Does this mean that CA derivations are never garbage-collected?

They aren't until their output-path is (like if keep-derivations is set to true in the config)

In fact, perhaps they should be kept around even if their output isn't.

Suppose we have:

( resolved-gcc.drv , "out" ) -> foo
( resolved-bash.drv [ foo / "gcc" ] , "out" ) -> bar

If we get rid of the row for foo, then if someone tries to rebuild (unresolved) bash.drv they'll end up rebuilding GCC!

We're going to need a pretty big migration I'm afraid to have the bar row preserve the foo row without also preserving the foo path itself.

In fact, perhaps they should be kept around even if their output isn't.

Suppose we have:

( resolved-gcc.drv , "out" ) -> foo
( resolved-bash.drv [ foo / "gcc" ] , "out" ) -> bar

If we get rid of the row for foo, then if someone tries to rebuild (unresolved) bash.drv they'll end up rebuilding GCC!

We're going to need a pretty big migration I'm afraid to have the bar row preserve the foo row without also preserving the foo path itself.

I'm not 100% sure, but I think that the DerivationOutputs entries stay there as long as the corresponding derivation is there (even if their output is gc-ed), so in that case the first mapping would stay alive because resolved-gcc.drv would (as it's a dependency of resolve-bash.drv)

Ah, but resolved-gcc.drv is not a dependency of resolve-bash.drv, because resolved floating CA derivations always have empty inputDrvs. That's the difficulty.

Ah CI failed because of that annoying spurious failure. Do we have an issue for it?

Ah, but resolved-gcc.drv is not a dependency of resolve-bash.drv, because resolved floating CA derivations always have empty inputDrvs. That's the difficulty.

Oh yes because we only have this mapping for resolved drvs 🤔
I think we need to dig all that a little to properly fix this (but the workaround is worth keeping in the meantime)

In fact, perhaps they should be kept around even if their output isn't.

But then derivations will accumulate forever... Or they could be treated like a cache and have some expiration policy.