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` 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.

1 files changed, 21 insertions, 10 deletions

diff --git a/src/util/threadnames.cpp b/src/util/threadnames.cpp
index ea597dd05a..ba074e30e0 100644
--- a/src/util/threadnames.cpp
+++ b/src/util/threadnames.cpp
@@ -4,6 +4,7 @@
 
 #include <config/bitcoin-config.h> // IWYU pragma: keep
 
+#include <cstring>
 #include <string>
 #include <thread>
 #include <utility>
@@ -40,27 +41,37 @@ 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; }
+/**
+ * The name of the thread. We use char array instead of std::string to avoid
+ * complications with running a destructor when the thread exits. Avoid adding
+ * other thread_local variables.
+ * @see https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=278701
+ */
+static thread_local char g_thread_name[128]{'\0'};
+std::string 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(const std::string& name)
+{
+    const size_t copy_bytes{std::min(sizeof(g_thread_name) - 1, name.length())};
+    std::memcpy(g_thread_name, name.data(), copy_bytes);
+    g_thread_name[copy_bytes] = '\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; }
-static void SetInternalName(std::string name) { }
+std::string util::ThreadGetInternalName() { return ""; }
+static void SetInternalName(const std::string& name) { }
 #endif
 
-void util::ThreadRename(std::string&& name)
+void util::ThreadRename(const std::string& name)
 {
     SetThreadName(("b-" + name).c_str());
-    SetInternalName(std::move(name));
+    SetInternalName(name);
 }
 
-void util::ThreadSetInternalName(std::string&& name)
+void util::ThreadSetInternalName(const std::string& name)
 {
-    SetInternalName(std::move(name));
+    SetInternalName(name);
 }