Merge #16995: Fix gcc 9 warnings

ff9c671b11 refactor: Work around GCC 9 `-Wredundant-move` warning (Russell Yanofsky)
b837b334db net: Fail instead of truncate command name in CMessageHeader (Wladimir J. van der Laan)

Pull request description:

  Fixes all 3 from #16992 (see commits)

  - net: Fail instead of truncate command name in CMessageHeader
  - refactor: Use std::move workaround for unique_ptr upcast only when necessary

ACKs for top commit:
  practicalswift:
    ACK ff9c671b11 -- patch looks correct
  sipa:
    utACK ff9c671b11
  ryanofsky:
    Code review ACK ff9c671b11. Looks good and seems to pass travis, modulo a timeout on one build
  hebasto:
    ACK ff9c671b11, tested on Fedora 31:

Tree-SHA512: 52d8c13aaf0d56f9bc546a98d7f853eae21f7e325b202fdeb2286b19a9a0ee308634c644b039f60ad8043421e382381cbf1bce58d9f807547f928621c7d245d0

src/interfaces/chain.cpp

@@ -234,11 +234,10 @@ public:
     explicit ChainImpl(NodeContext& node) : m_node(node) {}
     std::unique_ptr<Chain::Lock> lock(bool try_lock) override
     {
-        auto result = MakeUnique<LockImpl>(::cs_main, "cs_main", __FILE__, __LINE__, try_lock);
-        if (try_lock && result && !*result) return {};
-        // std::move necessary on some compilers due to conversion from
-        // LockImpl to Lock pointer
-        return std::move(result);
+        auto lock = MakeUnique<LockImpl>(::cs_main, "cs_main", __FILE__, __LINE__, try_lock);
+        if (try_lock && lock && !*lock) return {};
+        std::unique_ptr<Chain::Lock> result = std::move(lock); // Temporary to avoid CWG 1579
+        return result;
     }
     bool findBlock(const uint256& hash, CBlock* block, int64_t* time, int64_t* time_max) override
     {

src/protocol.cpp

@@ -85,8 +85,13 @@ CMessageHeader::CMessageHeader(const MessageStartChars& pchMessageStartIn)
 CMessageHeader::CMessageHeader(const MessageStartChars& pchMessageStartIn, const char* pszCommand, unsigned int nMessageSizeIn)
 {
     memcpy(pchMessageStart, pchMessageStartIn, MESSAGE_START_SIZE);
-    memset(pchCommand, 0, sizeof(pchCommand));
-    strncpy(pchCommand, pszCommand, COMMAND_SIZE);
+
+    // Copy the command name, zero-padding to COMMAND_SIZE bytes
+    size_t i = 0;
+    for (; i < COMMAND_SIZE && pszCommand[i] != 0; ++i) pchCommand[i] = pszCommand[i];
+    assert(pszCommand[i] == 0); // Assert that the command name passed in is not longer than COMMAND_SIZE
+    for (; i < COMMAND_SIZE; ++i) pchCommand[i] = 0;
+
     nMessageSize = nMessageSizeIn;
     memset(pchChecksum, 0, CHECKSUM_SIZE);
 }

src/protocol.h

@@ -37,6 +37,10 @@ public:
     typedef unsigned char MessageStartChars[MESSAGE_START_SIZE];
 
     explicit CMessageHeader(const MessageStartChars& pchMessageStartIn);
+
+    /** Construct a P2P message header from message-start characters, a command and the size of the message.
+     * @note Passing in a `pszCommand` longer than COMMAND_SIZE will result in a run-time assertion error.
+     */
     CMessageHeader(const MessageStartChars& pchMessageStartIn, const char* pszCommand, unsigned int nMessageSizeIn);
 
     std::string GetCommand() const;