feat(sync): Phase 3 - resilience and health tracking

- Add RelayHealthTracker with DashMap
- Implement exponential backoff (5s -> 1h max)
- Handle dead relays (24h failures -> daily retry)
- Add startup jitter to prevent thundering herd
- Add NGIT_SYNC_MAX_BACKOFF_SECS config

1 files changed, 68 insertions, 12 deletions

diff --git a/src/sync/connection.rs b/src/sync/connection.rs
index 76cc8e8..319cbbd 100644
--- a/src/sync/connection.rs
+++ b/src/sync/connection.rs
@@ -9,6 +9,12 @@
 //!   1. Layer 1: kinds 30617 + 30618 (announcements)
 //!   2. Layer 2: A/a tags for repository events
 //!   3. Layer 3: E/e tags for related events (PRs, Issues, etc.)
+//!
+//! ## Phase 3 Features
+//!
+//! - Health tracking with success/failure reporting
+//! - Exponential backoff with health-aware delays
+//! - Dead relay detection and minimal retry
 
 use std::sync::Arc;
 use std::time::Duration;
@@ -17,6 +23,7 @@ use nostr_sdk::prelude::*;
 use tokio::sync::mpsc;
 
 use super::filter::FilterService;
+use super::health::RelayHealthTracker;
 
 /// Event received from the sync connection
 #[derive(Debug, Clone)]
@@ -169,47 +176,96 @@ impl SyncConnection {
     }
 }
 
-/// Reconnect loop with exponential backoff
+/// Reconnect loop with health-aware exponential backoff
+///
+/// This function manages the connection lifecycle with health tracking:
+/// - Checks health state before attempting connections
+/// - Reports success/failure to the health tracker
+/// - Respects backoff delays from the health tracker
+/// - Handles dead relay detection (24h+ failures)
 ///
 /// # Arguments
 /// * `url` - The relay URL to connect to
 /// * `tx` - Channel sender for synced events
 /// * `filter_service` - FilterService for building subscriptions
 /// * `our_domain` - Our relay's domain (used to extract remote domain)
+/// * `health_tracker` - Health tracker for managing connection state
 pub async fn connect_with_retry(
     url: &str,
     tx: mpsc::Sender<SyncedEvent>,
     filter_service: Arc<FilterService>,
     _our_domain: &str,
+    health_tracker: Arc<RelayHealthTracker>,
 ) {
-    let mut backoff = Duration::from_secs(1);
-    let max_backoff = Duration::from_secs(60);
-
     // Extract remote domain from URL
     let remote_domain = extract_domain_from_url(url).unwrap_or_else(|| url.to_string());
 
     loop {
+        // Check if we should attempt connection based on health state
+        if !health_tracker.should_attempt_connection(url) {
+            // Wait for remaining backoff
+            if let Some(remaining) = health_tracker.get_remaining_backoff(url) {
+                tracing::debug!(
+                    "Relay {} in backoff, waiting {:?} before retry",
+                    url,
+                    remaining
+                );
+                tokio::time::sleep(remaining).await;
+                continue;
+            }
+        }
+
+        // Log current health state for dead relays
+        if health_tracker.is_dead(url) {
+            tracing::info!(
+                "Attempting reconnection to dead relay {} (daily retry)",
+                url
+            );
+        }
+
         match SyncConnection::new(url, filter_service.clone(), &remote_domain).await {
             Ok(conn) => {
-                backoff = Duration::from_secs(1); // Reset backoff on successful connection
+                // Record successful connection
+                health_tracker.record_success(url);
+                tracing::info!("Sync connection established to {}", url);
+
+                // Run the connection (this blocks until disconnection)
                 conn.run(tx.clone()).await;
+
+                // Connection ended - record as failure for reconnection backoff
+                // (The connection ending is considered a failure even if it worked for a while)
+                health_tracker.record_failure(url);
                 tracing::warn!("Sync connection to {} ended, will reconnect", url);
             }
             Err(e) => {
+                // Record connection failure
+                health_tracker.record_failure(url);
+
+                let failure_count = health_tracker.get_failure_count(url);
+                let state = health_tracker.get_state(url);
+
                 tracing::error!(
-                    "Failed to connect to sync relay {}: {} (retrying in {:?})",
+                    "Failed to connect to sync relay {} (attempt #{}, state: {}): {}",
                     url,
-                    e,
-                    backoff
+                    failure_count,
+                    state,
+                    e
                 );
             }
         }
 
-        // Wait before reconnecting
-        tokio::time::sleep(backoff).await;
+        // Get the backoff duration from health tracker
+        // If the health tracker has no backoff set (shouldn't happen), use a small default
+        let wait_duration = health_tracker
+            .get_remaining_backoff(url)
+            .unwrap_or(Duration::from_secs(5));
 
-        // Exponential backoff
-        backoff = std::cmp::min(backoff * 2, max_backoff);
+        tracing::debug!(
+            "Waiting {:?} before reconnecting to {}",
+            wait_duration,
+            url
+        );
+        tokio::time::sleep(wait_duration).await;
     }
 }