next.js

0dd0f20a - [Turbopack] refactor persistent caching from log based to cow approach (#76234)

[Turbopack] refactor persistent caching from log based to cow approach (#76234) ### What? Instead of collecting all modifications to the task graph in a log structure (which ends up taking a lot of memory), use a modified and snapshot flag to avoid using more memory than needed. This way we don't store extra memory before a snapshot is requested. Once a snapshot is captured, we store at most twice the memory. We store only the memory of the modified tasks. It works this way: Idle Phase: (before a snapshot is requested) * When a task is modified, we only set the modified flag on that task. Capturing a snapshot want to avoid locking all tasks. So it first switches to the Snapshot Phase. Snapshot Phase: (after a snapshot is captured) * When a task is modified: * If there is already a snapshot, skip * If the task has the modified flag set, clone the task and store the snapshot. * If the task doesn't have the modified flag set, store a None snapshot. To actually capture the snapshot, go over all modified or snapshot tasks (which is stored in a separate map): * If the task has a snapshot (not None) stored, use that. * If the task has only the modified flag set, read the task state and capture a snapshot from that. * There is a race condition here, when the task is modified inbetween. We handle that by having a snapshot flag too. We have all snapshot captured now. To leave Snapshot Phase, we * First unset all modified flags. * Switch to Idle Phase. * Change all stored snapshots to modified.