net: only use the 15s block retry spacing at the tip

nRequestTime lives in mapAlreadyAskedFor, which is global to the inv, so
the spacing AskFor applies is not per-peer pacing: it is what strings the
getdatas for one block across its announcers. Shortening it to 15s was
right at the tip and wrong everywhere else, and the eight-entry cap that
made the failover table safe for catch-up does not cover this at all --
it is a different mechanism.

At two minutes a second announcer's queued request almost always fired
after the block had arrived, so AlreadyHave dropped it. At fifteen
seconds, with a peer working through a 500-deep queue and block N
legitimately minutes away, the second announcer is asked at +15s, the
third at +30s, and the bulk download arrives once per announcer -- up to
eight times over, with nothing bounding it.

So the fast spacing is now conditional on tracking the tip, measured off
the age of our own best block. Not off a peer-reported height:
nStartingHeight is optional in the version message and every node
released so far sends the short form, so it is -1 for essentially every
peer, and where it does get filled in later it comes from the height of
a block that peer served us, which during catch-up tracks our own height
and would answer "at the tip" for the entire download. Block timestamps
are the one clock a peer cannot move, being bounded above by CheckBlock
and below by AcceptBlock's median-time rule.

Thirty block intervals is the threshold: far longer than any stall worth
failing over from, far shorter than catching up. Failover at the tip
does not rest on this in any case -- mapBlockFetch re-issues a stalled
getdata to the next peer on its own 15s timer, and that is what actually
answers #33.

1 files changed, 23 insertions, 0 deletions

diff --git a/src/main.cpp b/src/main.cpp
index 9ab1f44..be89416 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -101,6 +101,29 @@ static const int BLOCK_FETCH_TRACK_MAX = 8;
 // forever; a re-announcement recreates it.
 static const int BLOCK_FETCH_GIVEUP_SECS = 10 * 60;
 
+// How stale our own best block may be before we stop calling this the tip.
+// Blocks target two minutes, so this is thirty intervals: far longer than any
+// stall worth failing over from -- the one in #33 ran sixteen minutes -- and
+// far shorter than catching up, which starts hours or days back.
+static const int BLOCK_FETCH_TIP_AGE_SECS = 60 * 60;
+
+// Whether this node is tracking the tip rather than still catching up.
+//
+// Measured off the age of our own best block, not off any height a peer told
+// us. nStartingHeight is optional in the version message and every node
+// released so far sends the short form, so it is -1 for essentially every
+// peer; and where it is filled in later it is filled in from the height of a
+// block that peer served us, which during catch-up tracks our own height and
+// would report "at the tip" for the whole download. Block timestamps are the
+// one clock here that peers cannot move: CheckBlock refuses anything more than
+// two hours ahead and AcceptBlock refuses anything at or before the previous
+// median, so the only way to make this read "at the tip" falsely is to mine,
+// and all that buys is the 15s spacing this code had unconditionally anyway.
+bool IsBlockFetchNearTip()
+{
+    return pindexBest && (int64)pindexBest->nTime > GetTime() - BLOCK_FETCH_TIP_AGE_SECS;
+}
+
 // Drop one entry so a hash announced by addrNew can be tracked.
 //
 // Refusing the insertion instead was a remote off-switch for the whole