fix library sync backfill O(N^2) hotspots on both sides of the protocol

core/src/domain/resource_manager.rs

@@ -269,6 +269,19 @@ impl ResourceManager {
 		resource_type: &str,
 		resource_ids: Vec<Uuid>,
 	) -> Result<()> {
+		// During backfill we skip per-record fan-out. For models like
+		// content_identity each UUID triggers 2 DB queries via dependency
+		// routing, which dominates apply time on large libraries. The backfill
+		// coordinator emits a single coarse invalidation after the scope ends.
+		if crate::infra::sync::is_in_backfill() {
+			tracing::trace!(
+				resource_type = %resource_type,
+				count = resource_ids.len(),
+				"Skipping per-record resource event emission during backfill"
+			);
+			return Ok(());
+		}
+
 		// For now, delegate to single-resource handler
 		// In future, could optimize by batching virtual resource construction
 		self.emit_resource_events(resource_type, resource_ids).await

core/src/infra/db/entities/content_identity.rs

@@ -246,36 +246,77 @@ impl Syncable for Model {
 
 		let results = query.all(db).await?;
 
-		let mut sync_results = Vec::new();
+		let mut sync_results: Vec<(Uuid, serde_json::Value, chrono::DateTime<chrono::Utc>)> =
+			Vec::with_capacity(results.len());
 		for content in results {
-			if content.uuid.is_none() {
-				continue;
-			}
+			let uuid = match content.uuid {
+				Some(u) => u,
+				None => continue,
+			};
 
-			let mut json = match content.to_sync_json() {
+			let json = match content.to_sync_json() {
 				Ok(j) => j,
 				Err(e) => {
 					tracing::warn!(error = %e, content_hash = %content.content_hash, "Failed to serialize content_identity for sync");
 					continue;
 				}
 			};
 
-			// Convert FK to UUID for cross-device compatibility
-			for fk in Self::foreign_key_mappings() {
-				if let Err(e) =
-					crate::infra::sync::fk_mapper::convert_fk_to_uuid(&mut json, &fk, db).await
+			sync_results.push((uuid, json, content.last_verified_at));
+		}
+
+		// Batch FK → UUID conversion across the whole batch: one DB round trip
+		// per FK type instead of one per (record × FK). Records that fail
+		// resolution are dropped so peers never see a sender-local int in the
+		// payload.
+		let fk_mappings = Self::foreign_key_mappings();
+		if !fk_mappings.is_empty() && !sync_results.is_empty() {
+			let mut payloads: Vec<serde_json::Value> = sync_results
+				.iter()
+				.map(|(_, json, _)| json.clone())
+				.collect();
+			let mut failed_indices: std::collections::HashSet<usize> =
+				std::collections::HashSet::new();
+
+			for fk in &fk_mappings {
+				match crate::infra::sync::fk_mapper::convert_fks_to_uuids_batch(
+					&mut payloads,
+					fk,
+					db,
+				)
+				.await
 				{
-					tracing::warn!(
-						error = %e,
-						uuid = %content.uuid.unwrap(),
-						fk_field = fk.local_field,
-						"Failed to convert FK to UUID, skipping content_identity"
-					);
-					continue;
+					Ok(failed) => failed_indices.extend(failed),
+					Err(e) => {
+						tracing::warn!(
+							error = %e,
+							fk_field = fk.local_field,
+							"Batch FK conversion failed for content_identity"
+						);
+						return Err(sea_orm::DbErr::Custom(format!(
+							"ContentIdentity FK batch conversion failed: {}",
+							e
+						)));
+					}
 				}
 			}
 
-			sync_results.push((content.uuid.unwrap(), json, content.last_verified_at));
+			sync_results = sync_results
+				.into_iter()
+				.zip(payloads.into_iter())
+				.enumerate()
+				.filter_map(|(idx, ((uuid, _, ts), resolved))| {
+					if failed_indices.contains(&idx) {
+						tracing::warn!(
+							uuid = %uuid,
+							"Dropping content_identity with unresolved FK from sync batch"
+						);
+						None
+					} else {
+						Some((uuid, resolved, ts))
+					}
+				})
+				.collect();
 		}
 
 		Ok(sync_results)

core/src/infra/db/entities/entry.rs

@@ -292,16 +292,31 @@ impl crate::infra::sync::Syncable for Model {
 				std::collections::HashMap::new()
 			};
 
-		// Convert to sync format with FK mapping
-		let mut sync_results = Vec::new();
+		// Serialize each row to JSON with its UUID and timestamp.
+		let mut staged: Vec<(Uuid, serde_json::Value, chrono::DateTime<chrono::Utc>)> =
+			Vec::with_capacity(results.len());
 
 		for entry in results {
 			let uuid = match entry.uuid {
 				Some(u) => u,
-				None => continue, // Skip entries without UUIDs
+				None => continue,
+			};
+
+			// The sync cursor filters/orders on `indexed_at`, so a row with a
+			// NULL `indexed_at` would emit a cursor the next query predicate
+			// doesn't represent. Skip them — the indexed_at backfill migration
+			// populated every existing row, so this should be unreachable.
+			let indexed_at = match entry.indexed_at {
+				Some(ts) => ts,
+				None => {
+					tracing::warn!(
+						uuid = %uuid,
+						"Entry has NULL indexed_at; skipping sync until it's populated"
+					);
+					continue;
+				}
 			};
 
-			// Serialize to JSON
 			let mut json = match entry.to_sync_json() {
 				Ok(j) => j,
 				Err(e) => {
@@ -311,9 +326,8 @@ impl crate::infra::sync::Syncable for Model {
 			};
 
 			// For directories, include the absolute path from directory_paths
-			// This ensures receiving devices get identical paths for universal addressing
+			// so receiving devices get identical paths for universal addressing.
 			if entry.kind == 1 {
-				// Directory
 				if let Some(path) = directory_paths_map.get(&entry.id) {
 					if let Some(obj) = json.as_object_mut() {
 						obj.insert(
@@ -324,27 +338,62 @@ impl crate::infra::sync::Syncable for Model {
 				}
 			}
 
-			// Convert FK integer IDs to UUIDs
-			for fk in <Model as Syncable>::foreign_key_mappings() {
-				if let Err(e) =
-					crate::infra::sync::fk_mapper::convert_fk_to_uuid(&mut json, &fk, db).await
+			staged.push((uuid, json, indexed_at));
+		}
+
+		// Batch-convert FK integer IDs to UUIDs one FK type at a time across
+		// the whole batch — single DB round trip per FK, not per record × FK.
+		// Any record that fails resolution (missing target, bad value) is
+		// dropped before we return so peers never see a sender-local int.
+		let fk_mappings = <Model as Syncable>::foreign_key_mappings();
+		if !fk_mappings.is_empty() && !staged.is_empty() {
+			let mut payloads: Vec<serde_json::Value> =
+				staged.iter().map(|(_, json, _)| json.clone()).collect();
+			let mut failed_indices: std::collections::HashSet<usize> =
+				std::collections::HashSet::new();
+
+			for fk in &fk_mappings {
+				match crate::infra::sync::fk_mapper::convert_fks_to_uuids_batch(
+					&mut payloads,
+					fk,
+					db,
+				)
+				.await
 				{
-					tracing::warn!(
-						error = %e,
-						uuid = %uuid,
-						fk_field = fk.local_field,
-						"Failed to convert FK to UUID, skipping entry"
-					);
-					continue;
+					Ok(failed) => failed_indices.extend(failed),
+					Err(e) => {
+						tracing::warn!(
+							error = %e,
+							fk_field = fk.local_field,
+							"Batch FK conversion failed for entries"
+						);
+						return Err(sea_orm::DbErr::Custom(format!(
+							"Entry FK batch conversion failed: {}",
+							e
+						)));
+					}
 				}
 			}
 
-			// Use indexed_at for checkpoint/watermark tracking, fallback to modified_at if NULL
-			let timestamp = entry.indexed_at.unwrap_or(entry.modified_at);
-			sync_results.push((uuid, json, timestamp));
+			staged = staged
+				.into_iter()
+				.zip(payloads.into_iter())
+				.enumerate()
+				.filter_map(|(idx, ((uuid, _, ts), resolved))| {
+					if failed_indices.contains(&idx) {
+						tracing::warn!(
+							uuid = %uuid,
+							"Dropping entry with unresolved FK from sync batch"
+						);
+						None
+					} else {
+						Some((uuid, resolved, ts))
+					}
+				})
+				.collect();
 		}
 
-		Ok(sync_results)
+		Ok(staged)
 	}
 
 	/// Apply state change - already implemented in Model impl block below
@@ -564,10 +613,12 @@ impl Model {
 			inserted.id
 		};
 
-		// Rebuild entry_closure for this synced entry
-		// Without this, the entry only has a self-reference and cannot be queried
-		// for descendants, breaking subtree operations, location scoping, etc.
-		Self::rebuild_entry_closure(entry_id, parent_id, db).await?;
+		// Rebuild entry_closure for this synced entry, unless we're inside a
+		// backfill apply loop — in that case the post_backfill_rebuild hook
+		// does a single bulk rebuild at the end, so per-entry work is wasted.
+		if !crate::infra::sync::is_in_backfill() {
+			Self::rebuild_entry_closure(entry_id, parent_id, db).await?;
+		}
 
 		// If this is a directory, create or update its entry in the directory_paths table
 		if EntryKind::from(kind) == EntryKind::Directory {

core/src/infra/db/migration/m20260417_000001_add_entries_sync_cursor_index.rs

@@ -0,0 +1,36 @@
+//! Add composite index on entries(indexed_at, uuid) to back the device-owned
+//! sync cursor.
+//!
+//! `Entry::query_for_sync` paginates by `ORDER BY indexed_at ASC, uuid ASC`
+//! with a tie-breaker filter of the same shape. Without this index, SQLite
+//! does a full table scan per batch request — O(N) per batch, O(N^2) across
+//! an initial backfill of a large library.
+
+use sea_orm_migration::prelude::*;
+
+#[derive(DeriveMigrationName)]
+pub struct Migration;
+
+#[async_trait::async_trait]
+impl MigrationTrait for Migration {
+	async fn up(&self, manager: &SchemaManager) -> Result<(), DbErr> {
+		manager
+			.get_connection()
+			.execute_unprepared(
+				"CREATE INDEX IF NOT EXISTS idx_entries_indexed_at_uuid \
+				 ON entries(indexed_at, uuid)",
+			)
+			.await?;
+
+		Ok(())
+	}
+
+	async fn down(&self, manager: &SchemaManager) -> Result<(), DbErr> {
+		manager
+			.get_connection()
+			.execute_unprepared("DROP INDEX IF EXISTS idx_entries_indexed_at_uuid")
+			.await?;
+
+		Ok(())
+	}
+}

core/src/infra/db/migration/mod.rs

@@ -38,6 +38,7 @@ mod m20260104_000001_replace_device_id_with_volume_id;
 mod m20260105_000001_add_volume_id_to_locations;
 mod m20260114_000001_fix_search_index_include_directories;
 mod m20260123_000001_remove_legacy_sync_columns;
+mod m20260417_000001_add_entries_sync_cursor_index;
 
 pub struct Migrator;
 
@@ -81,6 +82,7 @@ impl MigratorTrait for Migrator {
 			Box::new(m20260105_000001_add_volume_id_to_locations::Migration),
 			Box::new(m20260114_000001_fix_search_index_include_directories::Migration),
 			Box::new(m20260123_000001_remove_legacy_sync_columns::Migration),
+			Box::new(m20260417_000001_add_entries_sync_cursor_index::Migration),
 		]
 	}
 }

core/src/infra/sync/backfill_context.rs

@@ -0,0 +1,26 @@
+//! Task-local flag identifying code running inside a backfill apply loop.
+//!
+//! A few per-record hooks (closure table rebuild, resource event emission)
+//! are redundant during backfill because the coordinator does bulk work at
+//! the end. Models check this flag to skip that per-record work.
+
+tokio::task_local! {
+	static IN_BACKFILL: ();
+}
+
+/// Run `fut` with the in-backfill flag set. Nested scopes are allowed.
+pub async fn in_backfill<F, T>(fut: F) -> T
+where
+	F: std::future::Future<Output = T>,
+{
+	if is_in_backfill() {
+		fut.await
+	} else {
+		IN_BACKFILL.scope((), fut).await
+	}
+}
+
+/// True when the current task is inside an `in_backfill` scope.
+pub fn is_in_backfill() -> bool {
+	IN_BACKFILL.try_with(|_| ()).is_ok()
+}