cmake 3.15 adding JOB_POOL to add_custom_command SOMETIMES - cmake

Question

For users that are using cmake 3.15 or later and are also using Ninja as a generator, I want to set the new JOB_POOL argument to some large add_custom_command() blocks. For other users, I want to keep my add_custom_command() the same (no JOB_POOL).
In earlier steps, I check the version and the generator and set ${JOB_POOLS} and I also set a variable such that users who should use a pool will see (something like):
For historical reasons, I leave this here, although #Tsyvarev
points out that this is the source of my problem!
The double-quotes are NOT wanted here!
set(USE_POOL "JOB_POOL pool_A")
Users that are not using a pool will not have that variable set.
Now how to leverage that variable in my custom command...?
1.) Generator expressions don't work, just including the text with the previous line...
add_custom_command(
...
$<USE_POOL>
)
2.) I can't seem to simply place the variable in the command, again just including the variable contents on the previous line. For example, when ${JOB_POOL} is set to the string "JOB_POOL pool_A", this code...
For historical reasons, I leave this here, although #Tsyvarev
points out that this is the source of my problem!
Don't use a STRING! No double-quotes!
add_custom_command(
OUTPUT foo
DEPENDS bar
# Comment line here...
${USE_POOL}
COMMAND
${CMAKE_COMMAND} -E ...
)
gives this error...
ninja: error: '../path/to/src/dir/JOB_POOL pool_A', needed by 'path/to/src/dir/foo', missing and no known rule to make it
It simply considers the ${JOB_POOL} string to be another dependency!
3.) I can't use the "APPEND" feature of add_custom_command(). It's just ignored...
if (${USE_POOL})
add_custom_command(
...
APPEND
JOB_POOL pool_A
)
endif()
The only thing that seems to work is to put an "if" around my
entire command, which offends my sensibility since I don't like to duplicate so much code...
if(${USE_POOL})
add_custom_command(
...many lines...
JOB_POOL pool_A
)
else()
add_custom_command(
...many lines...
)
endif()
Do you have a better idea...?
Here's a standalone example for #tsyvarev:
cmake_minimum_required(VERSION 3.15)
project foo
set_property(GLOBAL PROPERTY JOB_POOLS pool_A=2)
# For historical reasons, I leave this here, although #Tsyvarev
# points out that this is the source of my problem!
# Don't use a STRING! No double-quotes!
set(USE_POOL "JOB_POOL pool_A")
add_custom_command(
OUTPUT foo.out
DEPENDS foo.in
${USE_POOL}
COMMAND
${CMAKE_COMMAND} -E copy foo.in foo.out
COMMENT "Converting foo.in -> foo.out"
VERBATIM
)
add_custom_target(foo-out
DEPENDS foo.out
)
% cmake -GNinja .
% ninja foo-out
ninja: error: 'JOB_POOL pool_A', needed by 'foo.out', missing and no known rule to make it
It considers the string to be a dependency... If I move the USE_POOL to after the comment, it considers it part of the comment... If I move it to after the command, it considers it part of the command...

Answer 1

Your JOB_POOL option serves for the user's choice. You may create another variable which contains sequence of related parameters for add_custom_command:
if(JOB_POOL)
set(JOB_POOL_PARAMS JOB_POOL pool_A) # Will add 'JOB_POOL pool_A' sequence of params
else()
set(JOB_POOL_PARAMS) # Will add nothing
endif()
Then use new variable directly in add_custom_command call:
add_custom_command(
...
${JOB_POOL_PARAMS} # Will expand to additional parameters when needed
)