A first backfill enqueues artists before albums at a single priority, so
the drain took them in that order. Artists resolve through a
rate-limited agent, and gate() waits for its permit while holding a
worker slot, so the whole pool sat asleep in the limiter with every
album queued behind it.
Measured on a 96k-track library (29,115 artists to 6,949 albums, 4:1):
zero albums resolved in seven minutes, and roughly 3.3 hours before the
first album cover would have appeared. Splitting the drain gives each
class its own slots: albums now finish in under eight minutes while
artists trickle at the same 2/s they were always limited to.
The two budgets are carved out of MaxOpenConns so a second pool cannot
take connections the scanner and the UI need. Dequeue filters by kind,
and the drain index leads with item_kind so each pool seeks to its own
work instead of scanning past the other's backlog.
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