[PATCH 1/2] GIT-VERSION-GEN: fix overriding version via environment

GIT-VERSION-GEN tries to derive the version that Git is being built from via multiple different sources in the following order:

  1. A file called "version" in the source tree's root directory, if it
     exists.

  2. The current commit in case Git is built from a Git repository.

  3. Otherwise, we use a fallback version stored in a variable which is
     bumped whenever a new Git version is getting tagged.

It used to be possible to override the version by overriding the `GIT_VERSION` Makefile variable (e.g. `make GIT_VERSION=foo`). This worked somewhat by chance, only: `GIT-VERSION-GEN` would write the actual Git version into `GIT-VERSION-FILE`, not the overridden value, but when including the file into our Makefile we would not override the `GIT_VERSION` variable because it has already been set by the user. And because our Makefile used the variable to propagate the version to our build tools instead of using `GIT-VERSION-FILE` the resulting build artifacts used the overridden version.

But that subtle mechanism broke with 4838deab65 (Makefile: refactor GIT-VERSION-GEN to be reusable, 2024-12-06) and subsequent commits because the version information is not propagated via the Makefile variable anymore, but instead via the files that `GIT-VERSION-GEN` started to write. And as the script never knew about the `GIT_VERSION` environment variable in the first place it uses one of the values listed above instead of the overridden value.

Fix this issue by making `GIT-VERSION-GEN` handle the case where `GIT_VERSION` has been set via the environment.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
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 GIT-VERSION-GEN | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)