util: avoid using thread_local variable that has a destructor

Store the thread name in a `thread_local` variable of type `char[]`
instead of `std::string`. This avoids calling the destructor when
the thread exits. This is a workaround for
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=278701

For type-safety, return `std::string_view` from
`util::ThreadGetInternalName()` instead of `char[]`.

As a side effect of this change, we no longer store a reference
to a `thread_local` variable in `CLockLocation`. This was
dangerous because if the thread quits while the reference still
exists (in the global variable `lock_data`, see inside `GetLockData()`)
then the reference will become dangling.

configure.ac

@@ -1049,11 +1049,6 @@ if test "$use_thread_local" = "yes" || test "$use_thread_local" = "auto"; then
           dnl https://gist.github.com/jamesob/fe9a872051a88b2025b1aa37bfa98605
           AC_MSG_RESULT([no])
           ;;
-        *freebsd*)
-          dnl FreeBSD's implementation of thread_local is also buggy (per
-          dnl https://groups.google.com/d/msg/bsdmailinglist/22ncTZAbDp4/Dii_pII5AwAJ)
-          AC_MSG_RESULT([no])
-          ;;
         *)
           AC_DEFINE([HAVE_THREAD_LOCAL], [1], [Define if thread_local is supported.])
           AC_MSG_RESULT([yes])

src/logging.cpp

@@ -360,8 +360,8 @@ void BCLog::Logger::LogPrintStr(const std::string& str, const std::string& loggi
     }
 
     if (m_log_threadnames && m_started_new_line) {
-        const auto& threadname = util::ThreadGetInternalName();
-        str_prefixed.insert(0, "[" + (threadname.empty() ? "unknown" : threadname) + "] ");
+        const auto threadname = util::ThreadGetInternalName();
+        str_prefixed.insert(0, "[" + (threadname.empty() ? "unknown" : std::string{threadname}) + "] ");
     }
 
     str_prefixed = LogTimestampStr(str_prefixed);

src/sync.cpp

@@ -37,7 +37,7 @@ struct CLockLocation {
         const char* pszFile,
         int nLine,
         bool fTryIn,
-        const std::string& thread_name)
+        std::string_view thread_name)
         : fTry(fTryIn),
           mutexName(pszName),
           sourceFile(pszFile),
@@ -60,7 +60,7 @@ struct CLockLocation {
     bool fTry;
     std::string mutexName;
     std::string sourceFile;
-    const std::string& m_thread_name;
+    const std::string m_thread_name;
     int sourceLine;
 };
 

src/test/util_threadnames_tests.cpp

@@ -33,7 +33,7 @@ std::set<std::string> RenameEnMasse(int num_threads)
     auto RenameThisThread = [&](int i) {
         util::ThreadRename(TEST_THREAD_NAME_BASE + ToString(i));
         std::lock_guard<std::mutex> guard(lock);
-        names.insert(util::ThreadGetInternalName());
+        names.emplace(util::ThreadGetInternalName());
     };
 
     threads.reserve(num_threads);

src/util/threadnames.cpp

@@ -4,6 +4,7 @@
 
 #include <config/bitcoin-config.h> // IWYU pragma: keep
 
+#include <cstring>
 #include <string>
 #include <thread>
 #include <utility>
@@ -40,17 +41,20 @@ static void SetThreadName(const char* name)
 // global.
 #if defined(HAVE_THREAD_LOCAL)
 
-static thread_local std::string g_thread_name;
-const std::string& util::ThreadGetInternalName() { return g_thread_name; }
+static thread_local char g_thread_name[128]{'\0'};
+std::string_view util::ThreadGetInternalName() { return g_thread_name; }
 //! Set the in-memory internal name for this thread. Does not affect the process
 //! name.
-static void SetInternalName(std::string name) { g_thread_name = std::move(name); }
+static void SetInternalName(std::string name)
+{
+    std::memcpy(g_thread_name, name.c_str(), std::min(sizeof(g_thread_name), name.length() + 1));
+    g_thread_name[sizeof(g_thread_name) - 1] = '\0';
+}
 
 // Without thread_local available, don't handle internal name at all.
 #else
 
-static const std::string empty_string;
-const std::string& util::ThreadGetInternalName() { return empty_string; }
+std::string_view util::ThreadGetInternalName() { return std::string_view{}; }
 static void SetInternalName(std::string name) { }
 #endif
 

src/util/threadnames.h

@@ -6,6 +6,7 @@
 #define BITCOIN_UTIL_THREADNAMES_H
 
 #include <string>
+#include <string_view>
 
 namespace util {
 //! Rename a thread both in terms of an internal (in-memory) name as well
@@ -19,7 +20,7 @@ void ThreadSetInternalName(std::string&&);
 
 //! Get the thread's internal (in-memory) name; used e.g. for identification in
 //! logging.
-const std::string& ThreadGetInternalName();
+std::string_view ThreadGetInternalName();
 
 } // namespace util