[mining] segwit option must be set in GBT

Calling getblocktemplate without the segwit rule specified is most
likely a client error, since it results in lower fees for the miner.
Prevent this client error by failing getblocktemplate if called without
the segwit rule specified.

5 files changed, 14 insertions, 19 deletions

diff --git a/src/bench/block_assemble.cpp b/src/bench/block_assemble.cpp
index 2def0b23e2..7cf27f9f5b 100644
--- a/src/bench/block_assemble.cpp
+++ b/src/bench/block_assemble.cpp
@@ -27,7 +27,7 @@ static std::shared_ptr<CBlock> PrepareBlock(const CScript& coinbase_scriptPubKey
 {
     auto block = std::make_shared<CBlock>(
         BlockAssembler{Params()}
-            .CreateNewBlock(coinbase_scriptPubKey, /* fMineWitnessTx */ true)
+            .CreateNewBlock(coinbase_scriptPubKey)
             ->block);
 
     block->nTime = ::chainActive.Tip()->GetMedianTimePast() + 1;
diff --git a/src/miner.cpp b/src/miner.cpp
index 96c9cd6d2a..ef48a86e32 100644
--- a/src/miner.cpp
+++ b/src/miner.cpp
@@ -95,7 +95,7 @@ void BlockAssembler::resetBlock()
     nFees = 0;
 }
 
-std::unique_ptr<CBlockTemplate> BlockAssembler::CreateNewBlock(const CScript& scriptPubKeyIn, bool fMineWitnessTx)
+std::unique_ptr<CBlockTemplate> BlockAssembler::CreateNewBlock(const CScript& scriptPubKeyIn)
 {
     int64_t nTimeStart = GetTimeMicros();
 
@@ -139,7 +139,7 @@ std::unique_ptr<CBlockTemplate> BlockAssembler::CreateNewBlock(const CScript& sc
     // not activated.
     // TODO: replace this with a call to main to assess validity of a mempool
     // transaction (which in most cases can be a no-op).
-    fIncludeWitness = IsWitnessEnabled(pindexPrev, chainparams.GetConsensus()) && fMineWitnessTx;
+    fIncludeWitness = IsWitnessEnabled(pindexPrev, chainparams.GetConsensus());
 
     int nPackagesSelected = 0;
     int nDescendantsUpdated = 0;
diff --git a/src/miner.h b/src/miner.h
index 8cdcf7133b..44c50b01ad 100644
--- a/src/miner.h
+++ b/src/miner.h
@@ -157,7 +157,7 @@ public:
     BlockAssembler(const CChainParams& params, const Options& options);
 
     /** Construct a new block template with coinbase to scriptPubKeyIn */
-    std::unique_ptr<CBlockTemplate> CreateNewBlock(const CScript& scriptPubKeyIn, bool fMineWitnessTx=true);
+    std::unique_ptr<CBlockTemplate> CreateNewBlock(const CScript& scriptPubKeyIn);
 
 private:
     // utility functions
diff --git a/src/rpc/mining.cpp b/src/rpc/mining.cpp
index 93fa3a2728..d4d1adbb50 100644
--- a/src/rpc/mining.cpp
+++ b/src/rpc/mining.cpp
@@ -311,7 +311,7 @@ static UniValue getblocktemplate(const JSONRPCRequest& request)
                 "    https://github.com/bitcoin/bips/blob/master/bip-0009.mediawiki#getblocktemplate_changes\n"
                 "    https://github.com/bitcoin/bips/blob/master/bip-0145.mediawiki\n",
                 {
-                    {"template_request", RPCArg::Type::OBJ, /* opt */ true, /* default_val */ "", "A json object in the following spec",
+                    {"template_request", RPCArg::Type::OBJ, /* opt */ false, /* default_val */ "", "A json object in the following spec",
                         {
                             {"mode", RPCArg::Type::STR, /* opt */ true, /* default_val */ "", "This must be set to \"template\", \"proposal\" (see BIP 23), or omitted"},
                             {"capabilities", RPCArg::Type::ARR, /* opt */ true, /* default_val */ "", "A list of strings",
@@ -319,7 +319,7 @@ static UniValue getblocktemplate(const JSONRPCRequest& request)
                                     {"support", RPCArg::Type::STR, /* opt */ true, /* default_val */ "", "client side supported feature, 'longpoll', 'coinbasetxn', 'coinbasevalue', 'proposal', 'serverlist', 'workid'"},
                                 },
                                 },
-                            {"rules", RPCArg::Type::ARR, /* opt */ true, /* default_val */ "", "A list of strings",
+                            {"rules", RPCArg::Type::ARR, /* opt */ false, /* default_val */ "", "A list of strings",
                                 {
                                     {"support", RPCArg::Type::STR, /* opt */ true, /* default_val */ "", "client side supported softfork deployment"},
                                 },
@@ -503,21 +503,17 @@ static UniValue getblocktemplate(const JSONRPCRequest& request)
     }
 
     const struct VBDeploymentInfo& segwit_info = VersionBitsDeploymentInfo[Consensus::DEPLOYMENT_SEGWIT];
-    // If the caller is indicating segwit support, then allow CreateNewBlock()
-    // to select witness transactions, after segwit activates (otherwise
-    // don't).
-    bool fSupportsSegwit = setClientRules.find(segwit_info.name) != setClientRules.end();
+    // GBT must be called with 'segwit' set in the rules
+    if (setClientRules.count(segwit_info.name) != 1) {
+        throw JSONRPCError(RPC_INVALID_PARAMETER, "getblocktemplate must be called with the segwit rule set (call with {\"rules\": [\"segwit\"]})");
+    }
 
     // Update block
     static CBlockIndex* pindexPrev;
     static int64_t nStart;
     static std::unique_ptr<CBlockTemplate> pblocktemplate;
-    // Cache whether the last invocation was with segwit support, to avoid returning
-    // a segwit-block to a non-segwit caller.
-    static bool fLastTemplateSupportsSegwit = true;
     if (pindexPrev != chainActive.Tip() ||
-        (mempool.GetTransactionsUpdated() != nTransactionsUpdatedLast && GetTime() - nStart > 5) ||
-        fLastTemplateSupportsSegwit != fSupportsSegwit)
+        (mempool.GetTransactionsUpdated() != nTransactionsUpdatedLast && GetTime() - nStart > 5))
     {
         // Clear pindexPrev so future calls make a new block, despite any failures from here on
         pindexPrev = nullptr;
@@ -526,11 +522,10 @@ static UniValue getblocktemplate(const JSONRPCRequest& request)
         nTransactionsUpdatedLast = mempool.GetTransactionsUpdated();
         CBlockIndex* pindexPrevNew = chainActive.Tip();
         nStart = GetTime();
-        fLastTemplateSupportsSegwit = fSupportsSegwit;
 
         // Create new block
         CScript scriptDummy = CScript() << OP_TRUE;
-        pblocktemplate = BlockAssembler(Params()).CreateNewBlock(scriptDummy, fSupportsSegwit);
+        pblocktemplate = BlockAssembler(Params()).CreateNewBlock(scriptDummy);
         if (!pblocktemplate)
             throw JSONRPCError(RPC_OUT_OF_MEMORY, "Out of memory");
 
@@ -682,7 +677,7 @@ static UniValue getblocktemplate(const JSONRPCRequest& request)
     result.pushKV("bits", strprintf("%08x", pblock->nBits));
     result.pushKV("height", (int64_t)(pindexPrev->nHeight+1));
 
-    if (!pblocktemplate->vchCoinbaseCommitment.empty() && fSupportsSegwit) {
+    if (!pblocktemplate->vchCoinbaseCommitment.empty()) {
         result.pushKV("default_witness_commitment", HexStr(pblocktemplate->vchCoinbaseCommitment.begin(), pblocktemplate->vchCoinbaseCommitment.end()));
     }
 
diff --git a/src/test/validation_block_tests.cpp b/src/test/validation_block_tests.cpp
index 4316f37999..a9d192e555 100644
--- a/src/test/validation_block_tests.cpp
+++ b/src/test/validation_block_tests.cpp
@@ -54,7 +54,7 @@ std::shared_ptr<CBlock> Block(const uint256& prev_hash)
     CScript pubKey;
     pubKey << i++ << OP_TRUE;
 
-    auto ptemplate = BlockAssembler(Params()).CreateNewBlock(pubKey, false);
+    auto ptemplate = BlockAssembler(Params()).CreateNewBlock(pubKey);
     auto pblock = std::make_shared<CBlock>(ptemplate->block);
     pblock->hashPrevBlock = prev_hash;
     pblock->nTime = ++time;