Zcashd will blindly request more block headers as long as it got 160
block headers in response to a previous query, EVEN IF THOSE HEADERS ARE
ALREADY KNOWN. To dodge this behavior, return slightly fewer than the
maximum, to get it to go away.
0ccc885371/src/main.cpp (L6274-L6280)
Without this change, communication between a partially-synced `zebrad`
and fully-synced `zcashd` looked like this:
1. `zebrad` connects to `zcashd`, which sends an initial `getheaders`
request;
2. `zebrad` correctly computes the intersection of the provided block
locator with the node's current chain and returns 160 following
headers;
3. `zcashd` does not check whether it already has those headers and
assumes that any provided headers are new and re-validates them;
4. `zcashd` assumes that because `zebrad` responded with 160 headers,
the `zebrad` node is ahead of it, and requests the next 160 headers.
5. Because block locators are sparse, the intersection between the
`zcashd` and `zebrad` chains is likely well behind the `zebrad` tip,
so this process continues for thousands of blocks.
To avoid this problem, we return slightly fewer than the protocol
maximum (158 rather than 160, to guard against off-by-one errors in
zcashd). This does not interfere with use of the returned headers by
peers that check the headers, but does prevent `zcashd` from trying to
download thousands of block headers it already has.
This problem does not occur in the `zcashd<->zcashd` case only because
`zcashd` does not respond to `getheaders` messages while it is syncing.
However, implementing this behavior in Zebra would be more complicated,
because we don't have a distinct "initial block sync" state (we do
poll-based syncing continuously) and we don't have shared global
variables to modify to set that state.
Relevant links (thanks @str4d):
- The PR that introduced this behavior: https://github.com/bitcoin/bitcoin/pull/4468/files#r17026905
- https://github.com/bitcoin/bitcoin/issues/6861
- https://github.com/bitcoin/bitcoin/issues/6755
- https://github.com/bitcoin/bitcoin/pull/8306#issuecomment-614916454
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