Merge bitcoin#21902: refactor: Remove useless extern keyword

fa4bbd3 refactor: Remove useless extern keyword (MarcoFalke)

Pull request description:

  It is redundant, confusing and useless.

  https://en.cppreference.com/w/cpp/language/storage_duration#external_linkage

ACKs for top commit:
  practicalswift:
    cr ACK fa4bbd3: patch looks correct
  Talkless:
    utACK fa4bbd3, built successfully on Debian Sid, looks OK.
  jonatack:
    Light code review ACK fa4bbd3
  hebasto:
    ACK fa4bbd3, I've verified that all of the remained `extern` keywords specify either (a) a variable with external linkage, or (b) a symbol with "C" language linkage.
  promag:
    Code review ACK fa4bbd3.

Tree-SHA512: 1d77d661132defa52ccb2046f7a287deb3669b68835e40ab75a0d9d08fe6efeaf3bea7c0e76c754fd18bfe45972c253a39462014080d014cc5d810498784e3e4

Author: fanquake

src/rpc/util.h

@@ -72,10 +72,10 @@ void RPCTypeCheckObj(const UniValue& o,
  * Utilities: convert hex-encoded Values
  * (throws error if not hex).
  */
-extern uint256 ParseHashV(const UniValue& v, std::string strName);
-extern uint256 ParseHashO(const UniValue& o, std::string strKey);
-extern std::vector<unsigned char> ParseHexV(const UniValue& v, std::string strName);
-extern std::vector<unsigned char> ParseHexO(const UniValue& o, std::string strKey);
+uint256 ParseHashV(const UniValue& v, std::string strName);
+uint256 ParseHashO(const UniValue& o, std::string strKey);
+std::vector<unsigned char> ParseHexV(const UniValue& v, std::string strName);
+std::vector<unsigned char> ParseHexO(const UniValue& o, std::string strKey);
 
 /**
  * Validate and return a CAmount from a UniValue number or string.
@@ -84,13 +84,13 @@ extern std::vector<unsigned char> ParseHexO(const UniValue& o, std::string strKe
  * @param[in] decimals  Number of significant digits (default: 8).
  * @returns a CAmount if the various checks pass.
  */
-extern CAmount AmountFromValue(const UniValue& value, int decimals = 8);
+CAmount AmountFromValue(const UniValue& value, int decimals = 8);
 
 using RPCArgList = std::vector<std::pair<std::string, UniValue>>;
-extern std::string HelpExampleCli(const std::string& methodname, const std::string& args);
-extern std::string HelpExampleCliNamed(const std::string& methodname, const RPCArgList& args);
-extern std::string HelpExampleRpc(const std::string& methodname, const std::string& args);
-extern std::string HelpExampleRpcNamed(const std::string& methodname, const RPCArgList& args);
+std::string HelpExampleCli(const std::string& methodname, const std::string& args);
+std::string HelpExampleCliNamed(const std::string& methodname, const RPCArgList& args);
+std::string HelpExampleRpc(const std::string& methodname, const std::string& args);
+std::string HelpExampleRpcNamed(const std::string& methodname, const RPCArgList& args);
 
 CPubKey HexToPubKey(const std::string& hex_in);
 CPubKey AddrToPubKey(const FillableSigningProvider& keystore, const std::string& addr_in);

src/test/base58_tests.cpp

@@ -16,7 +16,7 @@
 
 using namespace std::literals;
 
-extern UniValue read_json(const std::string& jsondata);
+UniValue read_json(const std::string& jsondata);
 
 BOOST_FIXTURE_TEST_SUITE(base58_tests, BasicTestingSetup)
 

src/test/key_io_tests.cpp

@@ -15,7 +15,7 @@
 
 #include <univalue.h>
 
-extern UniValue read_json(const std::string& jsondata);
+UniValue read_json(const std::string& jsondata);
 
 BOOST_FIXTURE_TEST_SUITE(key_io_tests, BasicTestingSetup)
 

src/test/script_tests.cpp

@@ -39,8 +39,7 @@ static const unsigned int gFlags = SCRIPT_VERIFY_P2SH | SCRIPT_VERIFY_STRICTENC;
 unsigned int ParseScriptFlags(std::string strFlags);
 std::string FormatScriptFlags(unsigned int flags);
 
-UniValue
-read_json(const std::string& jsondata)
+UniValue read_json(const std::string& jsondata)
 {
     UniValue v;
 

src/test/sighash_tests.cpp

@@ -21,7 +21,7 @@
 
 #include <univalue.h>
 
-extern UniValue read_json(const std::string& jsondata);
+UniValue read_json(const std::string& jsondata);
 
 // Old script.cpp SignatureHash function
 uint256 static SignatureHashOld(CScript scriptCode, const CTransaction& txTo, unsigned int nIn, int nHashType)

src/test/transaction_tests.cpp

@@ -39,7 +39,7 @@
 typedef std::vector<unsigned char> valtype;
 
 // In script_tests.cpp
-extern UniValue read_json(const std::string& jsondata);
+UniValue read_json(const std::string& jsondata);
 
 static std::map<std::string, unsigned int> mapFlagNames = {
     {std::string("P2SH"), (unsigned int)SCRIPT_VERIFY_P2SH},