Add warning of witness presistence race condition

tests/core/p2p-proto/test_wit_db.py

@@ -94,3 +94,10 @@ async def test_witness_union():
 
     expected = hash1_witnesses_unique1 | hash1_witnesses_both | hash1_witnesses_unique2
     assert set(stored_hashes) == expected
+
+
+# Things that should be true
+#   - saving two hash sets should never erase the results of one of them (be sure to handle race conditions)
+#   - eventually: get stats like Counter of the hashes
+#   - history should not shorten by getting multiple results for the same blockhash
+#   - get the union of all received hashes for a given blockhash

trinity/protocol/wit/db.py

@@ -69,6 +69,10 @@ def persist_witness_hashes(
             recent_blocks_with_witnesses)
 
         try:
+            # Note: if this call is converted to async, watch out for the race
+            #   condition of two persists that interleave. It would be
+            #   possible for one to overwrite the other. For now, the synchronous
+            #   approach means that that isn't a concern.
             existing_hashes = self.get_witness_hashes(block_hash)
         except WitnessHashesUnavailable:
             existing_hashes = ()