Do not inv old or missing blocks when pruning

When responding to a getblocks message, only return inv's as
long as we HAVE_DATA for blocks in the chain, and only for blocks
that we aren't likely to delete in the near future.

src/main.cpp

@@ -4153,6 +4153,14 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
                 LogPrint("net", "  getblocks stopping at %d %s\n", pindex->nHeight, pindex->GetBlockHash().ToString());
                 break;
             }
+            // If pruning, don't inv blocks unless we have on disk and are likely to still have
+            // for some reasonable time window (1 hour) that block relay might require.
+            const int nPrunedBlocksLikelyToHave = MIN_BLOCKS_TO_KEEP - 3600 / chainparams.GetConsensus().nPowTargetSpacing;
+            if (fPruneMode && (!(pindex->nStatus & BLOCK_HAVE_DATA) || pindex->nHeight <= chainActive.Tip()->nHeight - nPrunedBlocksLikelyToHave))
+            {
+                LogPrint("net", " getblocks stopping, pruned or too old block at %d %s\n", pindex->nHeight, pindex->GetBlockHash().ToString());
+                break;
+            }
             pfrom->PushInventory(CInv(MSG_BLOCK, pindex->GetBlockHash()));
             if (--nLimit <= 0)
             {