sync: a test exchanging until level, not one answer

delivered_messages_are_answered_with_changes_we_lack failed now and
then: after the answer, the node still lacked the peer's change.
Automerge's sync tells what each side has by a Bloom filter, whose
false positives take more rounds. The test now exchanges up to four
rounds more, each asking for what is still missing, until both are
level.

1 files changed, 17 insertions, 0 deletions

diff --git a/src/document/sync.rs b/src/document/sync.rs
index 440718c..a7a466b 100644
--- a/src/document/sync.rs
+++ b/src/document/sync.rs
@@ -1707,6 +1707,23 @@ mod tests {
         let answer = b.generate(&theirs, now, bytes).expect("b answers");
         b.send_succeeded(b.id());
         a.receive(&mut ours, decode(&answer).unwrap()).unwrap();
+        // The answer may not bring what we lack: Automerge's sync tells what each side has
+        // by a Bloom filter, whose false positives (about one in a hundred) take more
+        // rounds, each asking for what is still missing.
+        for _ in 0..4 {
+            if ours.get_heads() == theirs.get_heads() {
+                break;
+            }
+            let ask = a
+                .generate(&ours, now, bytes)
+                .expect("a asks for what it lacks");
+            a.send_succeeded(a.id());
+            b.receive(&mut theirs, decode(&ask).unwrap()).unwrap();
+            if let Some(lacking) = b.generate(&theirs, now, bytes) {
+                b.send_succeeded(b.id());
+                a.receive(&mut ours, decode(&lacking).unwrap()).unwrap();
+            }
+        }
         assert_eq!(ours.get_heads(), theirs.get_heads());
     }