kernel: Move MessageStartChars to its own file

The protocol.h file contains many non-consensus related definitions and
should thus not be part of the libbitcoinkernel. This commit makes
protocol.h no longer a required include for users of the
libbitcoinkernel.

This commit is part of the libbitcoinkernel project, namely its stage 1
step 3: Decouple most non-consensus headers from libbitcoinkernel.

Co-Authored-By: Cory Fields <cory-nospam-@coryfields.com>

3 files changed, 17 insertions, 3 deletions

diff --git a/src/kernel/chainparams.cpp b/src/kernel/chainparams.cpp
index c41559ad98..6cee379faf 100644
--- a/src/kernel/chainparams.cpp
+++ b/src/kernel/chainparams.cpp
@@ -10,6 +10,7 @@
 #include <consensus/merkle.h>
 #include <consensus/params.h>
 #include <hash.h>
+#include <kernel/messagestartchars.h>
 #include <logging.h>
 #include <primitives/block.h>
 #include <primitives/transaction.h>
diff --git a/src/kernel/chainparams.h b/src/kernel/chainparams.h
index 2d38af609c..f1df878f60 100644
--- a/src/kernel/chainparams.h
+++ b/src/kernel/chainparams.h
@@ -7,9 +7,9 @@
 #define BITCOIN_KERNEL_CHAINPARAMS_H
 
 #include <consensus/params.h>
+#include <kernel/messagestartchars.h>
 #include <netaddress.h>
 #include <primitives/block.h>
-#include <protocol.h>
 #include <uint256.h>
 #include <util/chaintype.h>
 #include <util/hash_type.h>
@@ -87,7 +87,7 @@ public:
     };
 
     const Consensus::Params& GetConsensus() const { return consensus; }
-    const CMessageHeader::MessageStartChars& MessageStart() const { return pchMessageStart; }
+    const MessageStartChars& MessageStart() const { return pchMessageStart; }
     uint16_t GetDefaultPort() const { return nDefaultPort; }
     uint16_t GetDefaultPort(Network net) const
     {
@@ -165,7 +165,7 @@ protected:
     CChainParams() {}
 
     Consensus::Params consensus;
-    CMessageHeader::MessageStartChars pchMessageStart;
+    MessageStartChars pchMessageStart;
     uint16_t nDefaultPort;
     uint64_t nPruneAfterHeight;
     uint64_t m_assumed_blockchain_size;
diff --git a/src/kernel/messagestartchars.h b/src/kernel/messagestartchars.h
new file mode 100644
index 0000000000..829616dc8b
--- /dev/null
+++ b/src/kernel/messagestartchars.h
@@ -0,0 +1,13 @@
+// Copyright (c) 2023 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_KERNEL_MESSAGESTARTCHARS_H
+#define BITCOIN_KERNEL_MESSAGESTARTCHARS_H
+
+#include <array>
+#include <cstdint>
+
+using MessageStartChars = std::array<uint8_t, 4>;
+
+#endif // BITCOIN_KERNEL_MESSAGESTARTCHARS_H