fix(storage): fix read-write race in Writer.Write (#6817)

Fixes #6816

w.monitorCancel() -> w.CloseWithError() reads the values of w.opened and w.pw concurrently with the calling routine initializing them; if the incoming context is cancelled before or during initialization.

The fix is simply to deploy starting the monitor until all fields are initialized. The older open() code from previous releases did this in the right order:

https://github.com/googleapis/google-cloud-go/blob/storage/v1.24.0/storage/writer.go#L130-L134

storage/writer.go

@@ -176,7 +176,6 @@ func (w *Writer) openWriter() (err error) {
 
 	isIdempotent := w.o.conds != nil && (w.o.conds.GenerationMatch >= 0 || w.o.conds.DoesNotExist == true)
 	opts := makeStorageOpts(isIdempotent, w.o.retry, w.o.userProject)
-	go w.monitorCancel()
 	params := &openWriterParams{
 		ctx:                w.ctx,
 		chunkSize:          w.ChunkSize,
@@ -191,11 +190,15 @@ func (w *Writer) openWriter() (err error) {
 		progress:           w.progress,
 		setObj:             func(o *ObjectAttrs) { w.obj = o },
 	}
+	if err := w.ctx.Err(); err != nil {
+		return err // short-circuit
+	}
 	w.pw, err = w.o.c.tc.OpenWriter(params, opts...)
 	if err != nil {
 		return err
 	}
 	w.opened = true
+	go w.monitorCancel()
 
 	return nil
 }