Allow block announcements with headers

This replaces using inv messages to announce new blocks, when a peer requests
(via the new "sendheaders" message) that blocks be announced with headers
instead of inv's.

Since headers-first was introduced, peers send getheaders messages in response
to an inv, which requires generating a block locator that is large compared to
the size of the header being requested, and requires an extra round-trip before
a reorg can be relayed.  Save time by tracking headers that a peer is likely to
know about, and send a headers chain that would connect to a peer's known
headers, unless the chain would be too big, in which case we revert to sending
an inv instead.

Based off of @sipa's commit to announce all blocks in a reorg via inv,
which has been squashed into this commit.

Rebased-by: Pieter Wuille

1 files changed, 3 insertions, 0 deletions

diff --git a/src/main.h b/src/main.h
index dfa2fcb537..3dec613fc7 100644
--- a/src/main.h
+++ b/src/main.h
@@ -98,6 +98,9 @@ static const unsigned int DEFAULT_BANSCORE_THRESHOLD = 100;
 
 static const bool DEFAULT_TESTSAFEMODE = false;
 
+/** Maximum number of headers to announce when relaying blocks with headers message.*/
+static const unsigned int MAX_BLOCKS_TO_ANNOUNCE = 8;
+
 struct BlockHasher
 {
     size_t operator()(const uint256& hash) const { return hash.GetCheapHash(); }