Remove restrictions on coinbase witness

This relaxes the restriction on the coinbase witness that limited it to
only having one entry that must be 32 bytes; instead there may be up to
255 items on the coinbase witness stack, and they may be any length.

In order to have multiple entries on the coinbase witness stack, the
number of entries must be specified in the coinbase commitment, by
changing the witness commitment from a 36 byte push:

  OP_RETURN [ 0xaa 0x21 0xa9 0xed HASH ]

to a 37 byte push:

  OP_RETURN [ 0xaa 0x21 0xa9 0xed HASH N ]

where N is the number of entries on the coinbase witness stack (from 0
to 255).

src/main.cpp

@@ -3063,9 +3063,17 @@ bool IsWitnessEnabled(const CBlock& block, const CBlockIndex* pindexPrev, const
 }
 
 
-bool CheckWitnessCommitAndNonce(const CBlock& block, int commitpos, CValidationState& state)
+bool CheckWitnessCommit(const CBlock& block, int commitpos, CValidationState& state)
 {
-    if (block.vtx[0].wit.vtxinwit.size() != 1 || block.vtx[0].wit.vtxinwit[0].scriptWitness.stack.size() != 1 || block.vtx[0].wit.vtxinwit[0].scriptWitness.stack[0].size() != 32) {
+    size_t stack_size;
+
+    if (block.vtx[0].vout[commitpos].scriptPubKey[1] == 0x24) {
+        stack_size = 1;
+    } else {
+        stack_size = block.vtx[0].vout[commitpos].scriptPubKey[38];
+    }
+
+    if (block.vtx[0].wit.vtxinwit.size() != 1 || block.vtx[0].wit.vtxinwit[0].scriptWitness.stack.size() != stack_size) {
         return state.DoS(100, error("%s : invalid witness commitment size", __func__), REJECT_INVALID, "bad-witness-merkle-size", true);
     }
 
@@ -3088,7 +3096,9 @@ int FindWitnessCommitPos(const CBlock& block)
     for (size_t o = 0; o < block.vtx[0].vout.size(); o++) {
         if (block.vtx[0].vout[o].scriptPubKey.size() >= 38
             && block.vtx[0].vout[o].scriptPubKey[0] == OP_RETURN
-            && block.vtx[0].vout[o].scriptPubKey[1] == 0x24
+            && (block.vtx[0].vout[o].scriptPubKey[1] == 0x24
+                || (block.vtx[0].vout[o].scriptPubKey[1] == 0x25
+                    && block.vtx[0].vout[o].scriptPubKey.size() >= 39))
             && block.vtx[0].vout[o].scriptPubKey[2] == 0xaa
             && block.vtx[0].vout[o].scriptPubKey[3] == 0x21
             && block.vtx[0].vout[o].scriptPubKey[4] == 0xa9
@@ -3222,7 +3232,7 @@ bool ContextualCheckBlock(const CBlock& block, CValidationState& state, CBlockIn
     if (IsWitnessEnabled(block, pindexPrev, consensusParams)) {
         int commitpos = FindWitnessCommitPos(block);
         if (commitpos != -1) {
-            if (!CheckWitnessCommitAndNonce(block, commitpos, state)) {
+            if (!CheckWitnessCommit(block, commitpos, state)) {
                 return false;
             }