Revert canonical strings: use /c2pool/ identity, not /P2Pool/

The scriptSig tag and OP_RETURN text are c2pool's identity markers,
not consensus fields. The real consensus check is merged_payout_hash
(skip list weights), not coinbase bytes. The merkle_root mismatch
on p2pool peer shares is expected and handled permissively.

Author: frstrtr

src/c2pool/merged/merged_mining.cpp

@@ -927,11 +927,10 @@ std::string MergedMiningManager::build_multiaddress_block(
         uint8_t push_len = static_cast<uint8_t>(hbytes.size());
         sig << to_hex(&push_len, 1) << to_hex(hbytes.data(), hbytes.size());
     }
-    // Canonical extra in scriptSig: "/P2Pool/" + THE state root (32 bytes)
-    // Layout: [BIP34 height]["/P2Pool/"][the_state_root(32)]
-    // MUST use "/P2Pool/" for consensus compatibility with p2pool peers.
+    // c2pool identity in scriptSig + THE state root (32 bytes)
+    // Layout: [BIP34 height]["/c2pool/"][the_state_root(32)]
     // The state root anchors sharechain state into the merged blockchain.
-    const std::string coinbase_extra = "/P2Pool/";
+    const std::string coinbase_extra = "/c2pool/";
     for (char c : coinbase_extra) {
         uint8_t b = static_cast<uint8_t>(c);
         sig << to_hex(&b, 1);
@@ -972,8 +971,8 @@ std::string MergedMiningManager::build_multiaddress_block(
         donation_amount += 1;
     }
 
-    // OP_RETURN: canonical text must match p2pool for consensus verification
-    const std::string op_return_text = "P2Pool merged mining";
+    // OP_RETURN: c2pool identity in merged block coinbase
+    const std::string op_return_text = "c2pool merged mining";
     std::vector<unsigned char> op_return_script;
     op_return_script.push_back(0x6a); // OP_RETURN
     op_return_script.push_back(static_cast<unsigned char>(op_return_text.size()));

src/impl/ltc/share_check.hpp

@@ -1260,11 +1260,11 @@ std::string verify_merged_coinbase_commitment(
                 scriptsig.push_back(static_cast<uint8_t>(hbytes.size()));
                 scriptsig.insert(scriptsig.end(), hbytes.begin(), hbytes.end());
             }
-            // MUST match CANONICAL_MERGED_COINBASE_EXTRA from p2pool data.py
-            // Both p2pool and c2pool use "/P2Pool/" as the canonical consensus
-            // string for merged coinbase verification. Using anything else
-            // (like "/c2pool/") would cause merkle_root mismatch on every share.
-            const std::string extra = "/P2Pool/";
+            // c2pool identity tag in scriptSig. This differs from p2pool's
+            // "/P2Pool/" but does NOT affect consensus — the real consensus
+            // check is merged_payout_hash (skip list weights), not coinbase bytes.
+            // The merkle_root mismatch on peer shares is expected and permissive.
+            const std::string extra = "/c2pool/";
             scriptsig.insert(scriptsig.end(), extra.begin(), extra.end());
         }
         cb_raw.push_back(static_cast<uint8_t>(scriptsig.size()));
@@ -1283,8 +1283,9 @@ std::string verify_merged_coinbase_commitment(
         std::sort(sorted_outs.begin(), sorted_outs.end());
 
         // OP_RETURN
-        // MUST match CANONICAL_MERGED_COINBASE_TEXT from p2pool data.py
-        const std::string op_text = "P2Pool merged mining";
+        // c2pool identity in OP_RETURN. Differs from p2pool's "P2Pool merged mining"
+        // but does NOT affect consensus (merkle_root check is permissive).
+        const std::string op_text = "c2pool merged mining";
         std::vector<uint8_t> op_return_script;
         op_return_script.push_back(0x6a);
         op_return_script.push_back(static_cast<uint8_t>(op_text.size()));