make Close safe, not letting Finalize block forever either

This commit was moved from ipld/go-car@c06b4f2

Author: mvdan

ipld/car/v2/blockstore/readonly.go

@@ -330,7 +330,18 @@ func (b *ReadOnly) Roots() ([]cid.Cid, error) {
 }
 
 // Close closes the underlying reader if it was opened by OpenReadOnly.
+//
+// Note that this call may block if any blockstore operations are currently in
+// progress, including an AllKeysChan that hasn't been fully consumed or
+// cancelled.
 func (b *ReadOnly) Close() error {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	return b.closeWithoutMutex()
+}
+
+func (b *ReadOnly) closeWithoutMutex() error {
 	if b.carv2Closer != nil {
 		return b.carv2Closer.Close()
 	}

ipld/car/v2/blockstore/readwrite.go

@@ -343,7 +343,12 @@ func (b *ReadWrite) Finalize() error {
 	defer b.mu.Unlock()
 	// TODO check if add index option is set and don't write the index then set index offset to zero.
 	b.header = b.header.WithDataSize(uint64(b.dataWriter.Position()))
-	defer b.Close()
+
+	// Note that we can't use b.Close here, as that tries to grab the same
+	// mutex we're holding here.
+	// TODO: should we check the error here? especially with OpenReadWrite,
+	// we should care about close errors.
+	defer b.closeWithoutMutex()
 
 	// TODO if index not needed don't bother flattening it.
 	fi, err := b.idx.flatten()