[CMake] Circular dependencies with Ninja, not with Unix Makefiles

[Ignacio Fernández Galván]

Well, in my real case this happens even with out-of-source builds, which 
also occurs in the test case if I change:

set (OUT ${CMAKE_BINARY_DIR}/ext)

in ext/CMakeLists.txt.

Sure, I can avoid it by renaming the file or target, but I thought this 
is something that could be avoided by either (a) generating unique ninja 
target names when possible, or (b) warning at the cmake stage that 
target names are duplicated.

 From my point of view as a CMake user, I see no reason why the 
combination of add_custom_command and add_custom_target should have a 
conflict.

On 05/05/17 14:22, Robert Maynard wrote:
> Hi,
>
> This is caused by doing an in source build and thereby causing the
> issue. The custom command is outputting a file called 'foo', which is
> also the chosen name of a target in the ninja file. All of this results
> in ninja conflating the two and causing a circular dependency.
>
> Your options are to not to in source builds, or to choose a different
> name for your custom target.
>
> On Thu, May 4, 2017 at 3:55 AM, Ignacio Fernández Galván via CMake
> <cmake at cmake.org <mailto:cmake at cmake.org>> wrote:
>
>     Hello,
>
>     I have a project where circular dependencies are generated when I
>     use the Ninja generator, but not the default Unix Makefiles. I could
>     create a minimal test like this:
>
>     $ cat CMakeLists.txt
>     add_subdirectory (ext ${PROJECT_BINARY_DIR}/ext)
>
>     $ cat ext/CMakeLists.txt
>     set (OUT ${CMAKE_CURRENT_LIST_DIR})
>
>     add_custom_command (OUTPUT ${OUT}/foo
>         COMMAND touch ${OUT}/foo
>         )
>
>     add_custom_target (foo ALL
>         DEPENDS ${OUT}/foo
>         )
>
>     add_custom_command (TARGET foo
>         POST_BUILD
>         COMMAND cp ${OUT}/foo ${CMAKE_BINARY_DIR}/foo
>         )
>
>     $ touch ext/bar
>
>     $ cmake -G Ninja .
>     [...]
>
>     $ ninja
>     ninja: warning: multiple rules generate ext/foo. builds involving
>     this target will not be correct; continuing anyway [-w dupbuild=warn]
>     ninja: error: dependency cycle: ext/foo -> ext/CMakeFiles/foo.util
>     -> ext/CMakeFiles/foo -> ext/foo
>
>     I used a freshly compiled CMake 3.8 and the latest binary ninja from
>     https://github.com/Kitware/ninja/releases
>     <https://github.com/Kitware/ninja/releases>.
>
>     Ignacio
>     --
>
>     Powered by www.kitware.com <http://www.kitware.com>
>
>     Please keep messages on-topic and check the CMake FAQ at:
>     http://www.cmake.org/Wiki/CMake_FAQ
>     <http://www.cmake.org/Wiki/CMake_FAQ>
>
>     Kitware offers various services to support the CMake community. For
>     more information on each offering, please visit:
>
>     CMake Support: http://cmake.org/cmake/help/support.html
>     <http://cmake.org/cmake/help/support.html>
>     CMake Consulting: http://cmake.org/cmake/help/consulting.html
>     <http://cmake.org/cmake/help/consulting.html>
>     CMake Training Courses: http://cmake.org/cmake/help/training.html
>     <http://cmake.org/cmake/help/training.html>
>
>     Visit other Kitware open-source projects at
>     http://www.kitware.com/opensource/opensource.html
>     <http://www.kitware.com/opensource/opensource.html>
>
>     Follow this link to subscribe/unsubscribe:
>     http://public.kitware.com/mailman/listinfo/cmake
>     <http://public.kitware.com/mailman/listinfo/cmake>
>
>