[CMake] Parallel builds do not work correctly when using "cmake -E copy" to copy files

[Brad King]

Alan W. Irwin wrote:
> I am struggling with understanding the recursive make system that
> CMake normally employs

CMake employs a 2-level make recursion system that is independent of the
directory structure.  The first level never builds anything...it just
evaluates target-level dependencies with phony targets.  That determines
the order in which targets must be built.  The second level is the
build.make for each target.  This is where file-level dependencies are
evaluated.

In your example the file1...fileN rules are showing up in target1's
build.make and target2's build.make but they should never be evaluated
in the second target.  They are pulled in through the additional_file
rule's dependencies on them (see below), but they should always be up to
date if target2 doesn't build until after target1 finishes.  Then only
the additional_file rule will be invoked.  However if there is no
dependency from target2->target1 then both build.make files may be built
simultaneously and you get race conditions causing the double evaluations.

CMake traces through the dependencies of custom commands in each target.
 When it is constructing target2 it doesn't know that target1 will also
provide rules for the files.  If you place the targets in different
directories it would not be able to make this extra connection, but then
the build would not work correctly unless you add the target-level
dependency.  Any further explanation here will just duplicate my
previous message so I'll stop.

-Brad