Bazel support for multiple emscripten caches

[DoDoENT]

I'm experimenting with Bazel with the goal of replacing my CMake+Conan setup with Bazel. I'm using LTO in release builds and in debug and fastbuild I'm not using LTO. However, when using Bazel, this doesn't work.

Namely, the default configuration does not ship with LTO-enabled cache, so when I enable LTO, I get the error Exception: FROZEN_CACHE is set, but cache file is missing: "sysroot/lib/wasm32-emscripten/thinlto/crtbegin.o".

Then, after reading the instructions for enabling LTO, I added the declaration for cache as given in the instructions and my LTO build still didn't work.

After a couple of hours of trial and error, I discovered that the documentation does not list all minimum targets that need to be built in cache - for my project to work, I had to add at least these targets:

emscripten_cache.configuration(flags = ["--lto=thin"])
emscripten_cache.targets(targets = [
    "crtbegin",
    "libal",
    "libc-mt",
    "libc-mt-debug",
    "libc",
    "libc++-mt-noexcept",
    "libc++-noexcept",
    "libc++-debug-mt-noexcept",
    "libc++abi-mt-noexcept",
    "libc++abi-debug-mt-noexcept",
    "libc++abi-debug-noexcept",
    "libcompiler_rt-mt",
    "libcompiler_rt",
    "libembind-rtti",
    "libemmalloc-mt",
    "libemmalloc-mt-debug",
    "libemmalloc",
    "libfetch-mt",
    "libfetch",
    "libGL-getprocaddr",
    "libGL-mt-getprocaddr",
    "libhtml5",
    "libnoexit",
    "libsockets-mt",
    "libsockets",
    "libprintf_long_double-mt",
    "libprintf_long_double-mt-debug",
    "libprintf_long_double-debug",
    "libstubs",
    "libstubs-debug",
    "libc-debug",
    "libdlmalloc",
    "libc++abi-debug-noexcept",
])

Now the LTO build started working, but dbg and fastbuild builds stopped working, throwing the very same exception:

Exception: FROZEN_CACHE is set, but cache file is missing: "sysroot/lib/wasm32-emscripten/crtbegin.o" because now the LTO-enabled cache is used, but dbg and fastbuild don't have LTO enabled.

My current workaround is to simply also enable LTO in dbg and fastbuild modes, but I don't see this as ideal, considering that I didn't have this requirement with my cmake-based setup.

I'm wondering whether it's possible to configure multiple emscripten caches, so that different build configurations could use different caches? Or maybe, treat the cache as "build artefact" instead of a "repository artefact" so it would be built as part of a regular Bazel build and cached on caching servers?

[DoDoENT]

Also, is there a way to persist the cache instead of rebuilding it every time?

My CI machines occasionally need to rebuild this cache, and this results in different binaries (different hashes), which completely invalidates the build cache, and the result is a slow and long rebuild of the entire codebase.