[williamjakfar]

Problem

Importing one big text insert made right after a concurrent change takes time quadratic in the insert's length:

Insert	Import (1.17.1)
100k chars	0.05 s
300k chars	0.43 s
1M chars	5.1 s
3M chars	51.5 s

Repro: peer A imports a small edit from peer B, then inserts a big string and exports that one update; B imports it. Inserting the same text as many small commits, each imported on its own, takes milliseconds. We hit it running Loro in a Cloudflare Durable Object, where a 3M paste hung the object for over a minute. It reproduces natively, in Node (loro-crdt 1.17.1) and in WASM.

Cause

Almost all the time (98% of samples) is in IdToCursor::update_insert_batch, called from Tracker::update_insert_by_split.

IdToCursor stores the insert in fragments of 256 ops, so forward sends the rope one LeafUpdate per fragment, all naming the same leaf. CrdtRope::update returns a leaf index per update, so update_insert_by_split gets the same leaf about n/256 times, each with the same id_span() covering the whole insert. update_insert_batch then walks every fragment of the insert once per entry. With instrumentation, a 100k insert gave 390 updates, each 11~100011, and 152k fragments walked; at 3M, 11,718 updates and 137M fragments.

Fix

Skip leaves already seen in update_insert_by_split. A repeated entry is the same (leaf.id_span(), leaf) pair, so updating it again changes nothing.

Insert	Before	After
100k chars	0.05 s	0.001 s
1M chars	5.1 s	0.02 s
3M chars	51.5 s	0.13 s

Tests

• crates/loro/tests/big_insert_import.rs:
  • a correctness test (both peers end with the same text);
  • an #[ignore]d timing test in the style of dead_container_cache_perf.rs, which imports 1M characters and fails above 1 s. It takes about 0.02 s with the fix and 5.1 s without.
• cargo test --release --no-fail-fast -p loro-internal -p loro: 1,533 passed, 1 failed. The failure, tests::op_count::len_ops_prefers_exact_oplog_value_when_lock_order_allows, fails on main without this change too (macOS, Apple silicon).
• I didn't run the fuzz targets; please say if you'd like any run.

A changeset for loro-crdt (patch) is included.

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