feat(sync): validate negentropy event receipt and retry missing events

Add validation that all events requested by ID during negentropy sync
are actually received from the relay. When events are missing:

- Log detailed information (requested/received/missing counts and IDs)
- Create retry subscriptions for missing events (chunked by 300)
- Update batch to track only missing events in next round
- Only complete batch after all events received or retry fails

This handles relays that have limits on ID-based queries (e.g., max 150
events per query) by automatically retrying in smaller chunks.

Also excludes purgatory and rejected announcement events from negentropy
requests to avoid re-requesting events we know we can't/won't store.

Note: Current implementation lacks retry limit - infinite loop protection
needed (tracked as future work).

2 files changed, 248 insertions, 5 deletions

diff --git a/src/sync/algorithms.rs b/src/sync/algorithms.rs
index f4b1f5c..4679986 100644
--- a/src/sync/algorithms.rs
+++ b/src/sync/algorithms.rs
@@ -402,6 +402,8 @@ mod tests {
                 outstanding_subs: HashSet::new(),
                 sync_method: SyncMethod::ReqEose,
                 pagination_state: HashMap::new(),
+                requested_event_ids: None,
+                received_event_ids: None,
             }],
         );
 
@@ -514,6 +516,8 @@ mod tests {
                 outstanding_subs: HashSet::new(),
                 sync_method: SyncMethod::ReqEose,
                 pagination_state: HashMap::new(),
+                requested_event_ids: None,
+                received_event_ids: None,
             }],
         );
 
diff --git a/src/sync/mod.rs b/src/sync/mod.rs
index ecfd020..d33364f 100644
--- a/src/sync/mod.rs
+++ b/src/sync/mod.rs
@@ -196,6 +196,12 @@ pub struct PendingBatch {
     /// Pagination tracking for REQ+EOSE subscriptions (empty for Negentropy)
     /// Maps subscription ID to its pagination state
     pub pagination_state: HashMap<SubscriptionId, PaginationState>,
+    /// Event IDs requested via negentropy ID-based fetch (None for REQ+EOSE)
+    /// Used to validate that all requested events were received
+    pub requested_event_ids: Option<HashSet<EventId>>,
+    /// Event IDs actually received for this batch (None for REQ+EOSE)
+    /// Compared against requested_event_ids to detect missing events
+    pub received_event_ids: Option<HashSet<EventId>>,
 }
 
 /// Items included in a pending batch
@@ -633,7 +639,119 @@ impl SyncManager {
             return;
         }
 
-        // 2. Batch complete - extract and remove
+        // 2. Batch complete - validate negentropy ID fetches before confirming
+        // For negentropy batches, check if all requested events were received
+        if batch.sync_method == SyncMethod::Negentropy {
+            if let (Some(requested), Some(received)) =
+                (&batch.requested_event_ids, &batch.received_event_ids)
+            {
+                let missing: Vec<EventId> =
+                    requested.difference(received).cloned().collect();
+
+                if !missing.is_empty() {
+                    let requested_count = requested.len();
+                    let received_count = received.len();
+
+                    tracing::warn!(
+                        relay = %relay_url,
+                        batch_id = batch.batch_id,
+                        requested_count = requested_count,
+                        received_count = received_count,
+                        missing_count = missing.len(),
+                        missing_ids = ?missing.iter().map(|id| id.to_hex()).collect::<Vec<_>>(),
+                        "Negentropy sync incomplete - relay returned fewer events than requested. \
+                         This may indicate a relay limit on ID-based queries. \
+                         Retrying missing events."
+                    );
+
+                    // Create retry subscription for missing events
+                    // Chunk by 300 to avoid overly large filters
+                    let relay_url_for_retry = relay_url.to_string();
+                    let batch_id = batch.batch_id;
+
+                    // Drop the lock before async operations
+                    drop(pending);
+
+                    // Create new subscriptions for missing events
+                    let retry_filters: Vec<_> = missing
+                        .chunks(300)
+                        .map(|c| Filter::new().ids(c.iter().copied()))
+                        .collect();
+
+                    let mut new_sub_ids = HashSet::new();
+                    if let Some(conn) = self.connections.get(&relay_url_for_retry) {
+                        for filter in retry_filters {
+                            match conn.subscribe_filter(filter, true).await {
+                                Ok(sub_id) => {
+                                    new_sub_ids.insert(sub_id);
+                                }
+                                Err(e) => {
+                                    tracing::error!(
+                                        relay = %relay_url_for_retry,
+                                        batch_id = batch_id,
+                                        error = %e,
+                                        "Failed to create retry subscription for missing events"
+                                    );
+                                }
+                            }
+                        }
+                    }
+
+                    if !new_sub_ids.is_empty() {
+                        // Re-acquire lock and update batch with new subscriptions
+                        let mut pending = self.pending_sync_index.write().await;
+                        if let Some(batches) = pending.get_mut(&relay_url_for_retry) {
+                            if let Some(batch) =
+                                batches.iter_mut().find(|b| b.batch_id == batch_id)
+                            {
+                                batch.outstanding_subs.extend(new_sub_ids.clone());
+                                // Update requested_event_ids to only include missing ones
+                                batch.requested_event_ids =
+                                    Some(missing.iter().cloned().collect());
+                                // Clear received_event_ids for fresh tracking
+                                batch.received_event_ids = Some(HashSet::new());
+
+                                tracing::info!(
+                                    relay = %relay_url_for_retry,
+                                    batch_id = batch_id,
+                                    retry_subs = new_sub_ids.len(),
+                                    missing_events = missing.len(),
+                                    "Created retry subscriptions for missing negentropy events"
+                                );
+                            }
+                        }
+                        // Early return - batch not complete yet, waiting for retry EOSE
+                        return;
+                    } else {
+                        // Failed to create retry subscriptions, log and continue to confirm
+                        // with partial results
+                        tracing::error!(
+                            relay = %relay_url_for_retry,
+                            batch_id = batch_id,
+                            missing_count = missing.len(),
+                            "Failed to retry missing events - confirming batch with partial results"
+                        );
+
+                        // Re-acquire lock to extract the batch
+                        let mut pending = self.pending_sync_index.write().await;
+                        if let Some(batches) = pending.get_mut(&relay_url_for_retry) {
+                            if let Some(idx) = batches.iter().position(|b| b.batch_id == batch_id)
+                            {
+                                let completed_batch = batches.remove(idx);
+                                if batches.is_empty() {
+                                    pending.remove(&relay_url_for_retry);
+                                }
+                                drop(pending);
+                                self.confirm_batch(&relay_url_for_retry, completed_batch).await;
+                            }
+                        }
+                        return;
+                    }
+                }
+            }
+        }
+
+        // 3. Batch complete - extract and remove
         let completed_batch = batches.remove(batch_idx);
 
         // Clean up empty relay entry
@@ -644,7 +762,7 @@ impl SyncManager {
         // Drop the pending lock before confirm_batch
         drop(pending);
 
-        // 3. Confirm the batch (moves items to RelayState)
+        // 4. Confirm the batch (moves items to RelayState)
         self.confirm_batch(relay_url, completed_batch).await;
     }
 
@@ -1104,11 +1222,13 @@ impl SyncManager {
                             }
                         }
 
-                        // Track pagination state for this subscription
+                        // Track pagination state for this subscription (REQ+EOSE)
+                        // and received event IDs for negentropy batches
                         if result == ProcessResult::Saved || result == ProcessResult::Duplicate {
                             let mut pending = pending_sync_index.write().await;
                             if let Some(batches) = pending.get_mut(&relay_url_clone) {
                                 for batch in batches.iter_mut() {
+                                    // Track pagination state (REQ+EOSE path)
                                     if let Some(state) =
                                         batch.pagination_state.get_mut(&subscription_id)
                                     {
@@ -1122,6 +1242,17 @@ impl SyncManager {
                                             _ => {}
                                         }
                                     }
+
+                                    // Track received event IDs (negentropy path)
+                                    // Only track if this batch has requested_event_ids set
+                                    // and the subscription is one we're waiting on
+                                    if batch.requested_event_ids.is_some()
+                                        && batch.outstanding_subs.contains(&subscription_id)
+                                    {
+                                        if let Some(ref mut received) = batch.received_event_ids {
+                                            received.insert(event.id);
+                                        }
+                                    }
                                 }
                             }
                         }
@@ -2240,6 +2371,8 @@ impl SyncManager {
                 outstanding_subs: HashSet::new(),
                 sync_method: SyncMethod::Negentropy,
                 pagination_state: HashMap::new(), // Negentropy doesn't use pagination
+                requested_event_ids: None,        // Will be set after negentropy diff
+                received_event_ids: None,         // Will be set after negentropy diff
             };
 
             // Add to pending_sync_index
@@ -2395,6 +2528,10 @@ impl SyncManager {
                 if let Some(relay_batches) = pending.get_mut(relay_url) {
                     if let Some(batch) = relay_batches.iter_mut().find(|b| b.batch_id == batch_id) {
                         batch.outstanding_subs.extend(subscription_ids.clone());
+                        // Store requested event IDs for validation after EOSE
+                        batch.requested_event_ids =
+                            Some(all_remote_ids.iter().cloned().collect());
+                        batch.received_event_ids = Some(HashSet::new());
                     }
                 }
             }
@@ -2403,7 +2540,7 @@ impl SyncManager {
                 batch_id = batch_id,
                 subscription_ids = subscription_ids.len(),
                 events = all_remote_ids.len(),
-                "historic_sync (Negentropy) created subscritions to fetch missing events by id, awaiting EOSE"
+                "historic_sync (Negentropy) created subscriptions to fetch missing events by id, awaiting EOSE"
             );
         } else {
             // Traditional REQ+EOSE path
@@ -2471,6 +2608,8 @@ impl SyncManager {
                 outstanding_subs: subscription_ids,
                 sync_method: SyncMethod::ReqEose,
                 pagination_state,
+                requested_event_ids: None, // Not used for REQ+EOSE
+                received_event_ids: None,  // Not used for REQ+EOSE
             };
 
             // Add to pending_sync_index
@@ -2540,7 +2679,6 @@ impl SyncManager {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use nostr_sdk::prelude::*;
 
     #[tokio::test]
     async fn test_rejected_events_index_tracks_announcements() {
@@ -2607,4 +2745,105 @@ mod tests {
         assert!(filtered_ids.contains(&valid_id));
         assert_eq!(filtered_ids.len(), 1);
     }
+
+    #[test]
+    fn test_negentropy_missing_event_detection() {
+        // Simulate scenario where relay returns fewer events than requested
+        // This tests the core logic for detecting missing events
+
+        // Requested 5 events from negentropy diff
+        let mut requested: HashSet<EventId> = HashSet::new();
+        for i in 1u8..=5 {
+            let id = EventId::from_hex(&format!(
+                "{:0>64}",
+                format!("{:x}", i)
+            ))
+            .unwrap();
+            requested.insert(id);
+        }
+
+        // Only received 3 events (simulating relay limit)
+        let mut received: HashSet<EventId> = HashSet::new();
+        for i in 1u8..=3 {
+            let id = EventId::from_hex(&format!(
+                "{:0>64}",
+                format!("{:x}", i)
+            ))
+            .unwrap();
+            received.insert(id);
+        }
+
+        // Calculate missing events
+        let missing: Vec<EventId> = requested.difference(&received).cloned().collect();
+
+        // Should have 2 missing events (IDs 4 and 5)
+        assert_eq!(missing.len(), 2);
+        assert_eq!(requested.len(), 5);
+        assert_eq!(received.len(), 3);
+
+        // Verify the specific missing IDs
+        let id_4 = EventId::from_hex(&format!("{:0>64}", format!("{:x}", 4u8))).unwrap();
+        let id_5 = EventId::from_hex(&format!("{:0>64}", format!("{:x}", 5u8))).unwrap();
+        assert!(missing.contains(&id_4));
+        assert!(missing.contains(&id_5));
+    }
+
+    #[test]
+    fn test_negentropy_all_events_received() {
+        // Simulate scenario where all requested events are received
+        let mut requested: HashSet<EventId> = HashSet::new();
+        for i in 1u8..=3 {
+            let id = EventId::from_hex(&format!(
+                "{:0>64}",
+                format!("{:x}", i)
+            ))
+            .unwrap();
+            requested.insert(id);
+        }
+
+        // Received all 3 events
+        let received = requested.clone();
+
+        // Calculate missing events
+        let missing: Vec<EventId> = requested.difference(&received).cloned().collect();
+
+        // Should have no missing events
+        assert!(missing.is_empty());
+    }
+
+    #[test]
+    fn test_pending_batch_negentropy_fields() {
+        // Test that PendingBatch properly tracks negentropy-specific fields
+        let batch = PendingBatch {
+            batch_id: 1,
+            items: PendingItems::default(),
+            outstanding_subs: HashSet::new(),
+            sync_method: SyncMethod::Negentropy,
+            pagination_state: HashMap::new(),
+            requested_event_ids: Some(HashSet::new()),
+            received_event_ids: Some(HashSet::new()),
+        };
+
+        assert!(batch.requested_event_ids.is_some());
+        assert!(batch.received_event_ids.is_some());
+        assert_eq!(batch.sync_method, SyncMethod::Negentropy);
+    }
+
+    #[test]
+    fn test_pending_batch_req_eose_fields() {
+        // Test that REQ+EOSE batches don't use negentropy fields
+        let batch = PendingBatch {
+            batch_id: 1,
+            items: PendingItems::default(),
+            outstanding_subs: HashSet::new(),
+            sync_method: SyncMethod::ReqEose,
+            pagination_state: HashMap::new(),
+            requested_event_ids: None,
+            received_event_ids: None,
+        };
+
+        assert!(batch.requested_event_ids.is_none());
+        assert!(batch.received_event_ids.is_none());
+        assert_eq!(batch.sync_method, SyncMethod::ReqEose);
+    }
 }