Randomize repair requests (#1059)

* randomize packet repair requests

* exponential random repair requests

* use gen_range to get a uniform distribution

src/window.rs

@@ -8,6 +8,7 @@ use erasure;
 use ledger::Block;
 use log::Level;
 use packet::{BlobRecycler, SharedBlob, SharedBlobs, BLOB_SIZE};
+use rand::{thread_rng, Rng};
 use result::{Error, Result};
 use signature::Pubkey;
 use std::cmp;
@@ -92,6 +93,27 @@ fn calculate_highest_lost_blob_index(num_peers: u64, consumed: u64, received: u6
     cmp::min(consumed + WINDOW_SIZE - 1, highest_lost)
 }
 
+fn repair_backoff(last: &mut u64, times: &mut usize, consumed: u64) -> bool {
+    //exponential backoff
+    if *last != consumed {
+        //start with a 50% chance of asking for repairs
+        *times = 1;
+    }
+    *last = consumed;
+    *times += 1;
+
+    // Experiment with capping repair request duration.
+    // Once nodes are too far behind they can spend many
+    // seconds without asking for repair
+    if *times > 128 {
+        // 50% chance that a request will fire between 64 - 128 tries
+        *times = 64;
+    }
+
+    //if we get lucky, make the request, which should exponentially get less likely
+    thread_rng().gen_range(0, *times as u64) == 0
+}
+
 fn repair_window(
     debug_id: u64,
     window: &SharedWindow,
@@ -103,22 +125,7 @@ fn repair_window(
     received: u64,
 ) -> Result<()> {
     //exponential backoff
-    if *last != consumed {
+    if !repair_backoff(last, times, consumed) {
-        *times = 0;
-    }
-    *last = consumed;
-    *times += 1;
-
-    // Experiment with capping repair request duration.
-    // Once nodes are too far behind they can spend many
-    // seconds without asking for repair
-    if *times > 128 {
-        *times = 65;
-    }
-
-    //if times flips from all 1s 7 -> 8, 15 -> 16, we retry otherwise return Ok
-    if *times & (*times - 1) != 0 {
-        trace!("repair_window counter {} {} {}", *times, consumed, received);
         return Ok(());
     }
 
@@ -697,7 +704,8 @@ mod test {
     use std::time::Duration;
     use streamer::{blob_receiver, receiver, responder, BlobReceiver, PacketReceiver};
     use window::{
-        blob_idx_in_window, calculate_highest_lost_blob_index, default_window, window, WINDOW_SIZE,
+        blob_idx_in_window, calculate_highest_lost_blob_index, default_window, repair_backoff,
+        window, WINDOW_SIZE,
     };
 
     fn get_msgs(r: PacketReceiver, num: &mut usize) {
@@ -1048,4 +1056,25 @@ mod test {
         assert_eq!(wrap_blob_idx_in_window(0, 91, 90, 100), (true, 100));
         assert_eq!(wrap_blob_idx_in_window(0, 101, 90, 100), (true, 101));
     }
+    #[test]
+    pub fn test_repair_backoff() {
+        let mut last = 0;
+        let mut times = 0;
+        let total: usize = (0..127)
+            .map(|x| {
+                let rv = repair_backoff(&mut last, &mut times, 1) as usize;
+                assert_eq!(times, x + 2);
+                rv
+            })
+            .sum();
+        assert_eq!(times, 128);
+        assert_eq!(last, 1);
+        assert!(total > 0);
+        assert!(total < 127);
+        repair_backoff(&mut last, &mut times, 1);
+        assert_eq!(times, 64);
+        repair_backoff(&mut last, &mut times, 2);
+        assert_eq!(times, 2);
+        assert_eq!(last, 2);
+    }
 }