Move tests to separate directories, and document

Today, with the tests inside a `tests` intermingled with the
corresponding library's source code, we have a few problems:

- We have to be careful that wildcards don't end up with tests being
  built as part of Nix proper, or test headers being installed as part
  of Nix proper.

- Tests in libraries but not executables is not right:

  - It means each executable runs the previous unit tests again, because
    it needs the libraries.

  - It doesn't work right on Windows, which doesn't want you to load a
    DLL just for the side global variable . It could be made to work
    with the dlopen equivalent, but that's gross!

This reorg solves these problems.

There is a remaining problem which is that sibbling headers (like
`hash.hh` the test header vs `hash.hh` the main `libnixutil` header) end
up shadowing each other. This PR doesn't solve that. That is left as
future work for a future PR.

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

.gitignore

@@ -43,14 +43,14 @@ perl/Makefile.config
 /src/libexpr/parser-tab.hh
 /src/libexpr/parser-tab.output
 /src/libexpr/nix.tbl
-/src/libexpr/tests/libnixexpr-tests
+/tests/unit/libexpr/libnixexpr-tests
 
 # /src/libstore/
 *.gen.*
-/src/libstore/tests/libnixstore-tests
+/tests/unit/libstore/libnixstore-tests
 
 # /src/libutil/
-/src/libutil/tests/libnixutil-tests
+/tests/unit/libutil/libnixutil-tests
 
 /src/nix/nix
 

Makefile

@@ -25,11 +25,13 @@ makefiles = \
 endif
 
 ifeq ($(ENABLE_BUILD)_$(ENABLE_TESTS), yes_yes)
-UNIT_TEST_ENV = _NIX_TEST_UNIT_DATA=unit-test-data
 makefiles += \
-  src/libutil/tests/local.mk \
-  src/libstore/tests/local.mk \
-  src/libexpr/tests/local.mk
+  tests/unit/libutil/local.mk \
+  tests/unit/libutil-support/local.mk \
+  tests/unit/libstore/local.mk \
+  tests/unit/libstore-support/local.mk \
+  tests/unit/libexpr/local.mk \
+  tests/unit/libexpr-support/local.mk
 endif
 
 ifeq ($(ENABLE_TESTS), yes)

doc/internal-api/doxygen.cfg.in

@@ -39,17 +39,21 @@ INPUT                  = \
   src/libcmd \
   src/libexpr \
   src/libexpr/flake \
-  src/libexpr/tests \
-  src/libexpr/tests/value \
+  tests/unit/libexpr \
+  tests/unit/libexpr/value \
+  tests/unit/libexpr/test \
+  tests/unit/libexpr/test/value \
   src/libexpr/value \
   src/libfetchers \
   src/libmain \
   src/libstore \
   src/libstore/build \
   src/libstore/builtins \
-  src/libstore/tests \
+  tests/unit/libstore \
+  tests/unit/libstore/test \
   src/libutil \
-  src/libutil/tests \
+  tests/unit/libutil \
+  tests/unit/libutil/test \
   src/nix \
   src/nix-env \
   src/nix-store

doc/manual/src/contributing/testing.md

@@ -20,6 +20,7 @@ The unit tests are defined using the [googletest] and [rapidcheck] frameworks.
 
 [googletest]: https://google.github.io/googletest/
 [rapidcheck]: https://github.com/emil-e/rapidcheck
+[property testing]: https://en.wikipedia.org/wiki/Property_testing
 
 ### Source and header layout
 
@@ -28,34 +29,50 @@ The unit tests are defined using the [googletest] and [rapidcheck] frameworks.
 > ```
 > src
 > ├── libexpr
+> │   ├── local.mk
 > │   ├── value/context.hh
 > │   ├── value/context.cc
-> │   │
 > │   …
->     └── tests
-> │       ├── value/context.hh
-> │       ├── value/context.cc
-> │       │
-> │       …
 > │
-> ├── unit-test-data
-> │   ├── libstore
-> │   │   ├── worker-protocol/content-address.bin
-> │   │   …
+> ├── tests
+> │   │
 > │   …
+> │   └── unit
+> │       ├── libutil
+> │       │   ├── local.mk
+> │       │   …
+> │       │   └── data
+> │       │       ├── git/tree.txt
+> │       │       …
+> │       │
+> │       ├── libexpr-support
+> │       │   ├── local.mk
+> │       │   └── tests
+> │       │       ├── value/context.hh
+> │       │       ├── value/context.cc
+> │       │       …
+> │       │
+> │       ├── libexpr
+> │       …   ├── local.mk
+> │           ├── value/context.cc
+> │           …
 > …
 > ```
 
-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 tests for each Nix library (`libnixexpr`, `libnixstore`, etc..) live inside a directory `tests/unit/${library_name_without-nix}`.
+Given a interface (header) and implementation pair in the original library, say, `src/libexpr/value/context.{hh,cc}`, we write tests for it in `tests/unit/libexpr/tests/value/context.cc`, and (possibly) declare/define additional interfaces for testing purposes in `tests/unit/libexpr-support/tests/value/context.{hh,cc}`.
 
-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`.
+Data for unit tests is stored in a `data` subdir of the directory for each unit test executable.
+For example, `libnixstore` code is in `src/libstore`, and its test data is in `tests/unit/libstore/data`.
+The path to the `tests/unit/data` directory is passed to the unit test executable with the environment variable `_NIX_TEST_UNIT_DATA`.
+Note that each executable only gets the data for its tests.
 
-> **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.
+The unit test libraries are in `tests/unit/${library_name_without-nix}-lib`.
+All headers are in a `tests` subdirectory so they are included with `#include "tests/"`.
+
+The use of all these separate directories for the unit tests might seem inconvenient, as for example the tests are not "right next to" the part of the code they are testing.
+But organizing the tests this way has one big benefit:
+there is no risk of any build-system wildcards for the library accidentally picking up test code that should not built and installed as part of the library.
 
 ### Running tests
 
@@ -69,7 +86,7 @@ See [functional characterisation testing](#characterisation-testing-functional)
 Like with the functional characterisation, `_NIX_TEST_ACCEPT=1` is also used.
 For example:
 ```shell-session
-$ _NIX_TEST_ACCEPT=1 make libstore-tests-exe_RUN
+$ _NIX_TEST_ACCEPT=1 make libstore-tests_RUN
 ...
 [  SKIPPED ] WorkerProtoTest.string_read
 [  SKIPPED ] WorkerProtoTest.string_write
@@ -80,6 +97,18 @@ $ _NIX_TEST_ACCEPT=1 make libstore-tests-exe_RUN
 will regenerate the "golden master" expected result for the `libnixstore` characterisation tests.
 The characterisation tests will mark themselves "skipped" since they regenerated the expected result instead of actually testing anything.
 
+### Unit test support libraries
+
+There are headers and code which are not just used to test the library in question, but also downstream libraries.
+For example, we do [property testing] with the [rapidcheck] library.
+This requires writing `Arbitrary` "instances", which are used to describe how to generate values of a given type for the sake of running property tests.
+Because types contain other types, `Arbitrary` "instances" for some type are not just useful for testing that type, but also any other type that contains it.
+Downstream types frequently contain upstream types, so it is very important that we share arbitrary instances so that downstream libraries' property tests can also use them.
+
+It is important that these testing libraries don't contain any actual tests themselves.
+On some platforms they would be run as part of every test executable that uses them, which is redundant.
+On other platforms they wouldn't be run at all.
+
 ## Functional tests
 
 The functional tests reside under the `tests/functional` directory and are listed in `tests/functional/local.mk`.

flake.nix

@@ -93,7 +93,7 @@
           ./misc
           ./precompiled-headers.h
           ./src
-          ./unit-test-data
+          ./tests/unit
           ./COPYING
           ./scripts/local.mk
           functionalTestFiles

mk/common-test.sh

@@ -1,7 +1,7 @@
 # Remove overall test dir (at most one of the two should match) and
 # remove file extension.
 test_name=$(echo -n "$test" | sed \
-    -e "s|^unit-test-data/||" \
+    -e "s|^tests/unit/[^/]*/data/||" \
     -e "s|^tests/functional/||" \
     -e "s|\.sh$||" \
     )

mk/programs.mk

@@ -87,6 +87,6 @@ define build-program
   # Phony target to run this program (typically as a dependency of 'check').
   .PHONY: $(1)_RUN
   $(1)_RUN: $$($(1)_PATH)
-	$(trace-test) $$(UNIT_TEST_ENV) $$($(1)_PATH)
+	$(trace-test) $$($(1)_ENV) $$($(1)_PATH)
 
 endef

tests/unit/libexpr-support/local.mk

@@ -0,0 +1,23 @@
+libraries += libexpr-test-support
+
+libexpr-test-support_NAME = libnixexpr-test-support
+
+libexpr-test-support_DIR := $(d)
+
+ifeq ($(INSTALL_UNIT_TESTS), yes)
+  libexpr-test-support_INSTALL_DIR := $(checklibdir)
+else
+  libexpr-test-support_INSTALL_DIR :=
+endif
+
+libexpr-test-support_SOURCES := \
+    $(wildcard $(d)/tests/*.cc) \
+    $(wildcard $(d)/tests/value/*.cc)
+
+libexpr-test-support_CXXFLAGS += $(libexpr-tests_EXTRA_INCLUDES)
+
+libexpr-test-support_LIBS = \
+    libstore-test-support libutil-test-support \
+    libexpr libstore libutil
+
+libexpr-test-support_LDFLAGS := -lrapidcheck

tests/unit/libexpr-support/tests/value/context.cc

@@ -0,0 +1,30 @@
+#include <rapidcheck.h>
+
+#include "tests/path.hh"
+#include "tests/value/context.hh"
+
+namespace rc {
+using namespace nix;
+
+Gen<NixStringContextElem::DrvDeep> Arbitrary<NixStringContextElem::DrvDeep>::arbitrary()
+{
+    return gen::just(NixStringContextElem::DrvDeep {
+        .drvPath = *gen::arbitrary<StorePath>(),
+    });
+}
+
+Gen<NixStringContextElem> Arbitrary<NixStringContextElem>::arbitrary()
+{
+    switch (*gen::inRange<uint8_t>(0, std::variant_size_v<NixStringContextElem::Raw>)) {
+    case 0:
+        return gen::just<NixStringContextElem>(*gen::arbitrary<NixStringContextElem::Opaque>());
+    case 1:
+        return gen::just<NixStringContextElem>(*gen::arbitrary<NixStringContextElem::DrvDeep>());
+    case 2:
+        return gen::just<NixStringContextElem>(*gen::arbitrary<NixStringContextElem::Built>());
+    default:
+        assert(false);
+    }
+}
+
+}

src/libexpr/tests/value/context.hh → tests/unit/libexpr-support/tests/value/context.hh

@@ -3,7 +3,7 @@
 
 #include <rapidcheck/gen/Arbitrary.h>
 
-#include <value/context.hh>
+#include "value/context.hh"
 
 namespace rc {
 using namespace nix;

tests/unit/libexpr/local.mk

@@ -0,0 +1,36 @@
+check: libexpr-tests_RUN
+
+programs += libexpr-tests
+
+libexpr-tests_NAME := libnixexpr-tests
+
+libexpr-tests_ENV := _NIX_TEST_UNIT_DATA=$(d)/data
+
+libexpr-tests_DIR := $(d)
+
+ifeq ($(INSTALL_UNIT_TESTS), yes)
+  libexpr-tests_INSTALL_DIR := $(checkbindir)
+else
+  libexpr-tests_INSTALL_DIR :=
+endif
+
+libexpr-tests_SOURCES := \
+    $(wildcard $(d)/*.cc) \
+    $(wildcard $(d)/value/*.cc)
+
+libexpr-tests_EXTRA_INCLUDES = \
+    -I tests/unit/libexpr-support \
+    -I tests/unit/libstore-support \
+    -I tests/unit/libutil-support \
+    -I src/libexpr \
+    -I src/libfetchers \
+    -I src/libstore \
+    -I src/libutil
+
+libexpr-tests_CXXFLAGS += $(libexpr-tests_EXTRA_INCLUDES)
+
+libexpr-tests_LIBS = \
+    libexpr-test-support libstore-test-support libutils-test-support \
+    libexpr libfetchers libstore libutil
+
+libexpr-tests_LDFLAGS := -lrapidcheck $(GTEST_LIBS) -lgmock