Simplify systhreads thread-local state management

Use a global key to access the per-thread `caml_thread_t` rather than
have a key per domain. This fixes the issue reported in ocaml#11226.

otherlibs/systhreads/st_stubs.c

@@ -88,11 +88,13 @@ struct caml_thread_struct {
 
 typedef struct caml_thread_struct* caml_thread_t;
 
+/* Thread-local key for accessing the current thread's [caml_thread_t] */
+st_tlskey caml_thread_key;
+
 /* overall table for threads across domains */
 struct caml_thread_table {
   caml_thread_t all_threads;
   caml_thread_t current_thread;
-  st_tlskey thread_key;
   st_masterlock thread_lock;
   int tick_thread_running;
   st_thread_id tick_thread_id;
@@ -116,10 +118,6 @@ static struct caml_thread_table thread_table[Max_domains];
 /* The thread identifier of the "tick" thread for this domain */
 #define Tick_thread_id thread_table[Caml_state->id].tick_thread_id
 
-/* The key used for storing the thread descriptor in the specific data
-   of the corresponding system thread. */
-#define Thread_key thread_table[Caml_state->id].thread_key
-
 /* Identifier for next thread creation */
 static atomic_uintnat thread_next_id = 0;
 
@@ -214,7 +212,7 @@ static void caml_thread_leave_blocking_section(void)
   st_masterlock_acquire(&Thread_main_lock);
   /* Update Current_thread to point to the thread descriptor corresponding to
      the thread currently executing */
-  Current_thread = st_tls_get(Thread_key);
+  Current_thread = st_tls_get(caml_thread_key);
   /* Restore the runtime state from the curr_thread descriptor */
   caml_thread_restore_runtime_state();
 }
@@ -365,8 +363,7 @@ CAMLprim value caml_thread_initialize_domain(value v)
   new_thread->prev = new_thread;
   new_thread->backtrace_last_exn = Val_unit;
 
-  st_tls_newkey(&Thread_key);
-  st_tls_set(Thread_key, (void *) new_thread);
+  st_tls_set(caml_thread_key, new_thread);
 
   All_threads = new_thread;
   Current_thread = new_thread;
@@ -402,6 +399,9 @@ CAMLprim value caml_thread_initialize(value unit)
     caml_failwith("caml_thread_initialize: cannot initialize Thread "
                   "while several domains are running.");
 
+  /* Initialize the key to the [caml_thread_t] structure */
+  st_tls_newkey(&caml_thread_key);
+
   /* First initialise the systhread chain on this domain */
   caml_thread_initialize_domain(Val_unit);
 
@@ -476,12 +476,12 @@ static void * caml_thread_start(void * v)
 
   caml_init_domain_self(th->domain_id);
 
-  st_tls_set(Thread_key, th);
+  st_tls_set(caml_thread_key, th);
 
   caml_domain_set_name("Domain");
 
   st_masterlock_acquire(&Thread_main_lock);
-  Current_thread = st_tls_get(Thread_key);
+  Current_thread = st_tls_get(caml_thread_key);
   caml_thread_restore_runtime_state();
 
 #ifdef POSIX_SIGNALS
@@ -586,7 +586,7 @@ CAMLexport int caml_c_thread_register(void)
   if (Caml_state == NULL) {
     caml_init_domain_self(0);
   };
-  if (st_tls_get(Thread_key) != NULL) return 0;
+  if (st_tls_get(caml_thread_key) != NULL) return 0;
   /* Take master lock to protect access to the runtime */
   st_masterlock_acquire(&Thread_main_lock);
   /* Create a thread info block */
@@ -608,7 +608,7 @@ CAMLexport int caml_c_thread_register(void)
     All_threads->next = th;
   }
   /* Associate the thread descriptor with the thread */
-  st_tls_set(Thread_key, (void *) th);
+  st_tls_set(caml_thread_key, (void *) th);
   /* Allocate the thread descriptor on the heap */
   th->descr = caml_thread_new_descriptor(Val_unit);  /* no closure */
 
@@ -633,13 +633,13 @@ CAMLexport int caml_c_thread_unregister(void)
   /* If Caml_state is not set, this thread was likely not registered */
   if (Caml_state == NULL) return 0;
 
-  th = st_tls_get(Thread_key);
+  th = st_tls_get(caml_thread_key);
   /* Not registered? */
   if (th == NULL) return 0;
   /* Wait until the runtime is available */
   st_masterlock_acquire(&Thread_main_lock);
   /*  Forget the thread descriptor */
-  st_tls_set(Thread_key, NULL);
+  st_tls_set(caml_thread_key, NULL);
   /* Remove thread info block from list of threads, and free it */
   caml_thread_remove_info(th);
 
@@ -699,7 +699,7 @@ CAMLprim value caml_thread_yield(value unit)
   caml_raise_if_exception(caml_process_pending_signals_exn());
   caml_thread_save_runtime_state();
   st_thread_yield(&Thread_main_lock);
-  Current_thread = st_tls_get(Thread_key);
+  Current_thread = st_tls_get(caml_thread_key);
   caml_thread_restore_runtime_state();
   caml_raise_if_exception(caml_process_pending_signals_exn());