[v4,2/2] gdb: Enable early init of thread pool size

diff --git a/gdb/main.c b/gdb/main.c
index eb11d6f2e46..688db7655a9 100644
--- a/gdb/main.c
+++ b/gdb/main.c
@@ -1058,6 +1058,10 @@  captured_main_1 (struct captured_main_args *context)
   execute_cmdargs (&cmdarg_vec, CMDARG_EARLYINIT_FILE,
 		   CMDARG_EARLYINIT_COMMAND, &ret);
 
+  /* Set the thread pool size here, so the size can be influenced by the
+     early initialization commands.  */
+  update_thread_pool_size ();
+
   /* Initialize the extension languages.  */
   ext_lang_initialization ();
 
diff --git a/gdb/maint.c b/gdb/maint.c
index c1154d0a54e..2e6754c9558 100644
--- a/gdb/maint.c
+++ b/gdb/maint.c
@@ -844,15 +844,23 @@  maintenance_set_profile_cmd (const char *args, int from_tty,
 
 static int n_worker_threads = -1;
 
-/* Update the thread pool for the desired number of threads.  */
-static void
+/* See maint.h.  */
+
+void
 update_thread_pool_size ()
 {
 #if CXX_STD_THREAD
   int n_threads = n_worker_threads;
 
   if (n_threads < 0)
-    n_threads = std::thread::hardware_concurrency ();
+    {
+      const int hardware_threads = std::thread::hardware_concurrency ();
+      /* Testing in #29959 indicates that parallel efficiency drops between
+	 n_threads=5 to 8.  Therefore, clamp the default value to 8 to avoid an
+	 excessive number of threads in the pool on many-core systems.  */
+      const int throttle = 8;
+      n_threads = std::clamp (hardware_threads, hardware_threads, throttle);
+    }
 
   gdb::thread_pool::g_thread_pool->set_thread_count (n_threads);
 #endif
@@ -874,7 +882,7 @@  maintenance_show_worker_threads (struct ui_file *file, int from_tty,
   if (n_worker_threads == -1)
     {
       gdb_printf (file, _("The number of worker threads GDB "
-			  "can use is unlimited (currently %zu).\n"),
+			  "can use is the default (currently %zu).\n"),
 		  gdb::thread_pool::g_thread_pool->thread_count ());
       return;
     }
@@ -1474,6 +1482,4 @@  such as demangling symbol names."),
 					     maintenance_selftest_option_defs,
 					     &set_selftest_cmdlist,
 					     &show_selftest_cmdlist);
-
-  update_thread_pool_size ();
 }
diff --git a/gdb/maint.h b/gdb/maint.h
index 1741d132567..2ec2493d43d 100644
--- a/gdb/maint.h
+++ b/gdb/maint.h
@@ -26,6 +26,10 @@  extern void set_per_command_time (int);
 
 extern void set_per_command_space (int);
 
+/* Update the thread pool for the desired number of threads.  */
+
+extern void update_thread_pool_size ();
+
 /* Records a run time and space usage to be used as a base for
    reporting elapsed time or change in space.  */
 
diff --git a/gdb/testsuite/gdb.base/early-init-file.exp b/gdb/testsuite/gdb.base/early-init-file.exp
index 6426da8dacf..d09fd145cbf 100644
--- a/gdb/testsuite/gdb.base/early-init-file.exp
+++ b/gdb/testsuite/gdb.base/early-init-file.exp
@@ -99,6 +99,25 @@  proc check_gdb_startups_up_quietly { message } {
     }
 }
 
+# Restart GDB and check that the size of the thread pool has not been
+# adjusted to match the number of machine cores at early init time.
+proc check_gdb_maint_show { message } {
+    global gdb_prompt
+    global gdb_sanitizer_msg_re
+    gdb_exit
+    gdb_spawn
+    set setshowprefix "The number of worker threads GDB can use is"
+    set unset "$setshowprefix the default \\\(currently 0\\\)."
+    set final "$setshowprefix 1."
+    # Output when CXX_STD_THREAD is undefined.
+    set off "$setshowprefix 0."
+    gdb_test_multiple "" $message {
+	-re "^(${gdb_sanitizer_msg_re})?($unset|$off)\r\n($final|$off)\r\n$gdb_prompt $" {
+	    pass $gdb_test_name
+	}
+    }
+}
+
 with_ansi_styling_terminal {
 
     # Start GDB and confirm that the version string is styled.
@@ -164,4 +183,25 @@  with_ansi_styling_terminal {
 	check_gdb_startups_up_quietly \
 	    "check GDB starts quietly using XDG_CONFIG_HOME"
     }
+
+    # Create fake home directory for the thread pool size check.
+    set dirs [setup_home_directories "maint-show" \
+		  [multi_line_input \
+		       "set startup-quietly on" \
+		       "maint show worker-threads" \
+		       "maint set worker-threads 1" \
+		       "maint show worker-threads"]]
+
+    set home_dir [lindex $dirs 0]
+
+    # Now arrange to use the fake home directory startup file.
+    save_vars { INTERNAL_GDBFLAGS env(HOME) env(XDG_CONFIG_HOME) } {
+	set INTERNAL_GDBFLAGS [string map {"-nx" "" "-q" ""} $INTERNAL_GDBFLAGS]
+
+	# Now test GDB when using the HOME directory.
+	set env(HOME) $home_dir
+	unset -nocomplain env(XDG_CONFIG_HOME)
+	check_gdb_maint_show \
+	    "check early init of thread pool size"
+    }
 }
diff --git a/gdbsupport/thread-pool.cc b/gdbsupport/thread-pool.cc
index bbe043dc0a3..0b11213729f 100644
--- a/gdbsupport/thread-pool.cc
+++ b/gdbsupport/thread-pool.cc
@@ -154,6 +154,7 @@  thread_pool::set_thread_count (size_t num_threads)
 {
 #if CXX_STD_THREAD
   std::lock_guard<std::mutex> guard (m_tasks_mutex);
+  m_sized_at_least_once = true;
 
   /* If the new size is larger, start some new threads.  */
   if (m_thread_count < num_threads)
@@ -197,6 +198,9 @@  thread_pool::set_thread_count (size_t num_threads)
 void
 thread_pool::do_post_task (std::packaged_task<void ()> &&func)
 {
+  /* This assert is here to check that no tasks are posted to the pool between
+     its initialization and sizing.  */
+  gdb_assert (m_sized_at_least_once);
   std::packaged_task<void ()> t (std::move (func));
 
   if (m_thread_count != 0)
diff --git a/gdbsupport/thread-pool.h b/gdbsupport/thread-pool.h
index d5e1dc7fce1..6959414476f 100644
--- a/gdbsupport/thread-pool.h
+++ b/gdbsupport/thread-pool.h
@@ -204,6 +204,7 @@  class thread_pool
      between the main thread and the worker threads.  */
   std::condition_variable m_tasks_cv;
   std::mutex m_tasks_mutex;
+  bool m_sized_at_least_once = false;
 #endif /* CXX_STD_THREAD */
 };