runtime: fix the equality check in AddCleanup

This fixes the check that ensures that arg is not equal to ptr in
AddCleanup. This also changes any use of throw to panic in AddCleanup.

Fixes #71316

Change-Id: Ie5a3e0163b254dff44b7fefedf75207ba587b771
Reviewed-on: https://go-review.googlesource.com/c/go/+/643655
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/runtime/mcleanup.go

@@ -70,19 +70,19 @@ func AddCleanup[T, S any](ptr *T, cleanup func(S), arg S) Cleanup {
 
 	// The pointer to the object must be valid.
 	if ptr == nil {
-		throw("runtime.AddCleanup: ptr is nil")
+		panic("runtime.AddCleanup: ptr is nil")
 	}
 	usptr := uintptr(unsafe.Pointer(ptr))
 
 	// Check that arg is not equal to ptr.
-	// TODO(67535) this does not cover the case where T and *S are the same
-	// type and ptr and arg are equal.
-	if unsafe.Pointer(&arg) == unsafe.Pointer(ptr) {
-		throw("runtime.AddCleanup: ptr is equal to arg, cleanup will never run")
+	if kind := abi.TypeOf(arg).Kind(); kind == abi.Pointer || kind == abi.UnsafePointer {
+		if unsafe.Pointer(ptr) == *((*unsafe.Pointer)(unsafe.Pointer(&arg))) {
+			panic("runtime.AddCleanup: ptr is equal to arg, cleanup will never run")
+		}
 	}
 	if inUserArenaChunk(usptr) {
 		// Arena-allocated objects are not eligible for cleanup.
-		throw("runtime.AddCleanup: ptr is arena-allocated")
+		panic("runtime.AddCleanup: ptr is arena-allocated")
 	}
 	if debug.sbrk != 0 {
 		// debug.sbrk never frees memory, so no cleanup will ever run
@@ -105,7 +105,7 @@ func AddCleanup[T, S any](ptr *T, cleanup func(S), arg S) Cleanup {
 			// Cleanup is a noop.
 			return Cleanup{}
 		}
-		throw("runtime.AddCleanup: ptr not in allocated block")
+		panic("runtime.AddCleanup: ptr not in allocated block")
 	}
 
 	// Ensure we have a finalizer processing goroutine running.

src/runtime/mcleanup_test.go

@@ -269,3 +269,30 @@ func TestCleanupStopAfterCleanupRuns(t *testing.T) {
 	<-ch
 	stop()
 }
+
+func TestCleanupPointerEqualsArg(t *testing.T) {
+	// See go.dev/issue/71316
+	defer func() {
+		want := "runtime.AddCleanup: ptr is equal to arg, cleanup will never run"
+		if r := recover(); r == nil {
+			t.Error("want panic, test did not panic")
+		} else if r == want {
+			// do nothing
+		} else {
+			t.Errorf("wrong panic: want=%q, got=%q", want, r)
+		}
+	}()
+
+	// allocate struct with pointer to avoid hitting tinyalloc.
+	// Otherwise we can't be sure when the allocation will
+	// be freed.
+	type T struct {
+		v int
+		p unsafe.Pointer
+	}
+	v := &new(T).v
+	*v = 97531
+	runtime.AddCleanup(v, func(x *int) {}, v)
+	v = nil
+	runtime.GC()
+}