Cache Kotlin/Native library caches in iOS KMP PR checks

[matejsemancik]

Why

Debug framework links build a native cache for every library dependency in ~/.konan/kotlin-native-prebuilt-*/klib/cache. KMP projects run on self-hosted runners that start from a fresh VM, so every PR check rebuilds all of them inside the link task.

Measured on a project that renders Compose Multiplatform on iOS (126 library dependencies, linkDebugFrameworkIosSimulatorArm64 only):

Link	Time
Cold (fresh VM, as on CI today)	246 s locally, ~4m40s on CI
Warm (library caches present)	28 s
Native cache disabled (disableNativeCache)	107 s

The Kotlin docs recommend preserving ~/.konan between CI builds: Preserve downloaded and cached components between builds.

What

ios-kmp-selfhosted-test gets a kotlin_native_cache input, enabled by default, so every KMP project gets the cache once it bumps to the release with this change. It adds an actions/cache@v6 step before Build KMP Package:

• Path: ~/.konan/kotlin-native-prebuilt-*/klib/cache/*-user*, only the library caches built during the link (~340 MB for the measured project's simulator slice). The compiler distribution and LLVM are left out: downloading them takes ~16 s, and restoring ~1.3 GB more would cost about as much.
• Key: OS, CPU architecture and libs.versions.toml, with no restore-keys fallback. Library caches live in per-version folders, so a partial restore would carry outdated versions into every new cache.
• Saving: actions/cache saves at the end of the job, only on a key miss and only when the job succeeds.

Release builds don't use these caches, so nightly and release workflows are unaffected.

Impact on projects: no changes needed beyond the version bump. Each project stores one cache entry per libs.versions.toml state (a few hundred MB, depending on its dependencies) in its 10 GB repo cache budget. A project can opt out with kotlin_native_cache: false.

Sharing the cache across PRs

A cache saved in a PR run is only visible to later runs of that PR. To let every PR restore it, projects can also run their check on push to develop (push: branches: [ develop ], with the concurrency group using github.head_ref || github.ref). Change detection (dorny/paths-filter), the iOS Fastlane test lane and the Android Danger step all handle push events.

Testing

Planned on a consumer project's PR pointed at this branch (no input needed now that it's on by default): the first run should miss and save, and a second run after a UI-only change should restore the cache and link in ~30–40 s.