auto peer discovery: gossip known peers during sync, discover from_hash automatically

tests/test_sync.py

@@ -334,6 +334,50 @@ def test_sync_updated_content():
         shutil.rmtree(dir_b)
 
 
+def test_sync_peer_discovery():
+    """Peers should discover each other through sync gossip."""
+    dir_a = tempfile.mkdtemp()
+    dir_b = tempfile.mkdtemp()
+    dir_c = tempfile.mkdtemp()
+    try:
+        db_a, ha = make_handler(dir_a)
+        db_b, hb = make_handler(dir_b)
+        db_c, hc = make_handler(dir_c)
+
+        now = "2026-06-05T12:00:00"
+        db_a.create_thread("t1", "From A", "", "", "", "aaa", "", now)
+        db_b.create_thread("t2", "From B", "", "", "", "bbb", "", now)
+        db_c.create_thread("t3", "From C", "", "", "", "ccc", "", now)
+
+        # Seed: A knows B, B knows C
+        db_a.add_known_peer("bbb")
+        db_b.add_known_peer("ccc")
+        # C doesn't know anyone yet
+
+        # B syncs with C — B sends its known peers (ccc's hash not in B's known peers since B is talking to C)
+        # Actually, B knows C (bbb -> ccc), so when B sends sync request to C,
+        # B includes known_peers. C learns about B.
+        hc.handle_sync_request({
+            "query": {},
+            "threads": [dict(db_b.get_thread("t2"))],
+            "posts": [], "upvotes": [],
+            "from_hash": "bbb",
+            "blocks": {"mine": [], "peers": []},
+            "retractions": [],
+            "known_peers": ["aaa"],  # B tells C about A
+        })
+
+        # C should now know about B (from auto-discovery of from_hash) and A (from known_peers)
+        known = [r["instance_hash"] for r in db_c.get_synced_instances()]
+        assert "bbb" in known, "C should auto-discover B from from_hash"
+        assert "aaa" in known, "C should discover A from known_peers gossip"
+    finally:
+        import shutil
+        shutil.rmtree(dir_a)
+        shutil.rmtree(dir_b)
+        shutil.rmtree(dir_c)
+
+
 if __name__ == "__main__":
     test_sync_thread()
     test_sync_reply()
@@ -343,4 +387,5 @@ if __name__ == "__main__":
     test_sync_bidirectional()
     test_sync_block_gossip()
     test_sync_updated_content()
+    test_sync_peer_discovery()
     print("all sync tests passed")