Add CoroutineContext configuration option to DiskCache.Builder

[alexvanyo]

I was trying to convert my injectable Dispatchers holder to inject CoroutineContexts, instead of CoroutineDispatchers in the spirit of https://code.cash.app/dispatchers-unconfined.

A case preventing doing this easily is configuring DiskCache, since the DiskCache.Builder only has a cleanupDispatcher configuration option: https://coil-kt.github.io/coil/api/coil-core/coil3.disk/-disk-cache/-builder/index.html#226104852%2FFunctions%2F1772253480

Could a method that accepts a CoroutineContext be added as well?

[alexvanyo]

It looks like the internals do call a CoroutineDispatcher method on the provided dispatcher, so it wouldn't be a super straightforward change if limitedParallelism is necessary:

coil/coil-core/src/commonMain/kotlin/coil3/disk/DiskLruCache.kt

Line 143 in d287c75

CoroutineScope(SupervisorJob() + cleanupDispatcher.limitedParallelism(1))

[colinrtwhite]

Great point! If I remember correctly I think limitedParallelism was used as the DiskLruCache was migrated from OkHttp, which used to use Executors.newSingleThreadExecutor.

I think we could probably refactor the code to remove limitedParallelism as the cleanup code is already guarded by a lock. Instead we could check if a clean up job is already scheduled/in progress and skip launch if it already is. After that we can deprecate DiskCache.Builder.cleanupDispatcher and replace it with DiskCache.Builder.cleanupCoroutineContext. PR welcome!

[colinrtwhite]

Actually an easier change would be to add DiskCache.Builder.cleanupCoroutineContext then grab the dispatcher from the context with (cleanupCoroutineContext.get(CoroutineDispatcher) ?: Dispatchers.Default).limitedParallelism(1) when creating the cleanupScope.

EDIT: Implemented here: #2797