Add PacketBufferPtr.ID() method to get a unique ID

Fuchsia uses the pointer value of the buffer for logging purposes[1]. The recent
refactor to hide the pointer behind the value type PacketBufferPtr broke this.
Add a method to expose an ID without exposing the raw pointer value.

[1]: https://cs.opensource.google/fuchsia/fuchsia/+/main:src/connectivity/network/netstack/link/netdevice/client.go;l=405;drc=c1ef7fc5a62946351e696705017366815014fc09

PiperOrigin-RevId: 485940941

pkg/tcpip/stack/packet_buffer_test.go

@@ -544,6 +544,35 @@ func TestPacketBufferData(t *testing.T) {
 	}
 }
 
+func TestPacketBufferId(t *testing.T) {
+	pk := NewPacketBuffer(PacketBufferOptions{
+		ReserveHeaderBytes: 12,
+	})
+
+	id := pk.ID()
+	// The ID should be stable
+	if idAgain := pk.ID(); idAgain != id {
+		t.Errorf("pk.ID() = %d, want %d", idAgain, id)
+	}
+
+	// Shallow copies have the same ID.
+	pkShallowCopy := pk
+	if shallowCopyID := pkShallowCopy.ID(); shallowCopyID != id {
+		t.Errorf("pkShallowCopy.ID() = %d, want %d", shallowCopyID, id)
+	}
+
+	// Clones have different IDs.
+	pkClone := pk.Clone()
+	if cloneID := pkClone.ID(); cloneID == id {
+		t.Errorf("pkClone.ID() = %d = pk.ID(), but pk = %#v, pkClone = %#v", cloneID, pk, pkClone)
+	}
+
+	pk2 := NewPacketBuffer(PacketBufferOptions{ReserveHeaderBytes: 12})
+	if id2 := pk2.ID(); id2 == id {
+		t.Errorf("pk2.ID() = %d = pk.ID(), but pk = %#v, pk2 = %#v", id2, pk, pk2)
+	}
+}
+
 type packetContents struct {
 	link      []byte
 	network   []byte

pkg/tcpip/stack/packet_buffer_unsafe.go

@@ -18,3 +18,11 @@ import "unsafe"
 
 // PacketBufferStructSize is the minimal size of the packet buffer overhead.
 const PacketBufferStructSize = int(unsafe.Sizeof(packetBuffer{}))
+
+// ID returns a unique ID for the underlying storage of the packet.
+//
+// Two PacketBufferPtrs have the same IDs if and only if they point to the same
+// location in memory.
+func (pk PacketBufferPtr) ID() uintptr {
+	return uintptr(unsafe.Pointer(pk.packetBuffer))
+}