[CMake] Addition to FindGit

[Michael Hertling]

On 07/12/2011 09:35 AM, Eric Noulard wrote:
> 2011/7/12 Thomas Petazzoni <thomas.petazzoni at free-electrons.com>:
>> Hello,
>>
>> Le Sun, 05 Jun 2011 11:02:00 +0200,
>> Quintus <sutniuq at gmx.net> a écrit :
>>
>>> I'm working on a git-versioned project that I'd like to display it's
>>> version number for development versions like this:
>>>
>>> 1.2.3-dev (commit abc1234 on devel, 12/4/10)
>>
>> For  similar need I did something like :
>>
>> execute_process(COMMAND sh ../getlocalversion OUTPUT_VARIABLE MYPROG_VERSION)
>> add_definitions(-DMYPROG_VERSION="${MYPROG_VERSION}")
>>
>> where getlocalversion is a shell script that does :
>>
>> printf "%s%s" $(git rev-parse --verify --short HEAD) $(if test `git
>> diff-index --name-only HEAD | wc -l` -ne 0 ; then echo "-dirty"; fi)
>>
>> However, I am not very happy with this, because the version is
>> determined at configuration time, and not at compile time. Therefore,
>> if I make some changes and commit them to the Git repository without
>> re-doing the CMake configuration step, the program version isn't
>> changed. Any idea on how to make sure that the version of the program
>> is updated at every compilation ?
> 
> If you want to execute **build time** scripts you should use
> 
> add_custom_command/add_custom_target
> 
> You'll have to make your script create a header that will be included
> in the concerned source file(s), something like:
> 
> add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/version.h
>                                    COMMAND  getlocalversion --output
> ${CMAKE_CURRENT_BINARY_DIR}/version.h)
> 
> If the script is smart enough not to re-create the file if the version
> did not change
> then it should work without too much overhead. If not this will trigger
> a undue recompilation.

For the custom command, use a CMake script which

(1) invokes the desired git command via EXECUTE_PROCESS()
(2) uses the results to convert a version.h.in template to
    the final version.h header via CONFIGURE_FILE(... @ONLY).

The CONFIGURE_FILE() command does not touch its output file if it
already exists and wouldn't change, so there won't be unneccesary
recompilations.

> if the version.h concerns a single target you could use:
> 
> add_custom_command(TARGET versionedTarget
>                                    PRE_BUILD
>                                    COMMAND  getlocalversion --output
> ${CMAKE_CURRENT_BINARY_DIR}/version.h)
> 
> this build of the "versionedTarget" should trigger the command.

This doesn't work for Makefiles if the version.h header is to be
incorporated in the versiondTarget because PRE_BUILD == PRE_LINK,
i.e. all of versiondTarget's compilations have already taken place
when the custom command is run and regenerates the version.h header.

Regards,

Michael