Add DomainThrottle for per-domain rate limiting

Implement per-domain throttling for purgatory sync operations:
- Concurrent request limit (max in-flight requests per domain)
- Rate limit (max requests per minute via sliding window)
- Fair round-robin queue processing across identifiers
- In-progress tracking to prevent duplicate fetches
- Tried URL tracking per identifier

Add indexmap dependency for ordered iteration in round-robin queue.

Includes 6 unit tests covering:
- Concurrent limit enforcement
- Rate limit enforcement (sliding window)
- Round-robin fair processing
- In-progress identifier skipping
- Round-robin index adjustment on removal
- Tried URL merging on re-enqueue

2 files changed, 393 insertions, 0 deletions

diff --git a/src/purgatory/sync/mod.rs b/src/purgatory/sync/mod.rs
index 7b6d64a..3c0d875 100644
--- a/src/purgatory/sync/mod.rs
+++ b/src/purgatory/sync/mod.rs
@@ -7,5 +7,7 @@
 //! - Debouncing for burst event arrivals
 
 mod queue;
+mod throttle;
 
 pub use queue::SyncQueueEntry;
+pub use throttle::DomainThrottle;
diff --git a/src/purgatory/sync/throttle.rs b/src/purgatory/sync/throttle.rs
new file mode 100644
index 0000000..6f01c86
--- /dev/null
+++ b/src/purgatory/sync/throttle.rs
@@ -0,0 +1,391 @@
+//! Domain-based rate limiting and identifier queue management.
+//!
+//! This module provides per-domain throttling to prevent overwhelming remote
+//! git servers during purgatory sync operations. Each domain has:
+//! - Concurrent request limit (max in-flight requests)
+//! - Rate limit (max requests per minute)
+//! - Queue of identifiers waiting for capacity (with round-robin processing)
+
+use indexmap::IndexMap;
+use std::collections::{HashSet, VecDeque};
+use std::time::{Duration, Instant};
+
+/// State for an identifier waiting in a domain's queue.
+///
+/// Tracks which URLs from this domain have been tried and whether
+/// a fetch is currently in progress for this identifier.
+#[derive(Debug, Clone)]
+struct IdentifierQueueState {
+    /// URLs from this domain that have been tried.
+    tried_urls: HashSet<String>,
+
+    /// Whether a fetch is currently in progress for this identifier on this domain.
+    ///
+    /// Prevents starting multiple concurrent fetches for the same identifier,
+    /// which is important when the queue is small (e.g., 2 identifiers with 5
+    /// concurrent slots would otherwise try to process the same identifier multiple times).
+    in_progress: bool,
+}
+
+impl IdentifierQueueState {
+    fn new(tried_urls: HashSet<String>) -> Self {
+        Self {
+            tried_urls,
+            in_progress: false,
+        }
+    }
+}
+
+/// Per-domain rate limiting and identifier queue.
+///
+/// Handles:
+/// - Rate limiting (concurrent requests, requests per minute)
+/// - Queue of identifiers waiting for capacity (using IndexMap for round-robin order)
+/// - Tracking tried URLs per identifier (for this domain only)
+/// - In-progress flag per identifier (prevents concurrent fetches for same identifier
+///   on this domain, important when queue is small and we have multiple concurrent slots)
+#[derive(Debug)]
+pub struct DomainThrottle {
+    /// Domain this throttle manages (for debugging/logging).
+    #[allow(dead_code)]
+    domain: String,
+
+    /// Current in-flight request count.
+    in_flight: u32,
+
+    /// Request timestamps (sliding window for rate limiting).
+    request_times: VecDeque<Instant>,
+
+    /// Queued identifiers with their state.
+    /// IndexMap preserves insertion order for round-robin processing.
+    queue: IndexMap<String, IdentifierQueueState>,
+
+    /// Round-robin index for fair processing across identifiers.
+    round_robin_index: usize,
+
+    /// Maximum concurrent requests for this domain.
+    max_concurrent: u32,
+
+    /// Maximum requests per minute for this domain.
+    max_per_minute: u32,
+}
+
+impl DomainThrottle {
+    /// Create a new domain throttle with the specified limits.
+    ///
+    /// # Arguments
+    /// * `domain` - The domain name (for logging)
+    /// * `max_concurrent` - Maximum concurrent in-flight requests
+    /// * `max_per_minute` - Maximum requests per 60-second window
+    pub fn new(domain: String, max_concurrent: u32, max_per_minute: u32) -> Self {
+        Self {
+            domain,
+            in_flight: 0,
+            request_times: VecDeque::new(),
+            queue: IndexMap::new(),
+            round_robin_index: 0,
+            max_concurrent,
+            max_per_minute,
+        }
+    }
+
+    /// Check if domain has capacity for another request.
+    ///
+    /// Returns false if:
+    /// - Already at max concurrent requests
+    /// - Already at max requests per minute (sliding window)
+    pub fn has_capacity(&self) -> bool {
+        // Check concurrent limit
+        if self.in_flight >= self.max_concurrent {
+            return false;
+        }
+
+        // Check rate limit (sliding window of 60 seconds)
+        let now = Instant::now();
+        let window = Duration::from_secs(60);
+        let recent_count = self
+            .request_times
+            .iter()
+            .filter(|t| now.duration_since(**t) < window)
+            .count();
+
+        recent_count < self.max_per_minute as usize
+    }
+
+    /// Check if there are any identifiers in the queue.
+    pub fn has_queued_work(&self) -> bool {
+        !self.queue.is_empty()
+    }
+
+    /// Record that a request is starting.
+    ///
+    /// Increments in-flight count and records timestamp for rate limiting.
+    pub fn start_request(&mut self) {
+        self.in_flight += 1;
+        self.request_times.push_back(Instant::now());
+    }
+
+    /// Record that a request completed.
+    ///
+    /// Decrements in-flight count and cleans up old timestamps.
+    pub fn complete_request(&mut self) {
+        self.in_flight = self.in_flight.saturating_sub(1);
+
+        // Clean old timestamps outside the 60-second window
+        let now = Instant::now();
+        let window = Duration::from_secs(60);
+        while self
+            .request_times
+            .front()
+            .map_or(false, |t| now.duration_since(*t) >= window)
+        {
+            self.request_times.pop_front();
+        }
+    }
+
+    /// Add an identifier to the queue.
+    ///
+    /// If the identifier is already queued, merges the tried_urls sets.
+    ///
+    /// # Arguments
+    /// * `identifier` - The repository identifier
+    /// * `tried_urls` - URLs from this domain that have already been tried
+    pub fn enqueue_identifier(&mut self, identifier: String, tried_urls: HashSet<String>) {
+        self.queue
+            .entry(identifier)
+            .and_modify(|state| {
+                // Merge tried_urls if already exists
+                state.tried_urls.extend(tried_urls.iter().cloned());
+            })
+            .or_insert(IdentifierQueueState::new(tried_urls));
+    }
+
+    /// Get next identifier ready for processing (round-robin, not in_progress).
+    ///
+    /// Iterates through the queue starting from round_robin_index, skipping
+    /// any identifiers that are already in_progress. This ensures fair
+    /// distribution even when some identifiers have active fetches.
+    ///
+    /// Returns the identifier and marks it as in_progress.
+    pub fn next_ready_identifier(&mut self) -> Option<String> {
+        let len = self.queue.len();
+        if len == 0 {
+            return None;
+        }
+
+        // Try each identifier starting from round_robin_index
+        for i in 0..len {
+            let index = (self.round_robin_index + i) % len;
+            if let Some((identifier, state)) = self.queue.get_index_mut(index) {
+                if !state.in_progress {
+                    state.in_progress = true;
+                    self.round_robin_index = (index + 1) % len;
+                    return Some(identifier.clone());
+                }
+            }
+        }
+
+        None // All identifiers are in_progress
+    }
+
+    /// Get tried URLs for an identifier.
+    pub fn get_tried_urls(&self, identifier: &str) -> HashSet<String> {
+        self.queue
+            .get(identifier)
+            .map(|s| s.tried_urls.clone())
+            .unwrap_or_default()
+    }
+
+    /// Mark a URL as tried for an identifier.
+    pub fn mark_url_tried(&mut self, identifier: &str, url: String) {
+        if let Some(state) = self.queue.get_mut(identifier) {
+            state.tried_urls.insert(url);
+        }
+    }
+
+    /// Mark identifier as not in progress (fetch completed).
+    pub fn mark_identifier_not_in_progress(&mut self, identifier: &str) {
+        if let Some(state) = self.queue.get_mut(identifier) {
+            state.in_progress = false;
+        }
+    }
+
+    /// Remove an identifier from the queue entirely.
+    ///
+    /// Adjusts round_robin_index if needed to maintain fair processing.
+    pub fn remove_identifier(&mut self, identifier: &str) {
+        if let Some((index, _, _)) = self.queue.shift_remove_full(identifier) {
+            // Adjust round_robin_index if we removed an entry before it
+            if index < self.round_robin_index && self.round_robin_index > 0 {
+                self.round_robin_index -= 1;
+            }
+            // Clamp to valid range
+            if !self.queue.is_empty() {
+                self.round_robin_index %= self.queue.len();
+            } else {
+                self.round_robin_index = 0;
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn concurrent_limit_blocks_when_saturated() {
+        let mut throttle = DomainThrottle::new("example.com".to_string(), 3, 100);
+
+        // Initially has capacity
+        assert!(throttle.has_capacity());
+
+        // Start 3 requests (at limit)
+        throttle.start_request();
+        throttle.start_request();
+        throttle.start_request();
+
+        // Should be at capacity now
+        assert!(!throttle.has_capacity());
+
+        // Complete one request
+        throttle.complete_request();
+
+        // Should have capacity again
+        assert!(throttle.has_capacity());
+    }
+
+    #[test]
+    fn rate_limit_blocks_when_window_full() {
+        // Use a very small rate limit for testing
+        let mut throttle = DomainThrottle::new("example.com".to_string(), 100, 2);
+
+        // Initially has capacity
+        assert!(throttle.has_capacity());
+
+        // Make 2 requests (at rate limit)
+        throttle.start_request();
+        throttle.complete_request();
+        throttle.start_request();
+        throttle.complete_request();
+
+        // Should be at rate limit now (2 requests in last 60s)
+        assert!(!throttle.has_capacity());
+
+        // Note: In a real test we'd need to wait 60 seconds or mock time
+        // For this test, we just verify the blocking behavior
+    }
+
+    #[test]
+    fn round_robin_processes_identifiers_fairly() {
+        let mut throttle = DomainThrottle::new("example.com".to_string(), 5, 100);
+
+        // Enqueue A, B, C
+        throttle.enqueue_identifier("A".to_string(), HashSet::new());
+        throttle.enqueue_identifier("B".to_string(), HashSet::new());
+        throttle.enqueue_identifier("C".to_string(), HashSet::new());
+
+        // First round: should get A, B, C in order
+        let first = throttle.next_ready_identifier();
+        assert_eq!(first, Some("A".to_string()));
+        throttle.mark_identifier_not_in_progress("A");
+
+        let second = throttle.next_ready_identifier();
+        assert_eq!(second, Some("B".to_string()));
+        throttle.mark_identifier_not_in_progress("B");
+
+        let third = throttle.next_ready_identifier();
+        assert_eq!(third, Some("C".to_string()));
+        throttle.mark_identifier_not_in_progress("C");
+
+        // Second round: should cycle back to A, B, C
+        let fourth = throttle.next_ready_identifier();
+        assert_eq!(fourth, Some("A".to_string()));
+        throttle.mark_identifier_not_in_progress("A");
+
+        let fifth = throttle.next_ready_identifier();
+        assert_eq!(fifth, Some("B".to_string()));
+    }
+
+    #[test]
+    fn skips_in_progress_identifiers() {
+        let mut throttle = DomainThrottle::new("example.com".to_string(), 5, 100);
+
+        // Enqueue A, B, C
+        throttle.enqueue_identifier("A".to_string(), HashSet::new());
+        throttle.enqueue_identifier("B".to_string(), HashSet::new());
+        throttle.enqueue_identifier("C".to_string(), HashSet::new());
+
+        // Get A (marks it in_progress)
+        let first = throttle.next_ready_identifier();
+        assert_eq!(first, Some("A".to_string()));
+
+        // Get B (A is still in_progress)
+        let second = throttle.next_ready_identifier();
+        assert_eq!(second, Some("B".to_string()));
+
+        // Get C (A and B are in_progress)
+        let third = throttle.next_ready_identifier();
+        assert_eq!(third, Some("C".to_string()));
+
+        // All are in_progress now, should return None
+        let fourth = throttle.next_ready_identifier();
+        assert_eq!(fourth, None);
+
+        // Mark A as not in_progress
+        throttle.mark_identifier_not_in_progress("A");
+
+        // Should get A again (it's the only one not in_progress)
+        let fifth = throttle.next_ready_identifier();
+        assert_eq!(fifth, Some("A".to_string()));
+    }
+
+    #[test]
+    fn remove_identifier_adjusts_round_robin_index() {
+        let mut throttle = DomainThrottle::new("example.com".to_string(), 5, 100);
+
+        // Enqueue A, B, C, D
+        throttle.enqueue_identifier("A".to_string(), HashSet::new());
+        throttle.enqueue_identifier("B".to_string(), HashSet::new());
+        throttle.enqueue_identifier("C".to_string(), HashSet::new());
+        throttle.enqueue_identifier("D".to_string(), HashSet::new());
+
+        // Get A (round_robin_index now points to B)
+        let first = throttle.next_ready_identifier();
+        assert_eq!(first, Some("A".to_string()));
+        throttle.mark_identifier_not_in_progress("A");
+
+        // Get B (round_robin_index now points to C)
+        let second = throttle.next_ready_identifier();
+        assert_eq!(second, Some("B".to_string()));
+        throttle.mark_identifier_not_in_progress("B");
+
+        // Remove A (before current index)
+        throttle.remove_identifier("A");
+
+        // Next should be C (not B again, index was adjusted)
+        let third = throttle.next_ready_identifier();
+        assert_eq!(third, Some("C".to_string()));
+    }
+
+    #[test]
+    fn enqueue_merges_tried_urls() {
+        let mut throttle = DomainThrottle::new("example.com".to_string(), 5, 100);
+
+        // First enqueue with some tried URLs
+        let mut tried1 = HashSet::new();
+        tried1.insert("url1".to_string());
+        throttle.enqueue_identifier("A".to_string(), tried1);
+
+        // Second enqueue with different tried URLs
+        let mut tried2 = HashSet::new();
+        tried2.insert("url2".to_string());
+        throttle.enqueue_identifier("A".to_string(), tried2);
+
+        // Should have both URLs
+        let tried = throttle.get_tried_urls("A");
+        assert!(tried.contains("url1"));
+        assert!(tried.contains("url2"));
+        assert_eq!(tried.len(), 2);
+    }
+}