Merge #15990: Add tests and documentation for blocksonly

fa8ced32a6 doc: Mention blocksonly in reduce-traffic.md, unhide option (MarcoFalke)
fa320de79f test: Add test for p2p_blocksonly (MarcoFalke)
fa3872e7b4 test: Format predicate source as multiline on error (MarcoFalke)
fa1dce7329 net: Rename ::fRelayTxes to ::g_relay_txes (MarcoFalke)

Pull request description:

  This is de-facto no longer hidden

ACKs for commit fa8ced:
  jamesob:
    utACK https://github.com/bitcoin/bitcoin/commit/fa8ced32a60dea37ac169241cf9a1f708ef46c4b

Tree-SHA512: 474fbdee6cbd035ed9068a066b6056c1f909ec7520be0417820fcd1672ab3069b53f55c5147968978d9258fd3a3933fe1a9ef8e4f6e14fb6ebbd79701a0a1245

test/functional/test_framework/mininode.py

@@ -361,6 +361,14 @@ class P2PInterface(P2PConnection):
 
     # Message receiving helper methods
 
+    def wait_for_tx(self, txid, timeout=60):
+        def test_function():
+            if not self.last_message.get('tx'):
+                return False
+            return self.last_message['tx'].tx.rehash() == txid
+
+        wait_until(test_function, timeout=timeout, lock=mininode_lock)
+
     def wait_for_block(self, blockhash, timeout=60):
         test_function = lambda: self.last_message.get("block") and self.last_message["block"].block.rehash() == blockhash
         wait_until(test_function, timeout=timeout, lock=mininode_lock)

test/functional/test_framework/util.py

@@ -216,7 +216,7 @@ def wait_until(predicate, *, attempts=float('inf'), timeout=float('inf'), lock=N
         time.sleep(0.05)
 
     # Print the cause of the timeout
-    predicate_source = inspect.getsourcelines(predicate)
+    predicate_source = "''''\n" + inspect.getsource(predicate) + "'''"
     logger.error("wait_until() failed. Predicate: {}".format(predicate_source))
     if attempt >= attempts:
         raise AssertionError("Predicate {} not true after {} attempts".format(predicate_source, attempts))