Merge #19572: ZMQ: Create "sequence" notifier, enabling client-side mempool tracking

759d94e70f Update zmq notification documentation and sample consumer (Gregory Sanders)
68c3c7e1bd Add functional tests for zmq sequence topic and mempool sequence logic (Gregory Sanders)
e76fc2b84d Add 'sequence' zmq publisher to track all block (dis)connects, mempool deltas (Gregory Sanders)
1b615e61bf zmq test: Actually make reorg occur (Gregory Sanders)

Pull request description:

  This PR creates a new ZMQ notifier that gives a "total hash history" of block (dis)connection, mempool addition/substraction, all in one pipeline. It also exposes a "mempool sequence number" to both this notifier and `getrawmempool` results, which allows the consumer to use the results together without confusion about ordering of results and without excessive `getrawmempool` polling.

  See the functional test `interfaces_zmq.py::test_mempool_sync` which shows the proposed user flow for the client-side tracking of mempool contents and confirmations.

  Inspired by https://github.com/bitcoin/bitcoin/pull/19462#issuecomment-656140421
  Alternative to https://github.com/bitcoin/bitcoin/pull/19462 due to noted deficiencies in current zmq notification streams.

  Also fixes a legacy zmq test that didn't actually trigger a reorg because of identical blocks being generated on each side of the split(oops)

ACKs for top commit:
  laanwj:
    Code review ACK 759d94e70f

Tree-SHA512: 9daf0d7d996190f3a68ff40340a687519323d7a6c51dcb26be457fbc013217ea7b62fbd0700b74b654433d2e370704feb61e5584399290692464fcfcb72ce3b7

doc/zmq.md

@@ -63,6 +63,7 @@ Currently, the following notifications are supported:
     -zmqpubhashblock=address
     -zmqpubrawblock=address
     -zmqpubrawtx=address
+    -zmqpubsequence=address
 
 The socket type is PUB and the address must be a valid ZeroMQ socket
 address. The same address can be used in more than one notification.
@@ -74,6 +75,7 @@ The option to set the PUB socket's outbound message high water mark
     -zmqpubhashblockhwm=n
     -zmqpubrawblockhwm=n
     -zmqpubrawtxhwm=n
+    -zmqpubsequencehwm=address
 
 The high water mark value must be an integer greater than or equal to 0.
 
@@ -87,7 +89,15 @@ Each PUB notification has a topic and body, where the header
 corresponds to the notification type. For instance, for the
 notification `-zmqpubhashtx` the topic is `hashtx` (no null
 terminator) and the body is the transaction hash (32
-bytes).
+bytes) for all but `sequence` topic. For `sequence`, the body
+is structured as the following based on the type of message:
+
+    <32-byte hash>C :                 Blockhash connected
+    <32-byte hash>D :                 Blockhash disconnected
+    <32-byte hash>R<8-byte LE uint> : Transactionhash removed from mempool for non-block inclusion reason
+    <32-byte hash>A<8-byte LE uint> : Transactionhash added mempool
+
+Where the 8-byte uints correspond to the mempool sequence number.
 
 These options can also be provided in bitcoin.conf.
 
@@ -124,13 +134,20 @@ No authentication or authorization is done on connecting clients; it
 is assumed that the ZeroMQ port is exposed only to trusted entities,
 using other means such as firewalling.
 
-Note that when the block chain tip changes, a reorganisation may occur
-and just the tip will be notified. It is up to the subscriber to
-retrieve the chain from the last known block to the new tip. Also note
-that no notification occurs if the tip was in the active chain - this
-is the case after calling invalidateblock RPC.
+Note that for `*block` topics, when the block chain tip changes,
+a reorganisation may occur and just the tip will be notified.
+It is up to the subscriber to retrieve the chain from the last known
+block to the new tip. Also note that no notification will occur if the tip
+was in the active chain--as would be the case after calling invalidateblock RPC.
+In contrast, the `sequence` topic publishes all block connections and
+disconnections.
 
 There are several possibilities that ZMQ notification can get lost
 during transmission depending on the communication type you are
 using. Bitcoind appends an up-counting sequence number to each
 notification which allows listeners to detect lost notifications.
+
+The `sequence` topic refers specifically to the mempool sequence
+number, which is also published along with all mempool events. This
+is a different sequence value than in ZMQ itself in order to allow a total
+ordering of mempool events to be constructed.