In spite of the name FindLatestBefore used std::lower_bound to try
to find the earliest block with a nTime greater or equal to the
the requested value. But lower_bound uses bisection and requires
the input to be ordered with respect to the comparison operation.
Block times are not well ordered.
I don't know what lower_bound is permitted to do when the data
is not sufficiently ordered, but it's probably not good.
(I could construct an implementation which would infinite loop...)
To resolve the issue this commit introduces a maximum-so-far to the
block indexes and searches that.
For clarity the function is renamed to reflect what it actually does.
An issue that remains is that there is no grace period in importmulti:
If a address is created at time T and a send is immediately broadcast
and included by a miner with a slow clock there may not yet have been
any block with at least time T.
The normal rescan has a grace period of 7200 seconds, but importmulti
does not.
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
Instead of only checking height to decide whether to disable script checks,
actually check whether a block is an ancestor of a checkpoint, up to which
headers have been validated. This means that we don't have to prevent
accepting a side branch anymore - it will be safe, just less fast to
do.
We still need to prevent being fed a multitude of low-difficulty headers
filling up our memory. The mechanism for that is unchanged for now: once
a checkpoint is reached with headers, no headers chain branching off before
that point are allowed anymore.
Gregory Maxwell