Avoid strncpy for CMessageHeader::pchCommand #18367

In GCC 9.2.1, I get the warning:

In file included from /usr/include/string.h:494,
                 from /usr/include/c++/9/cstring:42,
                 from ./serialize.h:12,
                 from ./netaddress.h:13,
                 from ./protocol.h:13,
                 from protocol.cpp:6:
In function ‘char* strncpy(char*, const char*, size_t)’,
    inlined from ‘CMessageHeader::CMessageHeader(const unsigned char (&)[4], const char*, unsigned int)’ at protocol.cpp:89:12:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:34: warning: ‘char* __builtin_strncpy(char*, const char*, long unsigned int)’ specified bound 12 equals destination size [-Wstringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      |          ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The warning is spurious, as we exactly want the truncation behaviour here (no zero padding in pchCommand when the input has length exactly 12), but it seems hard to avoid. The reason is that strncopy is designed to operate on C strings, and our target isn't exactly a C string.

I'm changing it here to a simple loop that assigns to every byte in the output once, but if desirable, a solution with strlen + memcpy + memset is also possible.

Concept ACK

FWIW -- C string function usage:

$ man 3 string | tr " *()<>.," "\n" | grep ^str | grep -v ^string | \
      sort -u | tr "\n" "|" | sed 's/|$//g'; echo
strcasecmp|strcat|strchr|strcmp|strcoll|strcpy|strcspn|strdup|strfry|strlen|strncasecmp|strncat|strncmp|strncpy|strpbrk|strrchr|strsep|strspn|strstr|strtok|strxfrm
$ git grep -E $(man 3 string | tr " *()<>.," "\n" | grep ^str | \
      grep -v ^string | sort -u | tr "\n" "|" | sed 's/|$//g') -- "*.cpp" "*.h" \
      ":(exclude)src/leveldb/" ":(exclude)src/secp256k1/" ":(exclude)src/univalue/"
src/base58.cpp:    int size = strlen(psz) * 733 / 1000 + 1; // log(58) / log(256), rounded up.
src/chainparams.cpp:    txNew.vin[0].scriptSig = CScript() << 486604799 << CScriptNum(4) << std::vector<unsigned char>((const unsigned char*)pszTimestamp, (const unsigned char*)pszTimestamp + strlen(pszTimestamp));
src/core_read.cpp:            if (strcmp(name, "OP_UNKNOWN") == 0)
src/net.cpp:            if (strcmp(ifa->ifa_name, "lo") == 0) continue;
src/net.cpp:            if (strcmp(ifa->ifa_name, "lo0") == 0) continue;
src/protocol.cpp:    strncpy(pchCommand, pszCommand, COMMAND_SIZE);
src/randomenv.cpp:        hasher.Write((const unsigned char*)path, strlen(path) + 1);
src/randomenv.cpp:    hasher.Write((const unsigned char*)COMPILER_VERSION, strlen(COMPILER_VERSION) + 1);
src/randomenv.cpp:    if (platform_str) hasher.Write((const unsigned char*)platform_str, strlen(platform_str) + 1);
src/randomenv.cpp:    if (exec_str) hasher.Write((const unsigned char*)exec_str, strlen(exec_str) + 1);
src/randomenv.cpp:        hasher.Write((const unsigned char*)ifit->ifa_name, strlen(ifit->ifa_name) + 1);
src/randomenv.cpp:        hasher.Write((const unsigned char*)&name.sysname, strlen(name.sysname) + 1);
src/randomenv.cpp:        hasher.Write((const unsigned char*)&name.nodename, strlen(name.nodename) + 1);
src/randomenv.cpp:        hasher.Write((const unsigned char*)&name.release, strlen(name.release) + 1);
src/randomenv.cpp:        hasher.Write((const unsigned char*)&name.version, strlen(name.version) + 1);
src/randomenv.cpp:        hasher.Write((const unsigned char*)&name.machine, strlen(name.machine) + 1);
src/randomenv.cpp:            hasher.Write((const unsigned char*)environ[i], strlen(environ[i]));
src/rest.cpp:        if (strlen(rf_names[i].name) > 0) {
src/test/serialize_tests.cpp:                strcmp(charstrval, rhs.charstrval) == 0 && \
src/test/serialize_tests.cpp:    return strcmp(expectedException.what(), ex.what()) == 0;
src/test/streams_tests.cpp:        BOOST_CHECK(strstr(e.what(),
src/test/streams_tests.cpp:        BOOST_CHECK(strstr(e.what(),
src/test/streams_tests.cpp:        BOOST_CHECK(strstr(e.what(),
src/util/strencodings.cpp:    val.reserve(strlen(p));
src/util/strencodings.cpp:    val.reserve(strlen(p));
src/util/string.h:    return str.size() == strlen(str.c_str());
src/util/system.cpp:    if (pszHome == nullptr || strlen(pszHome) == 0)
src/wallet/db.cpp:    fReadOnly = (!strchr(pszMode, '+') && !strchr(pszMode, 'w'));
src/wallet/db.cpp:    bool fCreate = strchr(pszMode, 'c') != nullptr;
src/wallet/db.cpp:                                strncmp(ssKey.data(), pszSkip, std::min(ssKey.size(), strlen(pszSkip))) == 0)
src/wallet/db.cpp:                            if (strncmp(ssKey.data(), "\x07version", 8) == 0) {
src/zmq/zmqpublishnotifier.cpp:    int rc = zmq_send_multipart(psocket, command, strlen(command), data, size, msgseq, (size_t)sizeof(uint32_t), nullptr);

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #16995 (Fix gcc 9 warnings by laanwj)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

Closing in favor of #16995.