Support a custom FLUTTER_STORAGE_BASE_URL as the source for Flutter package downloads

[oravecz]

We cannot install Flutter from a Git cache, nor can we download Flutter from the https://storage.googleapis.com/flutter_infra_release/releases location. Instead, we maintain our own versions of Flutter SDKs on an internal location which we set our FLUTTER_STORAGE_BASE_URL to. We also maintain our own version of the Flutter metadata files with our own checksum values in the same manner as Google.

Looking at the source code, I thought that FVM would respect the FLUTTER_STORAGE_BASE_URL environment variable, but it does not.

Is it feasible to make a modification to the source code to ensure that FLUTTER_STORAGE_BASE_URL is used as the source for Flutter packages to be put in the local cache?

Originally posted by @oravecz in #571

[digoreis]

@oravecz this issue was resolved in this PR #593 , correct ?

[charles0122]

@digoreis I think this is another solution, the method of setting FLUTTER_STORAGE_BASE_URL and FLUTTER_GIT_URL two variables is different, the former is generally strong, but requires fvm to adjust more code, the latter is more relevant, the modification is only download flutter specific sdk's repository

[oravecz]

Setting FLUTTER_GIT_URL was the approach we needed to be able to point fvm at our custom Flutter git repository.

If I recall correctly, this ticket was made earlier before I understood the difference between the two environment variables.

[charles0122]

If that's the case, and you found that the solution to this problem was to set FLUTTER_GIT_URL, then you can change the environment variable in the title and close this issue. If you still have other problems, you can open another one.

[luissantos]

I believe the FLUTTER_GIT_URL is sufficient for developers that only care about the flutter framework. But if like me you need to change the flutter engine. Then we need to be able to set FLUTTER_STORAGE_BASE_URL in order to get a new pre-compiled flutter engine. This gets complex when you need to compile flutter engines for multiple platform, therefore being able to access pre-compiled flutter sdks is an important need.

[leoafarias]

Update from the current triage pass: this is still the right tracking issue for archive-based installs.

FLUTTER_GIT_URL covers custom Flutter git repositories, but it does not cover the mirrored SDK archive/custom engine use case described here. The current implementation path is PR #1013, which adds archive-based install support with staging-directory recovery, checksum validation, and mirrored archive URL handling.

Leaving this open until #1013 passes CI, is reviewed, merged, and released.