Hopefully final fix for the stuck blockchain issue

Immediately issue a "getblocks", instead of a "getdata" (which will
trigger the relevant "inv" to be sent anyway), and only do so when
the previous set of invs led us into a known and attached part of
the block tree.

src/main.cpp

@@ -2482,8 +2482,10 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv)
         // find last block in inv vector
         unsigned int nLastBlock = (unsigned int)(-1);
         for (unsigned int nInv = 0; nInv < vInv.size(); nInv++) {
-            if (vInv[vInv.size() - 1 - nInv].type == MSG_BLOCK)
+            if (vInv[vInv.size() - 1 - nInv].type == MSG_BLOCK) {
                 nLastBlock = vInv.size() - 1 - nInv;
+                break;
+            }
         }
         CTxDB txdb("r");
         for (unsigned int nInv = 0; nInv < vInv.size(); nInv++)
@@ -2498,19 +2500,19 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv)
             if (fDebug)
                 printf("  got inventory: %s  %s\n", inv.ToString().c_str(), fAlreadyHave ? "have" : "new");
 
-            // Always request the last block in an inv bundle (even if we already have it), as it is the
-            // trigger for the other side to send further invs. If we are stuck on a (very long) side chain,
-            // this is necessary to connect earlier received orphan blocks to the chain again.
-            if (fAlreadyHave && nInv == nLastBlock) {
-                // bypass mapAskFor, and send request directly; it must go through.
-                std::vector<CInv> vGetData(1,inv);
-                pfrom->PushMessage("getdata", vGetData);
-            }
-
             if (!fAlreadyHave)
                 pfrom->AskFor(inv);
-            else if (inv.type == MSG_BLOCK && mapOrphanBlocks.count(inv.hash))
+            else if (inv.type == MSG_BLOCK && mapOrphanBlocks.count(inv.hash)) {
                 pfrom->PushGetBlocks(pindexBest, GetOrphanRoot(mapOrphanBlocks[inv.hash]));
+            } else if (nInv == nLastBlock) {
+                // In case we are on a very long side-chain, it is possible that we already have
+                // the last block in an inv bundle sent in response to getblocks. Try to detect
+                // this situation and push another getblocks to continue.
+                std::vector<CInv> vGetData(1,inv);
+                pfrom->PushGetBlocks(mapBlockIndex[inv.hash], uint256(0));
+                if (fDebug)
+                    printf("force request: %s\n", inv.ToString().c_str());
+            }
 
             // Track requests for our stuff
             Inventory(inv.hash);