[gdb] Initialize main_thread_id earlier

I wrote a patch using is_main_thread (), and found it returning false in the
main thread due to main_thread_id not being initialized yet.

Initialization currently takes place in _initialize_run_on_main_thread, but
that's too late for earlier uses.

Fix this by initializing, either:
- when entering main, or
- on an earlier first use.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

gdb/gdb.c

@@ -19,10 +19,17 @@
 #include "defs.h"
 #include "main.h"
 #include "interps.h"
+#include "run-on-main-thread.h"
 
 int
 main (int argc, char **argv)
 {
+  /* The first call to is_main_thread () should be from the main thread.
+     If this is the first call, then that requirement is fulfilled here.
+     If this is not the first call, then this verifies that the first call
+     fulfilled that requirement.  */
+  gdb_assert (is_main_thread ());
+
   struct captured_main_args args;
 
   memset (&args, 0, sizeof args);

gdb/run-on-main-thread.c

@@ -94,12 +94,24 @@ run_on_main_thread (std::function<void ()> &&func)
   serial_event_set (runnable_event);
 }
 
+#if CXX_STD_THREAD
+static bool main_thread_id_initialized = false;
+#endif
+
 /* See run-on-main-thread.h.  */
 
 bool
 is_main_thread ()
 {
 #if CXX_STD_THREAD
+  /* Initialize main_thread_id on first use of is_main_thread.  */
+  if (!main_thread_id_initialized)
+    {
+      main_thread_id_initialized = true;
+
+      main_thread_id = std::this_thread::get_id ();
+    }
+
   return std::this_thread::get_id () == main_thread_id;
 #else
   return true;
@@ -111,7 +123,12 @@ void
 _initialize_run_on_main_thread ()
 {
 #if CXX_STD_THREAD
-  main_thread_id = std::this_thread::get_id ();
+  /* The variable main_thread_id should be initialized when entering main, or
+     at an earlier use, so it should already be initialized here.  */
+  gdb_assert (main_thread_id_initialized);
+
+  /* Assume that we execute this in the main thread.  */
+  gdb_assert (is_main_thread ());
 #endif
   runnable_event = make_serial_event ();
   add_file_handler (serial_event_fd (runnable_event), run_events, nullptr,