go: Return error from Handle.NewWriter

This occurs when Handle is closed.

go/resource/resource.go

@@ -3,6 +3,7 @@
 package possumResource
 
 import (
+	"fmt"
 	"github.com/anacrolix/missinggo/v2/resource"
 	possum "github.com/anacrolix/possum/go"
 	"io"
@@ -61,7 +62,11 @@ func (i *instance) Get() (_ io.ReadCloser, err error) {
 }
 
 func (i *instance) Put(reader io.Reader) (err error) {
-	w := i.handle.NewWriter()
+	w, err := i.handle.NewWriter()
+	if err != nil {
+		err = fmt.Errorf("creating new possum writer: %w", err)
+		return
+	}
 	defer func() {
 		// TODO: There's no Writer.Drop.
 		commitErr := w.Commit()
@@ -100,7 +105,7 @@ func (i *instance) ReadAt(p []byte, off int64) (n int, err error) {
 	return i.handle.SingleReadAt(i.key, off, p)
 }
 
-func (i *instance) WriteAt(bytes []byte, i2 int64) (int, error) {
+func (i *instance) WriteAt([]byte, int64) (int, error) {
 	//TODO implement me
 	panic("implement me")
 }

go/writer.go

@@ -10,15 +10,16 @@ type Writer struct {
 	handleRef *Rc[*possumC.Handle]
 }
 
-func (me *Handle) NewWriter() *Writer {
+func (me *Handle) NewWriter() (w *Writer, err error) {
 	rc, err := me.cloneRc()
 	if err != nil {
-		panic(err)
+		return
 	}
-	return &Writer{
+	w = &Writer{
 		c:         possumC.NewWriter(rc.Deref()),
 		handleRef: rc,
 	}
+	return
 }
 
 type ValueWriter struct {
@@ -59,7 +60,8 @@ func (me *ValueWriter) NewFile(name string) (f *os.File, err error) {
 	return
 }
 
-// This consumes the Writer.
+// This consumes the Writer. You must commit a Writer after it's created or it will leak a reference
+// to the Handle. There doesn't seem to be a way to abort it in the Go API.
 func (me *Writer) Commit() error {
 	err := possumC.CommitWriter(me.c)
 	me.c = nil