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## Problem GC info is an input to updating layer visibility. Currently, gc info is updated on timeline activation and visibility is computed on tenant attach, so we ignore branch points and compute visibility by taking all layers into account. Side note: gc info is also updated when timelines are created and dropped. That doesn't help because we create the timelines in topological order from the root. Hence the root timeline goes first, without context of where the branch points are. The impact of this in prod is that shards need to rehydrate layers after live migration since the non-visible ones were excluded from the heatmap. ## Summary of Changes Move the visibility calculation into tenant attachment instead of activation.