[cmake-developers] ninja enforces explicit dependencies before order-only

[Peter Kümmel]

On 02.04.2013 15:19, Brad King wrote:
> Hi Peter,
>
> We've come across a case where the Makefile, VS, and Xcode generators
> work but Ninja does not::
>
>   cmake_minimum_required(VERSION 2.8.10)
>   project(DependSideEffect C)
>   add_library(A a.c)
>   add_custom_command(
>     TARGET A POST_BUILD
>     COMMAND cp ${CMAKE_CURRENT_SOURCE_DIR}/a.c a.txt
>     )
>   add_custom_command(
>     OUTPUT b.txt
>     DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/a.txt
>     COMMAND cp a.txt b.txt
>     )
>   add_custom_target(B ALL DEPENDS b.txt)
>   add_dependencies(B A)
>
> CMake's rule for target-level dependencies is that "A" must be up to
> date before the build system evaluates the rules of "B".
>
> Building with Ninja fails with::
>
>   ninja: error: 'a.txt', needed by 'b.txt', missing and no known rule to make it
>
> Relevant portions of build.ninja::
>
>   build libA.a: C_STATIC_LIBRARY_LINKER CMakeFiles/A.dir/a.c.o
>     POST_BUILD = cd ".../build" && cp .../a.c a.txt
>   ...
>   build b.txt: CUSTOM_COMMAND a.txt || libA.a
>     COMMAND = cd ".../build" && cp a.txt b.txt
>   ...
>
> The rule for "b.txt" has an order-only dependency on "libA.a" and
> an explicit dependency on "a.txt".  Ninja does not wait for the
> order-only dependencies to be up to date before complaining that
> the explicit dependency is missing.
>
> Can ninja be taught to wait for order-only dependencies to be up to
> date to see if they produce any of the explicit dependencies as
> a side effect?  Is there another way to do this in CMake's Ninja gen?
>
> Thanks,
> -Brad
>


In build.ninja is no rule for coping a.c to a.txt at all.
Seems support for add_custom_command(TARGET) is missing or broken.

Could you open a ticket and assign to me?

Peter