ccache: Cache is not building/restoring/saving

[Gadgetoid]

I've been debugging loooooong CMake build times on pimoroni-pico with ccache being the culprit. I remembered I probably inherited the ccache code from here, so when I figure out how to fix it, should probably fix it here too!

Check any action with ccache and you'll see a cache miss, and a fail to upload the resulting cache, it looks like ccache has changed to not use /home/runner/.ccache (~/.cache) by default anymore.

[Gadgetoid]

On non-Windows systems, the default is $HOME/.ccache if such a directory exists

if such a directory exists

🤔 we have a good ol' fashioned paradox.

[Daft-Freak]

Maybe we should set CCACHE_DIR explicitly like the mac build does... though it seems that one isn't working either.

[Daft-Freak]

Wow, you can really see when this stopped working:

... without any actions changes... using a supposedly stable 2 year old OS version... sigh

[Gadgetoid]

Yeah you're not wrong- in pimoroni-pico it's taking >20min 💀 up from ~4min.

And it didn't occur to me that this should not happen 😬

I'm trying mkdir -p /home/runner/.ccache on a prayer the docs aren't lying to me.

[Gadgetoid]

Well that worked (and is extremely cursed!)

pimoroni/pimoroni-pico@9290243

[Gadgetoid]

My best sleuthing suggests that this change happened with ccache 4.0 on 2020-10-18.

I think we might have been riding on having an existing cache - which implicitly created the directory when restored - for much longer. It wouldn't break until cache misses started landing, and all branches get access to the default branch cache so that could have taken some time (long enough between main builds for the cache to expire?). That's my best theory for "why stable not stable" anyway.

[Daft-Freak]

This also seems to work: 9d6adb3

Relying on a fallback was probably always a bad idea, it's just weird that it suddenly broke now...

EDIT: It looks like your theory is correct though, the first build where this happened did fail to find an existing cache.