fix: use _Atomic flag for sel_alloc_thread_id registration, move reg before loop, add wrong-thread guard test

src/lib/testsuite/test_select.c

@@ -6,6 +6,7 @@
  */
 #define UNIT_TEST 1
 
+#include <pthread.h>
 #include <stdatomic.h>
 #include <stdint.h>
 #include <stdio.h>
@@ -2252,6 +2253,52 @@ test_sel_register_alloc_thread(void)
     sel_optimize_test_reset();
 }
 
+/* ── Task 8: wrong-thread ownership guard test ───────────────────────────*/
+
+/* shared state for the wrong-thread sweep test */
+static int g_wrong_thread_sweep_ran = 0;
+
+static void* wrong_thread_sweep(void* arg)
+{
+    (void)arg;
+    /* calling gc_sweep from the wrong thread must hit the ownership guard
+     * and return without sweeping — nodes must remain intact */
+    gc_sweep_select_nodes(NULL, 0);
+    g_wrong_thread_sweep_ran = 1;
+    return NULL;
+}
+
+static void
+test_sel_wrong_thread_guard(void)
+{
+    sel_optimize_test_reset();
+    sel_optimize_set(1);
+    sel_node_table_init();
+
+    /* Register THIS thread as the allocator */
+    sel_register_alloc_thread();
+
+    /* Allocate a node that should survive the wrong-thread sweep */
+    xce_select_node_t* n = makeSelectNode(SEL_TRUE, 0, 0);
+    TEST_ASSERT_NOT_NULL(n);
+
+    /* Spawn a different thread to call gc_sweep — guard must block it */
+    pthread_t tid;
+    g_wrong_thread_sweep_ran = 0;
+    int rc = pthread_create(&tid, NULL, wrong_thread_sweep, NULL);
+    TEST_ASSERT_EQUAL_INT(0, rc);
+    pthread_join(tid, NULL);
+    TEST_ASSERT_EQUAL_INT(1, g_wrong_thread_sweep_ran);
+
+    /* Node must still exist (wrong-thread sweep was blocked) */
+    /* GC from correct thread: mark then sweep normally */
+    gc_clear_marks();
+    gc_sweep_select_nodes(NULL, 0);
+
+    sel_node_table_fini();
+    sel_optimize_test_reset();
+}
+
 /* ── Top-level runner ─────────────────────────────────────────────────────*/
 
 void
@@ -2365,6 +2412,7 @@ test_select(void)
 
     /* ── Task 8: sel_alloc_thread_id ownership check ─────────────────── */
     RUN_TEST(test_sel_register_alloc_thread);
+    RUN_TEST(test_sel_wrong_thread_guard);
 
     xstring_table_fini();
 }

src/lib/xce_select.c

@@ -23,8 +23,9 @@
 
 /* ── optimize level ───────────────────────────────────────────────────── */
 
-static _Atomic int g_sel_optimize_level = -1;   /* -1 = unset */
-static pthread_t   sel_alloc_thread_id;          /* 0 = unregistered — skip checks */
+static _Atomic int g_sel_optimize_level   = -1;  /* -1 = unset */
+static pthread_t   sel_alloc_thread_id;
+static _Atomic int sel_alloc_thread_registered = 0; /* 1 after sel_register_alloc_thread() */
 
 void sel_optimize_set(int level)
 {
@@ -186,12 +187,14 @@ void sel_optimize_test_reset(void)
 {
     sel_node_table_fini();
     atomic_store_explicit(&g_sel_optimize_level, -1, memory_order_seq_cst);
-    sel_alloc_thread_id = 0;
+    atomic_store_explicit(&sel_alloc_thread_registered, 0, memory_order_seq_cst);
+    /* sel_alloc_thread_id left as-is; only checked when registered flag is set */
 }
 
 void sel_register_alloc_thread(void)
 {
     sel_alloc_thread_id = pthread_self();
+    atomic_store_explicit(&sel_alloc_thread_registered, 1, memory_order_release);
 }
 
 /* ── node construction ───────────────────────────────────────────────── */
@@ -651,7 +654,8 @@ int eval_select(const xce_select_node_t* node, select_eval_ctx_t* ctx)
     if (sel_get_optimize_level() <= 0) {
         ERROR("eval_select called with optimize_level <= 0"); abort();
     }
-    if (sel_alloc_thread_id != 0 && !pthread_equal(pthread_self(), sel_alloc_thread_id)) {
+    if (atomic_load_explicit(&sel_alloc_thread_registered, memory_order_acquire)
+        && !pthread_equal(pthread_self(), sel_alloc_thread_id)) {
         ERROR("eval_select called from non-allocator thread");
         return -1;
     }
@@ -875,7 +879,8 @@ void gc_sweep_select_nodes(const xce_sched_job_t** live_jobs, size_t njobs)
     if (sel_node_table.table == NULL) {
         return;
     }
-    if (sel_alloc_thread_id != 0 && !pthread_equal(pthread_self(), sel_alloc_thread_id)) {
+    if (atomic_load_explicit(&sel_alloc_thread_registered, memory_order_acquire)
+        && !pthread_equal(pthread_self(), sel_alloc_thread_id)) {
         ERROR("gc_sweep_select_nodes called from non-allocator thread");
         return;
     }

src/scheduler/src/main.c

@@ -117,16 +117,11 @@ int main(int argc, char* argv[])
 
     INFO("xce scheduler is running");
 
+    /* Register this thread as the sole eval/GC allocator thread (write-once) */
+    sel_register_alloc_thread();
+
     while (1) {
         sched_update(&sched);
-
-        /* Register this thread as the sole eval/GC allocator thread (write-once) */
-        static int sel_thread_registered = 0;
-        if (!sel_thread_registered) {
-            sel_register_alloc_thread();
-            sel_thread_registered = 1;
-        }
-
         sched_begin_session(&sched);
         sched_go(&sched);