validation: Add minimum witness commitment size constant

Per BIP 141, the witness commitment structure is atleast 38 bytes,
OP_RETURN (0x6a) + 36 (0x24) + 4 byte header (0xaa21a9ed) + 32 byte
SHA256 hash. It can be longer, however any additional data has no
consensus meaning.

Author: fanquake

src/validation.cpp

@@ -3386,7 +3386,14 @@ int GetWitnessCommitmentIndex(const CBlock& block)
     int commitpos = -1;
     if (!block.vtx.empty()) {
         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[2] == 0xaa && block.vtx[0]->vout[o].scriptPubKey[3] == 0x21 && block.vtx[0]->vout[o].scriptPubKey[4] == 0xa9 && block.vtx[0]->vout[o].scriptPubKey[5] == 0xed) {
+            const CTxOut& vout = block.vtx[0]->vout[o];
+            if (vout.scriptPubKey.size() >= MINIMUM_WITNESS_COMMITMENT &&
+                vout.scriptPubKey[0] == OP_RETURN &&
+                vout.scriptPubKey[1] == 0x24 &&
+                vout.scriptPubKey[2] == 0xaa &&
+                vout.scriptPubKey[3] == 0x21 &&
+                vout.scriptPubKey[4] == 0xa9 &&
+                vout.scriptPubKey[5] == 0xed) {
                 commitpos = o;
             }
         }
@@ -3417,7 +3424,7 @@ std::vector<unsigned char> GenerateCoinbaseCommitment(CBlock& block, const CBloc
             CHash256().Write(witnessroot.begin(), 32).Write(ret.data(), 32).Finalize(witnessroot.begin());
             CTxOut out;
             out.nValue = 0;
-            out.scriptPubKey.resize(38);
+            out.scriptPubKey.resize(MINIMUM_WITNESS_COMMITMENT);
             out.scriptPubKey[0] = OP_RETURN;
             out.scriptPubKey[1] = 0x24;
             out.scriptPubKey[2] = 0xaa;

src/validation.h

@@ -92,6 +92,8 @@ static const unsigned int DEFAULT_CHECKLEVEL = 3;
 // one 128MB block file + added 15% undo data = 147MB greater for a total of 545MB
 // Setting the target to >= 550 MiB will make it likely we can respect the target.
 static const uint64_t MIN_DISK_SPACE_FOR_BLOCK_FILES = 550 * 1024 * 1024;
+/** Minimum size of a witness commitment structure. Defined in BIP 141. **/
+static constexpr size_t MINIMUM_WITNESS_COMMITMENT{38};
 
 struct BlockHasher
 {