unique: don't retain uncloned input as key

Currently the unique package tries to clone strings that get stored in
its internal map to avoid retaining large strings.

However, this falls over entirely due to the fact that the original
string is *still* stored in the map as a key. Whoops. Fix this by
storing the cloned value in the map instead.

This change also adds a test which fails without this change.

Change-Id: I1a6bb68ed79b869ea12ab6be061a5ae4b4377ddb
Reviewed-on: https://go-review.googlesource.com/c/go/+/610738
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: mknyszek

src/unique/handle.go

@@ -50,21 +50,22 @@ func Make[T comparable](value T) Handle[T] {
 		toInsert     *T // Keep this around to keep it alive.
 		toInsertWeak weak.Pointer[T]
 	)
-	newValue := func() weak.Pointer[T] {
+	newValue := func() (T, weak.Pointer[T]) {
 		if toInsert == nil {
 			toInsert = new(T)
 			*toInsert = clone(value, &m.cloneSeq)
 			toInsertWeak = weak.Make(toInsert)
 		}
-		return toInsertWeak
+		return *toInsert, toInsertWeak
 	}
 	var ptr *T
 	for {
 		// Check the map.
 		wp, ok := m.Load(value)
 		if !ok {
 			// Try to insert a new value into the map.
-			wp, _ = m.LoadOrStore(value, newValue())
+			k, v := newValue()
+			wp, _ = m.LoadOrStore(k, v)
 		}
 		// Now that we're sure there's a value in the map, let's
 		// try to get the pointer we need out of it.

src/unique/handle_test.go

@@ -9,7 +9,10 @@ import (
 	"internal/abi"
 	"reflect"
 	"runtime"
+	"strings"
 	"testing"
+	"time"
+	"unsafe"
 )
 
 // Set up special types. Because the internal maps are sharded by type,
@@ -110,3 +113,22 @@ func checkMapsFor[T comparable](t *testing.T, value T) {
 	}
 	t.Errorf("failed to drain internal maps of %v", value)
 }
+
+func TestMakeClonesStrings(t *testing.T) {
+	s := strings.Clone("abcdefghijklmnopqrstuvwxyz") // N.B. Must be big enough to not be tiny-allocated.
+	ran := make(chan bool)
+	runtime.SetFinalizer(unsafe.StringData(s), func(_ *byte) {
+		ran <- true
+	})
+	h := Make(s)
+
+	// Clean up s (hopefully) and run the finalizer.
+	runtime.GC()
+
+	select {
+	case <-time.After(1 * time.Second):
+		t.Fatal("string was improperly retained")
+	case <-ran:
+	}
+	runtime.KeepAlive(h)
+}