internal/weak: add package implementing weak pointers

This change adds the internal/weak package, which exposes GC-supported
weak pointers to the standard library. This is for the upcoming weak
package, but may be useful for other future constructs.

For #62483.

Change-Id: I4aa8fa9400110ad5ea022a43c094051699ccab9d
Reviewed-on: https://go-review.googlesource.com/c/go/+/576297
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 130 insertions, 0 deletions

diff --git a/src/internal/weak/pointer_test.go b/src/internal/weak/pointer_test.go
new file mode 100644
index 0000000000..e143749230
--- /dev/null
+++ b/src/internal/weak/pointer_test.go
@@ -0,0 +1,130 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package weak_test
+
+import (
+	"internal/weak"
+	"runtime"
+	"testing"
+)
+
+type T struct {
+	// N.B. This must contain a pointer, otherwise the weak handle might get placed
+	// in a tiny block making the tests in this package flaky.
+	t *T
+	a int
+}
+
+func TestPointer(t *testing.T) {
+	bt := new(T)
+	wt := weak.Make(bt)
+	if st := wt.Strong(); st != bt {
+		t.Fatalf("weak pointer is not the same as strong pointer: %p vs. %p", st, bt)
+	}
+	// bt is still referenced.
+	runtime.GC()
+
+	if st := wt.Strong(); st != bt {
+		t.Fatalf("weak pointer is not the same as strong pointer after GC: %p vs. %p", st, bt)
+	}
+	// bt is no longer referenced.
+	runtime.GC()
+
+	if st := wt.Strong(); st != nil {
+		t.Fatalf("expected weak pointer to be nil, got %p", st)
+	}
+}
+
+func TestPointerEquality(t *testing.T) {
+	bt := make([]*T, 10)
+	wt := make([]weak.Pointer[T], 10)
+	for i := range bt {
+		bt[i] = new(T)
+		wt[i] = weak.Make(bt[i])
+	}
+	for i := range bt {
+		st := wt[i].Strong()
+		if st != bt[i] {
+			t.Fatalf("weak pointer is not the same as strong pointer: %p vs. %p", st, bt[i])
+		}
+		if wp := weak.Make(st); wp != wt[i] {
+			t.Fatalf("new weak pointer not equal to existing weak pointer: %v vs. %v", wp, wt[i])
+		}
+		if i == 0 {
+			continue
+		}
+		if wt[i] == wt[i-1] {
+			t.Fatalf("expected weak pointers to not be equal to each other, but got %v", wt[i])
+		}
+	}
+	// bt is still referenced.
+	runtime.GC()
+	for i := range bt {
+		st := wt[i].Strong()
+		if st != bt[i] {
+			t.Fatalf("weak pointer is not the same as strong pointer: %p vs. %p", st, bt[i])
+		}
+		if wp := weak.Make(st); wp != wt[i] {
+			t.Fatalf("new weak pointer not equal to existing weak pointer: %v vs. %v", wp, wt[i])
+		}
+		if i == 0 {
+			continue
+		}
+		if wt[i] == wt[i-1] {
+			t.Fatalf("expected weak pointers to not be equal to each other, but got %v", wt[i])
+		}
+	}
+	bt = nil
+	// bt is no longer referenced.
+	runtime.GC()
+	for i := range bt {
+		st := wt[i].Strong()
+		if st != nil {
+			t.Fatalf("expected weak pointer to be nil, got %p", st)
+		}
+		if i == 0 {
+			continue
+		}
+		if wt[i] == wt[i-1] {
+			t.Fatalf("expected weak pointers to not be equal to each other, but got %v", wt[i])
+		}
+	}
+}
+
+func TestPointerFinalizer(t *testing.T) {
+	bt := new(T)
+	wt := weak.Make(bt)
+	done := make(chan struct{}, 1)
+	runtime.SetFinalizer(bt, func(bt *T) {
+		if wt.Strong() != nil {
+			t.Errorf("weak pointer did not go nil before finalizer ran")
+		}
+		done <- struct{}{}
+	})
+
+	// Make sure the weak pointer stays around while bt is live.
+	runtime.GC()
+	if wt.Strong() == nil {
+		t.Errorf("weak pointer went nil too soon")
+	}
+	runtime.KeepAlive(bt)
+
+	// bt is no longer referenced.
+	//
+	// Run one cycle to queue the finalizer.
+	runtime.GC()
+	if wt.Strong() != nil {
+		t.Errorf("weak pointer did not go nil when finalizer was enqueued")
+	}
+
+	// Wait for the finalizer to run.
+	<-done
+
+	// The weak pointer should still be nil after the finalizer runs.
+	runtime.GC()
+	if wt.Strong() != nil {
+		t.Errorf("weak pointer is non-nil even after finalization: %v", wt)
+	}
+}