Add 'mempool' P2P command, and extend 'getdata' behavior

[jgarzik]

Add 'mempool' P2P command, and extend 'getdata' behavior to permit downloading of mempool transactions from the remote peer.

[jgarzik]

@gmaxwell was asking about testing. This was tested successfully with a one-line pynode patch (pynode HEAD deb07f51435cd0d18596bddfee28e337a5a6454e). pynode issues 'mempool' at startup, and fills its own mempool with the returned results.

--- a/node.py
+++ b/node.py
@@ -238,6 +238,7 @@ class NodeConn(asyncore.dispatcher):
                        self.send_message(msg_verack(self.ver_send))
                        if self.ver_send >= CADDR_TIME_VERSION:
                                self.send_message(msg_getaddr(self.ver_send))
+                       self.send_message(msg_mempool())
                        self.send_getblocks()

[jgarzik]

Random thought: miners may want this, to "prime the pump" after restarting their bitcoind. There is the incentive to capture every fee-paying transactions, after all.

[BitcoinPullTester]

Automatic sanity-testing: PASSED, see http://jenkins.bluematt.me/pull-tester/05a85b2b38d516a8701f684fcd2ab99f3e5b462d for binaries and test log.

[sipa]

ACK on the changes, but maybe there must at least be some discussion about adding this, and the protocol version change involved. An alternative would be using a service bit (I don't personally think that is preferable), but people may have other opinions.

I would expect the startup sequence to also send a mempool command to another node, in case it reports protocol version 60002 or higher?

[BitcoinPullTester]

Automatic sanity-testing: PASSED, see http://jenkins.bluematt.me/pull-tester/05f891636199c36e1639fb95f5e92445eea9e97f for binaries and test log.

[jgarzik]

Specification: https://en.bitcoin.it/wiki/BIP_0035

[sipa]

ACK

[gavinandresen]

ACK

[ghost]

1989