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diff --git a/tvix/nix-lang-test-suite/README.md b/tvix/nix-lang-test-suite/README.md new file mode 100644 index 000000000000..68f87f20f58f --- /dev/null +++ b/tvix/nix-lang-test-suite/README.md @@ -0,0 +1,140 @@ +# The Implementation Independent Nix Language Test Suite + +## Design Notes + +### Requirements + +- It should work with potentially any Nix implementation and with all serious + currently available ones (C++ Nix, hnix, Tvix, …). How much of it the + implementations pass, is of course an orthogonal question. +- It should be easy to add test cases, independent of any specific + implementation. +- It should be simple to ignore test cases and mark know failures + (similar to the notyetpassing mechanism in the Tvix test suite). + +### Test Case Types + +This is a summary of relevant kinds of test cases that can be found in the wild, +usually testing some kind of concrete implementation, but also doubling up as a +potential test case for _any_ Nix implementation. For the most part, this is the +`lang` test suite of C++ Nix which is also used by Tvix and hnix. + +- **parse** test cases: Parsing the given expression should either *succeed* or + *fail*. + + - C++ Nix doesn't have any expected output for the success cases while + `rnix-parser` checks them against its own textual AST representation. + - For the failure cases, `rnix-parser` and C++ Nix (as of recently) have + expected error messages/representations. + + Both error and failure cases probably are hard to implement against expected + output/error messages for a generic test suite. Even if standardized error + codes are implemented (see below), it is doubtful whether it'd be useful + to have a dedicated code for every kind of parse/lex failure. +- (strict) **eval** test cases: Evaluating the given expression should either + *fail* or *succeed* and yield a given result. + + - **eval-okay** (success) tests currently require three things: + + 1. Successful evaluation after deeply forcing and printing the evaluation + result (i.e. `nix-instantiate --eval --strict`) + 2. That the output matches an expected output exactly (string equality). + For this the output of `nix-instantiate(1)` is used, sometimes with + the addition of the `--xml --no-location` or `--json` flags. + 3. Optionally, stderr may need to be equal to an expected string exactly + which would test e.g. `builtins.trace` messages or deprecation warnings + (C++ Nix). + + This extra check is currently not supported by the Tvix test suite. + + - **eval-fail** tests require that the given expression fails to evaluate. C++ + Nix has recently started to also check the error messages via the stderr + mechanism described above. This is not supported by Tvix at the moment. +- _lazy_ eval test cases: This is currently only supported by the `nix_oracle` + test suite in Tvix which compares the evaluation result of expressions to the + output of `nix-instantiate(1)` without `--strict`. By relying on the fact + that the resulting value is not forced deeply before printing, it can be + observed whether certain expressions are thunked or not. + + This is somewhat fragile as permissible optimizations may prevent a thunk from + being created. However, this should not be an issue if the cases are chosen + carefully. Empirically, this test suite was useful for catching some instances + of overzealous evaluation early in development of Tvix. + +- **identity** test cases require that the given expression evaluates to a + value whose printed representation is the same (string equal to) the original + expression. Such test cases only exist in the Tvix test suite. + + Of course only a limited number of expression satisfy this, but it is + useful for testing `nix-instantiate(1)` style value printing. Consequently, + it is kind of on the edge of what you can call a language test. + +### Extra Dependencies of Some Test Cases + +- **Filesystem**: Some test cases `import` other files or use `builtins.readFile`, + `builtins.readDir` and friends. +- **Working and Home Directory**: Tests involving relative and home relative paths + need knowledge of the current and home directory to correctly interpret the output. + C++ Nix does a [search and replace on the test output for this purpose][cpp-nix-pwd-sed] +- **Nix Store**: Some tests add files to the store, either via path interpolation, + `builtins.toFile` or `builtins.derivation`. + + Additionally, it should be considered that Import-from-Derivation may be + interesting to test in the future. Currently, the Tvix and C++ Nix test + suites all pass with Import-from-Derivation disabled, i.e. a dummy store + implementation is enough. + + Note that the absence of a store dependency ideally also influences the test + execution: In Tvix, for example, store independent tests can be executed + with a store backend that immediately errors out, verifying that the test + is, in fact, store independent. +- **Environment**: The C++ Nix test suite sets a single environment variable, + `TEST_VAR=foo`. Additionally, `NIX_PATH` and `HOME` are sometimes set (the + latter is probably not a great idea, since it is not terribly reliable). +- **Nix Path**: A predetermined Nix Path (via `NIX_PATH` and/or command line + arguments) needs to be set for some test cases. +- **Nix flags**: Some tests need to have extra flags passed to `nix-instantiate(1)` + in order to work. This is done using a `.flags` file + +### Expected Output Considerations + +#### Success + +The expected output of `eval-okay` test cases (which are the majority of test +cases) uses the standard strict output of `nix-instantiate(1)` in most cases +which is nice to read and easy to work with. However, some more obscure aspects +of this output inevitably leak into the test cases, namely the cycle detection +and printing and (in the case of Tvix) the printing of thunks. Unfortunately, +the output has been changed after Nix 2.3, bringing it closer to the output of +`nix eval`, but in an inconsistent manner (e.g. `<CYCLE>` was changed to +`«repeated»`, but `<LAMBDA>` remained). As a consequence, it is not always +possible to write C++ Nix version independent test cases. + +It is unclear whether a satisfying solution (for a common test suite) can +be achieved here as it has become a somewhat contentious [issue whether +or not nix-instantiate should have a stable output](cpp-nix-attr-elision-printing-pr). + +A solution may be to use the XML output, specifically the `--xml --no-location` +flags to `nix-instantiate(1)` for some of these instances. As it (hopefully) +corresponds to `builtins.toXML`, there should be a greater incentive to keep it +stable. It does support (only via `nix-instantiate(1)`, though) printing +unevaluated thunks, but has no kind of cycle detection (which is fair enough for +its intended purpose). + +#### Failure + +C++ Nix has recently (some time after Nix 2.3, probably much later actually) +started checking error messages via expected stderr output. This naturally +won't work for a implementation independent language test suite: + +- It is fine to have differing phrasing for error messages or localize them. +- Printed error positions and stack traces may be slightly different depending + on implementation internals. +- Formatting will almost certainly differ. + +Consequently, just checking for failure when running the test suite should be +an option. Long term, it may be interesting to have standardized error codes +and portable error code reporting. + +[cpp-nix-pwd-sed]: https://github.com/NixOS/nix/blob/2cb9c7c68102193e7d34fabe6102474fc7f98010/tests/functional/lang.sh#L109 +[cpp-nix-attr-elision-printing-pr]: https://github.com/NixOS/nix/pull/9606 |