3 files changed, 49 insertions, 13 deletions
diff --git a/internal/thread.h b/internal/thread.h
index 00fcbfc560..928126c3b0 100644
--- a/internal/thread.h
+++ b/internal/thread.h
@@ -83,6 +83,8 @@ RUBY_SYMBOL_EXPORT_END
 int rb_threadptr_execute_interrupts(struct rb_thread_struct *th, int blocking_timing);
 bool rb_thread_mn_schedulable(VALUE thread);
 
+bool rb_thread_resolve_unblock_function(rb_unblock_function_t **unblock_function, void **data2, struct rb_thread_struct *thread);
+
 // interrupt exec
 
 typedef VALUE (rb_interrupt_exec_func_t)(void *data);
diff --git a/scheduler.c b/scheduler.c
index 11faca01d3..83b9681cc3 100644
--- a/scheduler.c
+++ b/scheduler.c
@@ -63,8 +63,10 @@ typedef enum {
 struct rb_fiber_scheduler_blocking_operation {
     void *(*function)(void *);
     void *data;
+
     rb_unblock_function_t *unblock_function;
     void *data2;
+
     int flags;
     struct rb_fiber_scheduler_blocking_operation_state *state;
 
@@ -208,7 +210,10 @@ rb_fiber_scheduler_blocking_operation_execute(rb_fiber_scheduler_blocking_operat
         return -1; // Invalid blocking operation
     }
 
-        // Atomically check if we can transition from QUEUED to EXECUTING
+    // Resolve sentinel values for unblock_function and data2:
+    rb_thread_resolve_unblock_function(&blocking_operation->unblock_function, &blocking_operation->data2, GET_THREAD());
+
+    // Atomically check if we can transition from QUEUED to EXECUTING
     rb_atomic_t expected = RB_FIBER_SCHEDULER_BLOCKING_OPERATION_STATUS_QUEUED;
     if (RUBY_ATOMIC_CAS(blocking_operation->status, expected, RB_FIBER_SCHEDULER_BLOCKING_OPERATION_STATUS_EXECUTING) != expected) {
         // Already cancelled or in wrong state
@@ -1124,25 +1129,33 @@ rb_fiber_scheduler_blocking_operation_cancel(rb_fiber_scheduler_blocking_operati
 
     rb_atomic_t current_state = RUBY_ATOMIC_LOAD(blocking_operation->status);
 
-        switch (current_state) {
+    switch (current_state) {
         case RB_FIBER_SCHEDULER_BLOCKING_OPERATION_STATUS_QUEUED:
-            // Work hasn't started - just mark as cancelled
+            // Work hasn't started - just mark as cancelled:
             if (RUBY_ATOMIC_CAS(blocking_operation->status, current_state, RB_FIBER_SCHEDULER_BLOCKING_OPERATION_STATUS_CANCELLED) == current_state) {
-                return 0; // Successfully cancelled before execution
+                // Successfully cancelled before execution:
+                return 0;
             }
             // Fall through if state changed between load and CAS
 
         case RB_FIBER_SCHEDULER_BLOCKING_OPERATION_STATUS_EXECUTING:
             // Work is running - mark cancelled AND call unblock function
-            RUBY_ATOMIC_SET(blocking_operation->status, RB_FIBER_SCHEDULER_BLOCKING_OPERATION_STATUS_CANCELLED);
-            if (blocking_operation->unblock_function) {
+            if (RUBY_ATOMIC_CAS(blocking_operation->status, current_state, RB_FIBER_SCHEDULER_BLOCKING_OPERATION_STATUS_CANCELLED) != current_state) {
+                // State changed between load and CAS - operation may have completed:
+                return 0;
+            }
+            // Otherwise, we successfully marked it as cancelled, so we can call the unblock function:
+            rb_unblock_function_t *unblock_function = blocking_operation->unblock_function;
+            if (unblock_function) {
+                RUBY_ASSERT(unblock_function != (rb_unblock_function_t *)-1 && "unblock_function is still sentinel value -1, should have been resolved earlier");
                 blocking_operation->unblock_function(blocking_operation->data2);
             }
-            return 1; // Cancelled during execution (unblock function called)
+            // Cancelled during execution (unblock function called):
+            return 1;
 
         case RB_FIBER_SCHEDULER_BLOCKING_OPERATION_STATUS_COMPLETED:
         case RB_FIBER_SCHEDULER_BLOCKING_OPERATION_STATUS_CANCELLED:
-            // Already finished or cancelled
+            // Already finished or cancelled:
             return 0;
     }
 
diff --git a/thread.c b/thread.c
index 232c677382..41bd6c9ec6 100644
--- a/thread.c
+++ b/thread.c
@@ -1540,6 +1540,29 @@ blocking_region_end(rb_thread_t *th, struct rb_blocking_region_buffer *region)
 #endif
 }
 
+/*
+ * Resolve sentinel unblock function values to their actual function pointers
+ * and appropriate data2 values. This centralizes the logic for handling
+ * RUBY_UBF_IO and RUBY_UBF_PROCESS sentinel values.
+ *
+ * @param unblock_function Pointer to unblock function pointer (modified in place)
+ * @param data2 Pointer to data2 pointer (modified in place)
+ * @param thread Thread context for resolving data2 when needed
+ * @return true if sentinel values were resolved, false otherwise
+ */
+bool
+rb_thread_resolve_unblock_function(rb_unblock_function_t **unblock_function, void **data2, struct rb_thread_struct *thread)
+{
+    rb_unblock_function_t *ubf = *unblock_function;
+
+    if ((ubf == RUBY_UBF_IO) || (ubf == RUBY_UBF_PROCESS)) {
+        *unblock_function = ubf_select;
+        *data2 = thread;
+        return true;
+    }
+    return false;
+}
+
 void *
 rb_nogvl(void *(*func)(void *), void *data1,
          rb_unblock_function_t *ubf, void *data2,
@@ -1566,11 +1589,9 @@ rb_nogvl(void *(*func)(void *), void *data1,
     bool is_main_thread = vm->ractor.main_thread == th;
     int saved_errno = 0;
 
-    if ((ubf == RUBY_UBF_IO) || (ubf == RUBY_UBF_PROCESS)) {
-        ubf = ubf_select;
-        data2 = th;
-    }
-    else if (ubf && rb_ractor_living_thread_num(th->ractor) == 1 && is_main_thread) {
+    rb_thread_resolve_unblock_function(&ubf, &data2, th);
+
+    if (ubf && rb_ractor_living_thread_num(th->ractor) == 1 && is_main_thread) {
         if (flags & RB_NOGVL_UBF_ASYNC_SAFE) {
             vm->ubf_async_safe = 1;
         }