Optimize -regtest setgenerate block generation

Speed up generating blocks in regression test mode, by moving
block-creating and nonce-finding directly into the setgenerate
RPC call (instead of starting up a mining thread and waiting for
it to find a block).

This makes the forknotify RPC test three times quicker, for
example (10 seconds runtime instead of 30 seconds, assuming
the initial blockchain cache is already built).

src/rpcmining.cpp

@@ -121,6 +121,8 @@ Value setgenerate(const Array& params, bool fHelp)
             "1. generate         (boolean, required) Set to true to turn on generation, off to turn off.\n"
             "2. genproclimit     (numeric, optional) Set the processor limit for when generation is on. Can be -1 for unlimited.\n"
             "                    Note: in -regtest mode, genproclimit controls how many blocks are generated immediately.\n"
+            "\nResult\n"
+            "[ blockhashes ]     (array, -regtest only) hashes of blocks generated\n"
             "\nExamples:\n"
             "\nSet the generation on with a limit of one processor\n"
             + HelpExampleCli("setgenerate", "true 1") +
@@ -154,26 +156,38 @@ Value setgenerate(const Array& params, bool fHelp)
         int nHeightEnd = 0;
         int nHeight = 0;
         int nGenerate = (nGenProcLimit > 0 ? nGenProcLimit : 1);
+        CReserveKey reservekey(pwalletMain);
+
         {   // Don't keep cs_main locked
             LOCK(cs_main);
             nHeightStart = chainActive.Height();
             nHeight = nHeightStart;
             nHeightEnd = nHeightStart+nGenerate;
         }
-        int nHeightLast = -1;
+        unsigned int nExtraNonce = 0;
+        Array blockHashes;
         while (nHeight < nHeightEnd)
         {
-            if (nHeightLast != nHeight)
+            auto_ptr<CBlockTemplate> pblocktemplate(CreateNewBlockWithKey(reservekey));
+            if (!pblocktemplate.get())
+                throw JSONRPCError(RPC_INTERNAL_ERROR, "Wallet keypool empty");
+            CBlock *pblock = &pblocktemplate->block;
             {
-                nHeightLast = nHeight;
-                GenerateBitcoins(fGenerate, pwalletMain, 1);
-            }
-            MilliSleep(1);
-            {   // Don't keep cs_main locked
                 LOCK(cs_main);
-                nHeight = chainActive.Height();
+                IncrementExtraNonce(pblock, chainActive.Tip(), nExtraNonce);
+            }
+            while (!CheckProofOfWork(pblock->GetHash(), pblock->nBits)) {
+                // Yes, there is a chance every nonce could fail to satisfy the -regtest
+                // target -- 1 in 2^(2^32). That ain't gonna happen.
+                ++pblock->nNonce;
             }
+            CValidationState state;
+            if (!ProcessNewBlock(state, NULL, pblock))
+                throw JSONRPCError(RPC_INTERNAL_ERROR, "ProcessNewBlock, block not accepted");
+            ++nHeight;
+            blockHashes.push_back(pblock->GetHash().GetHex());
         }
+        return blockHashes;
     }
     else // Not -regtest: start generate thread, return immediately
     {