Set short thread name for pthread_setname

Abbreviate thread name to "pika/<first char of context>/<first char of pool name>/<local thread
index>", e.g. "pika/w/d/3" for the fourth worker thread on the [d]efault pool, which is a [w]orker
thread.

libs/pika/runtime/src/runtime.cpp

@@ -1125,8 +1125,9 @@ namespace pika::detail {
             cond.notify_all();
         }
 
-        std::string thread_name("main-thread#wait_helper");
-        detail::set_thread_name(thread_name);
+        std::string fullname("pika/main-thread#wait_helper");
+        std::string shortname("pika/waithelper");
+        detail::set_thread_name(fullname, shortname);
 
 #if defined(PIKA_HAVE_APEX)
         // not registering helper threads - for now
@@ -1381,9 +1382,18 @@ namespace pika::detail {
         std::size_t global_thread_num, char const* pool_name, char const* postfix)
     // NOLINTEND(bugprone-easily-swappable-parameters)
     {
+        PIKA_ASSERT(context != nullptr);
+        PIKA_ASSERT(context[0]);
+
         std::ostringstream fullname;
+        std::ostringstream shortname;
         fullname << "pika/" << context;
-        if (pool_name && *pool_name) { fullname << "/pool:" << pool_name; }
+        shortname << "pika/" << context[0];
+        if (pool_name && *pool_name)
+        {
+            fullname << "/pool:" << pool_name;
+            shortname << '/' << pool_name[0];
+        }
         if (postfix && *postfix) { fullname << '/' << postfix; }
         if (global_thread_num != std::size_t(-1))
         {
@@ -1392,10 +1402,11 @@ namespace pika::detail {
         if (local_thread_num != std::size_t(-1))
         {
             fullname << "/local:" + std::to_string(local_thread_num);
+            shortname << '/' << std::to_string(local_thread_num);
         }
 
         PIKA_ASSERT(detail::get_thread_name_internal().empty());
-        detail::set_thread_name(PIKA_MOVE(fullname).str());
+        detail::set_thread_name(fullname.str(), shortname.str());
 #if defined(PIKA_HAVE_APEX) || defined(PIKA_HAVE_TRACY)
         char const* name = detail::get_thread_name().c_str();
 
@@ -1423,7 +1434,7 @@ namespace pika::detail {
         if (on_stop_func_) { on_stop_func_(global_thread_num, global_thread_num, "", context); }
 
         // reset thread local storage
-        detail::set_thread_name("");
+        detail::set_thread_name("", "");
     }
 
     void runtime::add_pre_startup_function(startup_function_type f)

libs/pika/thread_support/include/pika/thread_support/thread_name.hpp

@@ -17,5 +17,5 @@
 namespace pika::detail {
     PIKA_EXPORT std::string& get_thread_name_internal();
     PIKA_EXPORT std::string get_thread_name();
-    PIKA_EXPORT void set_thread_name(std::string_view name);
+    PIKA_EXPORT void set_thread_name(std::string_view name, std::string_view short_name);
 }    // namespace pika::detail

libs/pika/thread_support/src/thread_name.cpp

@@ -28,9 +28,9 @@ namespace pika::detail {
 
     std::string get_thread_name()
     {
-        std::string const& thread_name = detail::get_thread_name_internal();
-        if (thread_name.empty()) return "<unknown>";
-        return thread_name;
+        std::string const& name = detail::get_thread_name_internal();
+        if (name.empty()) return "<unknown>";
+        return name;
     }
 
 #if defined(WIN32) || defined(_WIN32) || defined(__WIN32__)
@@ -47,15 +47,15 @@ namespace pika::detail {
 # pragma pack(pop)
 
     // Set the name of the thread shown in the Visual Studio debugger
-    void set_thread_name(std::string_view thread_name)
+    void set_thread_name(std::string_view name, std::string_view)
     {
-        auto& name = get_thread_name_internal();
-        name = thread_name;
+        auto& name_internal = get_thread_name_internal();
+        name_internal = name;
 
         DWORD dwThreadID = -1;
         THREADNAME_INFO info;
         info.dwType = 0x1000;
-        info.szName = name.c_str();
+        info.szName = name_internal.c_str();
         info.dwThreadID = dwThreadID;
         info.dwFlags = 0;
 
@@ -69,15 +69,15 @@ namespace pika::detail {
         }
     }
 #else
-    void set_thread_name(std::string_view thread_name)
+    void set_thread_name(std::string_view name, std::string_view short_name)
     {
-        auto& name = get_thread_name_internal();
-        name = thread_name;
+        auto& name_internal = get_thread_name_internal();
+        name_internal = name;
 # if defined(PIKA_HAVE_PTHREAD_SETNAME_NP)
         // https://man7.org/linux/man-pages/man3/pthread_setname_np.3.html:
         // The thread name is a meaningful C language string, whose length is restricted to 16
         // characters, including the terminating null byte ('\0').
-        std::string pthread_name{thread_name, 0, 15};
+        std::string pthread_name{short_name, 0, 15};
         int rc = pthread_setname_np(pthread_self(), pthread_name.c_str());
         if (rc != 0)
         {