chore: regression testing for unique id (#875)

Add the ability to mock time in unique id generator so we can test it
better.

Author: levkk

pgdog/src/unique_id.rs

@@ -43,13 +43,20 @@ impl State {
     // Generate next unique ID in a distributed sequence.
     // The `node_id` argument must be globally unique.
     fn next_id(&mut self, node_id: u64, id_offset: u64) -> u64 {
-        let mut now = wait_until(self.last_timestamp_ms);
+        self.next_id_with(node_id, id_offset, &mut now_ms)
+    }
+
+    fn next_id_with<F>(&mut self, node_id: u64, id_offset: u64, now_ms: &mut F) -> u64
+    where
+        F: FnMut() -> u64,
+    {
+        let mut now = wait_until_with(self.last_timestamp_ms, now_ms);
 
         if now == self.last_timestamp_ms {
             self.sequence = (self.sequence + 1) & MAX_SEQUENCE;
             // Wraparound.
             if self.sequence == 0 {
-                now = wait_until(now + 1);
+                now = wait_until_with(now + 1, now_ms);
             }
         } else {
             // Reset sequence to zero once we reach next ms.
@@ -80,9 +87,10 @@ fn now_ms() -> u64 {
         .as_millis() as u64
 }
 
-// Get a monotonically increasing timestamp in ms.
-// Protects against clock drift.
-fn wait_until(target_ms: u64) -> u64 {
+fn wait_until_with<F>(target_ms: u64, now_ms: &mut F) -> u64
+where
+    F: FnMut() -> u64,
+{
     loop {
         let now = now_ms();
         if now >= target_ms {
@@ -150,6 +158,29 @@ mod test {
 
     use super::*;
 
+    fn decode_id(id: u64) -> (u64, u64, u64) {
+        let sequence = id & MAX_SEQUENCE;
+        let node_id = (id >> NODE_SHIFT) & MAX_NODE_ID;
+        let elapsed_ms = id >> TIMESTAMP_SHIFT;
+        (elapsed_ms, node_id, sequence)
+    }
+
+    fn fake_clock(readings: &[u64]) -> impl FnMut() -> u64 + '_ {
+        assert!(
+            !readings.is_empty(),
+            "fake clock requires at least one reading"
+        );
+
+        let mut index = 0;
+        let last = readings[readings.len() - 1];
+
+        move || {
+            let reading = readings.get(index).copied().unwrap_or(last);
+            index += 1;
+            reading
+        }
+    }
+
     #[test]
     fn test_unique_ids() {
         unsafe {
@@ -238,6 +269,65 @@ mod test {
         assert!(matches!(extracted_seq2, 1 | 0)); // Sequence incremented (or time advanced and reset to 0)
     }
 
+    #[test]
+    fn test_regression_fixed_clock_ids() {
+        let node = 42;
+        let now = 1_775_590_268_126;
+        let mut state = State::default();
+        let readings = [now, now, now + 1];
+        let mut now_ms = fake_clock(&readings);
+
+        let first = state.next_id_with(node, 0, &mut now_ms);
+        let second = state.next_id_with(node, 0, &mut now_ms);
+        let third = state.next_id_with(node, 0, &mut now_ms);
+
+        assert_eq!(first, 47_839_699_276_046_336);
+        assert_eq!(second, 47_839_699_276_046_337);
+        assert_eq!(third, 47_839_699_280_240_640);
+
+        assert_eq!(decode_id(first), (11_405_873_126, node, 0));
+        assert_eq!(decode_id(second), (11_405_873_126, node, 1));
+        assert_eq!(decode_id(third), (11_405_873_127, node, 0));
+    }
+
+    #[test]
+    fn test_regression_sequence_wrap_waits_for_next_millisecond() {
+        let node = 7;
+        let elapsed_ms = 99;
+        let now = PGDOG_EPOCH + elapsed_ms;
+        let mut state = State {
+            last_timestamp_ms: now,
+            sequence: MAX_SEQUENCE - 1,
+        };
+        let readings = [now, now, now, now + 1];
+        let mut now_ms = fake_clock(&readings);
+
+        let last_in_millisecond = state.next_id_with(node, 0, &mut now_ms);
+        let first_in_next_millisecond = state.next_id_with(node, 0, &mut now_ms);
+
+        assert_eq!(last_in_millisecond, 415_268_863);
+        assert_eq!(first_in_next_millisecond, 419_459_072);
+        assert!(
+            first_in_next_millisecond > last_in_millisecond,
+            "IDs must remain monotonic across sequence wrap"
+        );
+    }
+
+    #[test]
+    fn test_regression_offset_is_applied_after_packed_id() {
+        let node = 3;
+        let elapsed_ms = 321;
+        let offset = 1_000_000;
+        let now = PGDOG_EPOCH + elapsed_ms;
+        let mut state = State::default();
+        let readings = [now];
+        let mut now_ms = fake_clock(&readings);
+
+        let id = state.next_id_with(node, offset, &mut now_ms);
+
+        assert_eq!(id, 1_347_383_872);
+    }
+
     #[test]
     fn test_id_offset() {
         let offset: u64 = 1_000_000_000;