Speed up build by avoiding gradle-api.jar generation

[raphiz]

Not sure yet if that's a good Idea but Gradle initially generates a gradle-api.jar which is then stored in GRADLE_HOME/.gradle/caches/8.7/generated-gradle-jars/gradle-api-8.7.jar (depending on the version, here 8.7). This can take 10-20 seconds depending on your machine.

[raphiz]

I played a bit around with this and generating the api jar makes quite an impact.
Building the initial version (with Gradle 8.14.3) takes ~15s on my machine. When reusing the generated artifact, time goes down to ~10s.

 pkgs.runCommand "gradle" {
          nativeBuildInputs = [
            pkgs.gradle
          ];
        } ''
          export GRADLE_USER_HOME=$PWD/gradle
          mkdir -p "$GRADLE_USER_HOME"

          touch settings.gradle.kts
          echo 'rootProject.name = "warmup"' >> settings.gradle.kts
          touch build.gradle.kts

          gradle --offline --console=plain --no-watch-fs --no-daemon tasks

          mkdir -p $out
          ver=${pkgs.gradle.version}
          cache="$GRADLE_USER_HOME/caches/$ver"

          cp -r $cache/generated-gradle-jars/ $out/
          rm $cache/generated-gradle-jars/*.lock
        '';

      usage =
        pkgs.runCommand "gradle" {
          nativeBuildInputs = [pkgs.gradle];
        } ''
          export GRADLE_USER_HOME=$PWD/gradle
          mkdir -p "$GRADLE_USER_HOME"
          ver=${pkgs.gradle.version}
          cache="$GRADLE_USER_HOME/caches/$ver"
          mkdir -p $cache
          cp -r ${gradleCache}/* $cache
          chmod -R a+w $cache/*

          touch settings.gradle.kts
          echo 'rootProject.name = "warmup"' >> settings.gradle.kts
          touch build.gradle.kts

          gradle --offline --console=plain --no-watch-fs --no-daemon tasks

          touch $out
        '';

While the gradle-api-x.y.z.jar seems to reproducible inside the nix sandbox that's not the case for additional artifacts that would speed things further up. I tried out some directories and had good results when also caching fileHashes and kotlin-dsl. However, both are not reproducible.

 pkgs.runCommand "gradle" {
          nativeBuildInputs = [
            pkgs.gradle
          ];
        } ''
          # same as above ...

          cp -r $cache/generated-gradle-jars/ $out/
          rm $cache/generated-gradle-jars/*.lock

          cp -r $cache/fileHashes/ $out/
          cp -r $cache/kotlin-dsl/ $out/
        '';

The problem seem to be the *.bin and *.lock files. Remove *.lock files seems safe, but removing *.bin files breaks things.

So it might be worth to attempt writing a wrapper that copies the gradle-api-x.y.z.jar into the gradle user home if it does not exist. However, it might only be safe inside the sandbox.

Storing the entire Gradle cache in a derivation might be a good thing to save some time when iterating quickly - but since it's not reproducible, it's not an option for me in production code.