net: make room in the block fetch table instead of refusing new blocks

Nothing on the path from inv to insertion can tell a real block hash
from thirty-two random bytes. AlreadyHave only asks whether we already
hold the block, and with no header there is no work to check. So one inv
carrying eight hashes that never existed filled all eight slots, and
because a full table refused new entries outright, no genuinely new
block was tracked again until the give-up timer expired ten minutes
later. One message per ten minutes from a single connection switched
failover off for the whole node -- not eight wasted slots, an off-switch.

A full table now evicts. Eviction on its own would just be the same
attack in a new shape, so the victim is chosen by occupancy: a peer may
only take a slot from a peer holding strictly more slots than itself,
and otherwise recycles one of its own oldest. A peer that has filled the
table is the heaviest holder by definition, so its own entries are the
only ones it can displace; a peer announcing its first block holds
nothing, so it always evicts from the heaviest. A genuine announcement
is therefore never refused a slot, whatever an attacker holds or across
however many connections it holds it.

Entries carry the announcing peer for this, set once on creation --
the failover sweep asks other peers later and none of them chose to
announce the hash, so none of them should be charged for it.

The two timestamps also get member initialisers. The insertion path uses
operator[] and tests nTimeFirstAsked == 0 to spot a fresh entry, which
worked on value-initialisation alone, but only until somebody gave the
struct a constructor -- and this commit gives it a member that wants one.

0 files changed, 0 insertions, 0 deletions