austinried 7b17e33545 sync: batch the fallback getAlbum song fetches ...

The `search3`-less fallback emitted one batch per album, and
`SyncService.writeSongs` opens a transaction per batch, so syncing a large
library that way ran one transaction per album — thousands of fsyncs, and an
invalidation round for every list observing `songs` with each.

Accumulate what each `getAlbum` returns and flush at `PAGE_SIZE`, so the
fallback writes the same ~500-row batches every other fetch already does.
Albums are fetched concurrently and complete out of order, so the accumulator
sits behind a mutex and the send happens outside it.

Batches are in-flight memory, so the flush is per rows reached rather than
per album page. Measured per `Song` (strings, boxed longs and genre list
included): ~0.7 KiB, so a 500-row batch is ~334 KiB plus ~51 KiB of the
`names`/`genres` structures `writeSongs` derives, where a whole album page
(~5,500 rows) would be ~3.8 MiB. `albumSongs()` is also a bare `channelFlow`,
whose channel holds 64 elements whatever their size, so 500-row batches would
have made its worst case ~21 MiB; it now carries the same `.buffer(1)` that
`entityBatches()` already uses on itself. The rest of the in-flight rows
scale with `syncConcurrency`, so that leaves the default's batch in hand and
a few MiB at the highest setting.

`SubsonicSourceTest.albumSongsAreEmittedInBoundedBatches` asserts the emitted
sizes, [500, 100] for 600 songs spread over 200 three-song albums, which pins
the carried remainder as well as the flush: verified it fails on the
pre-change code (200 batches of 3) and against a mutant that drops the
remainder ([500, 99]). `docs/architecture.md`'s page cap was 20x out of date,
so it is corrected alongside. `ktlintCheck`, `testDebugUnitTest` and
`lintDebug` pass.

2026-10-16 12:39:42 +00:00

..

architecture.md

sync: batch the fallback getAlbum song fetches

2026-10-16 12:39:42 +00:00