fix a snapshot encoding error that could happen under concurrent mutation and snapshotting#92658
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lukesandberg merged 2 commits intocanaryfrom Apr 29, 2026
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fix a snapshot encoding error that could happen under concurrent mutation and snapshotting#92658lukesandberg merged 2 commits intocanaryfrom
lukesandberg merged 2 commits intocanaryfrom
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What?
Fixes a snapshot encoding panic introduced by #89370 in
turbopack/crates/turbo-tasks-backend/src/backend/storage.rs.Why?
#89370 refactored snapshot iteration and in doing so introduced two incorrect assertions in
SnapshotShardIter::nextfor tasks in themodifiedlist that were re-modified during snapshot iteration:.expect("snapshot entry for modified_during_snapshot task must contain a value")— this panics in the(true, false)branch oftrack_modification_internal, where a task was modified in one category before the snapshot and then modified in a different category during iteration. The second modification stores aNoneentry insnapshots(because there was no pre-existing data to copy for that category), but the iterator unconditionally unwrappedSome.debug_assert!(!inner.flags.any_modified())— after clearing the livedata_modified/meta_modifiedflags and before promoting_during_snapshotflags, the old code asserted that no modified flags remained. This fires in the(true, true)branch because clearing the flags happens after the snapshot copy was taken, so the assert races with the flag state.How?
direct_snapshotsfast-path and themodifiedlist into a single path that handles both(true, true)and(true, false)cases gracefully: ifany_modified_during_snapshot()is set, check thesnapshotsmap — use theSome(copy)if present, otherwise fall back to live data (which is correct for theNone/first-time-modified case).(true, false)branch (modify_different_category_during_snapshot).