A common idiom in CMake-based build systems is to have custom commands
that generate files not listed explicitly as outputs so that these
files do not have to be newer than the inputs. The file modification
times of such "byproducts" are updated only when their content changes.
Then other build rules can depend on the byproducts explicitly so that
their dependents rebuild when the content of the original byproducts
really does change.
This "undeclared byproduct" approach is necessary for Makefile, VS, and
Xcode build tools because if a byproduct were listed as an output of a
rule then the rule would always rerun when the input is newer than the
byproduct but the byproduct may never be updated.
Ninja solves this problem by offering a 'restat' feature to check
whether an output was really modified after running a rule and tracking
the fact that it is up to date separately from its timestamp. However,
Ninja also stats all dependencies up front and will only restat files
that are listed as outputs of rules with the 'restat' option enabled.
Therefore an undeclared byproduct that does not exist at the start of
the build will be considered missing and the build will fail even if
other dependencies would cause the byproduct to be available before its
dependents build.
CMake works around this limitation by adding 'phony' build rules for
custom command dependencies in the build tree that do not have any
explicit specification of what produces them. This is not optimal
because it prevents Ninja from reporting an error when an input to a
rule really is missing. A better approach is to allow projects to
explicitly specify the byproducts of their custom commands so that no
phony rules are needed for them. In order to work with the non-Ninja
generators, the byproducts must be known separately from the outputs.
Add a new "BYPRODUCTS" option to the add_custom_command and
add_custom_target commands to specify byproducts explicitly. Teach the
Ninja generator to specify byproducts as outputs of the custom commands.
In the case of POST_BUILD, PRE_LINK, and PRE_BUILD events on targets
that link, the byproducts must be specified as outputs of the link rule
that runs the commands. Activate 'restat' for such rules so that Ninja
knows it needs to check the byproducts, but not for link rules that have
no byproducts.
If you think about adding a new testcase then here is a small checklist you
can run through to find a proper place for it. Go through the list from the
beginning and stop once you find something that matches your tests needs,
i.e. if you will test a module and only need the configure mode use the
instructions from section 2, not 3.
1. Your testcase can run in CMake script mode, i.e. "cmake -P something"
Put your test in Tests/CMakeTests/ directory as a .cmake.in file. It will be
put into the test binary directory by configure_file(... @ONLY) and run from
there. Use the AddCMakeTest() macro in Tests/CMakeTests/CMakeLists.txt to add
your test to the test runs.
2. Your test needs CMake to run in configure mode, but will not build anything
This includes tests that will build something using try_compile() and friends,
but nothing that expects add_executable(), add_library(), or add_test() to run.
If the test configures the project only once and it must succeed then put it
into the Tests/CMakeOnly/ directory. Create a subdirectory named like your
test and write the CMakeLists.txt you need into that subdirectory. Use the
add_CMakeOnly_test() macro from Tests/CMakeOnly/CMakeLists.txt to add your
test to the test runs.
If the test configures the project with multiple variations and verifies
success or failure each time then put it into the Tests/RunCMake/ directory.
Read the instructions in Tests/RunCMake/CMakeLists.txt to add a test.
3. If you are testing something from the Modules directory
Put your test in the Tests/Modules/ directory. Create a subdirectory there
named after your test. Use the ADD_TEST_MACRO macro from Tests/CMakeLists.txt
to add your test to the test run. If you have put your stuff in
Tests/Modules/Foo then you call it using ADD_TEST_MACRO(Module.Foo Foo).
4. You are doing other stuff.
Find a good place ;) In doubt mail to cmake-developers@cmake.org and ask for
advise.
e15a7075b5