Add asynchronous block notification interface

To be implemented in later commits.

pkg/raptorq/interface.go

@@ -121,6 +121,7 @@ type Decoder interface {
  // returns; IsSourceBlockReady or IsSourceObjectReady may not
  // immediately return true even if the symbol made the source block or
  // the source object available.
+ // Use AddReadyBlockChan if immediate notification is needed.
  Decode(sbn uint8, esi uint32, symbol []byte)
 
  // IsSourceBlockReady returns whether the given source block has been fully
@@ -146,6 +147,24 @@ type Decoder interface {
  // been freed, calling Encode with its SBN may return an error.
  FreeSourceBlock(sbn uint8)
 
+ // AddReadyBlockChan adds a channel through which the decoder shall avail
+ // source blocks ready for retrieval as soon as they become available.
+ //
+ // If any block is currently available,
+ // their source block number is immediately sent into the given channel.
+ //
+ // The added channel is closed when no more blocks are available,
+ // e.g. when the decoder is closed or destroyed.
+ //
+ // If more than one channel is added,
+ // newly available source block numbers are sent to all of them,
+ // in no specific order.
+ AddReadyBlockChan(chan<- uint8) (err error)
+
+ // RemoveReadyBlockChan removes a channel previously added via
+ // AddReadyBlockChan. It does not close the removed channel.
+ RemoveReadyBlockChan(chan<- uint8) (err error)
+
  // Close closes the Decoder. After a Decoder is closed, all methods but
  // Close() will panic if called.
  Close() error