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## Problem The current implementation of struct Layer supports canceled read requests, but those will leave the internal state such that a following `Layer::keep_resident` call will need to repair the state. In pathological cases seen during generation numbers resetting in staging or with too many in-progress on-demand downloads, this repair activity will need to wait for the download to complete, which stalls disk usage-based eviction. Similar stalls have been observed in staging near disk-full situations, where downloads failed because the disk was full. Fixes #6028 or the "layer is present on filesystem but not evictable" problems by: 1. not canceling pending evictions by a canceled `LayerInner::get_or_maybe_download` 2. completing post-download initialization of the `LayerInner::inner` from the download task Not canceling evictions above case (1) and always initializing (2) lead to plain `LayerInner::inner` always having the up-to-date information, which leads to the old `Layer::keep_resident` never having to wait for downloads to complete. Finally, the `Layer::keep_resident` is replaced with `Layer::is_likely_resident`. These fix #7145. ## Summary of changes - add a new test showing that a canceled get_or_maybe_download should not cancel the eviction - switch to using a `watch` internally rather than a `broadcast` to avoid hanging eviction while a download is ongoing - doc changes for new semantics and cleanup - fix `Layer::keep_resident` to use just `self.0.inner.get()` as truth as `Layer::is_likely_resident` - remove `LayerInner::wanted_evicted` boolean as no longer needed Builds upon: #7185. Cc: #5331.