Installing no longer existing files - cmake

Question

I have a list of various files that I want to install, by executing a resulting INSTALL project. This works, but sometimes files are no longer available, when the install operation is executed. Environment is a build server, where files get moved around -> this causes the build to be faulty.
An easy way to fix this behaviour is in the OPTIONAL parameter of the install command. So my question is: Is there a way to output warnings at runtime, if the install command failed?
Here is my code, to recreate the issue. In the src directory there are files "1.txt" and "2.txt". I build the cmakelists.txt and then delete "2.txt". After that, I execute the INSTALL solution I got.
cmake_minimum_required(VERSION 3.0)
project(documentation)
set(SOURCEDOCUMENTATION "D:/projects/side_master/src/documentation/src")
set(TARGETDOCUMENTATION "D:/projects/side_master/src/documentation/tgt")
file (GLOB files_to_install "${SOURCEDOCUMENTATION}/*")
foreach(file_to_install ${files_to_install})
install(FILES ${file_to_install} DESTINATION ${TARGETDOCUMENTATION} OPTIONAL)
endforeach()
The error (without the OPTIONAL parameter):
-- Install configuration: "Debug"
-- Installing: D:/projects/side_master/src/documentation/tgt/1.txt
CMake Error at cmake_install.cmake:56 (file):
file INSTALL cannot find
"D:/projects/side_master/src/documentation/src/2.txt".
What I want to get is a generated message, like this:
File "D:/projects/side_master/src/documentation/src/2.txt" not found.

Answer 1

You could make use of the install(SCRIPT ...) or install(CODE ...) signatures of the CMake install command, to run custom installation steps specific to your use case. The custom steps here would check for the existence of the files (using CMake's EXISTS logic) to be installed, and print a warning message if the file does not exist. The custom installation command could look something like this:
install(CODE "
if(NOT EXISTS ${file_to_install})
message(WARNING \"File ${file_to_install} not found during installation.\")
endif()
")