Added .git/index to the CMAKE_CONFIGURE_DEPENDS

[daschuer]

… to update the version if the git branch or revision changes.

Fixes https://bugs.launchpad.net/mixxx/+bug/1861548

[Holzhaus]

I'm not sure that .git/index is the correct file. It also includes the staging area, so that changes to staged file would also trigger a version change:

$ sha256sum .git/index
2eca1e2be4ab51c3fe1a1e3fb96fd1261b1796dbec8caaf29a7e73ba2362fbd7  .git/index
$ touch foo
$ git add foo
$ sha256sum .git/index
db2c251b4e78a8d5a4d1629ee5a011c7ba5a933ccdc7ddf3c9cccd681de4dd91  .git/index

Do we want that behaviour?

If it's just about branches and commits, I think we need to read .git/HEAD and then use both .git/HEAD and the referenced file:

# When on a branch
$ cat .git/HEAD
ref: refs/heads/dont-stop-track-on-unconfigured-passthru
$ cat .git/refs/heads/dont-stop-track-on-unconfigured-passthru
bed4a876d8f37ce11896b3b610f1a0acbe2ff0b9

# In "detached HEAD" mode
$ cat .git/HEAD
67f7cc05d564647d6534ef21ec60e5fb6fc90211

[daschuer]

./git/index is just the easy solution that works.
Reading one file and check on other is too much effort for just to save a second build time after messing with the index for me.

However this is actually not a full solution, because modified files are not considered to create the + prefix. Do you have an idea how to fix that?

[Holzhaus]

The problem is that the git commands are run at configure time, not build time. I could rewrite it a bit. By the way, is there a reason we don't use git describe --always --dirty?

[daschuer]

No, except that changing this will disturb our version counting.

[Holzhaus]

By the way, you can work around it by using:

$ cmake .. && cmake --build .

The reconfiguration should be quick because most of the variables are cached.

[daschuer]

Yes, right. That is also an argument to just merge this. Because compared to configure every time, it does it only if the git index changes.
.. anoying wrong revision bug solved and we can proceed.

[daschuer]

@Holzhaus what do you think? Merge?

[Holzhaus]

I'm sure there a better way by adding a custom command target, but as a temporary workaround we can merge this.

[daschuer]

Ok, thanks. Than please merge. :-)