the getgo. Fundamentals may have changed since.
Here's the build command I used to do this on and for x86_64-darwin:
