Add exchange.SessionExchange interface for exchanges that support sessions.

Blockservice has an explicit dependency on bitswap so it can
call NewSession. It should rely on the exchange interfaces though, not
on specific implementations.

License: MIT
Signed-off-by: Hector Sanjuan <hector@protocol.ai>

blockservice.go

@@ -11,7 +11,6 @@ import (
 
 	"github.com/ipfs/go-ipfs/blocks/blockstore"
 	exchange "github.com/ipfs/go-ipfs/exchange"
-	bitswap "github.com/ipfs/go-ipfs/exchange/bitswap"
 
 	logging "gx/ipfs/QmRb5jh8z2E8hMGN2tkvs1yHynUanqnZ3UeKwgN1i9P1F8/go-log"
 	cid "gx/ipfs/QmcZfnkapfECQGcLZaf9B79NRg7cRa9EnZh4LSbkCzwNvY/go-cid"
@@ -107,19 +106,22 @@ func (s *blockService) Exchange() exchange.Interface {
 	return s.exchange
 }
 
-// NewSession creates a bitswap session that allows for controlled exchange of
-// wantlists to decrease the bandwidth overhead.
+// NewSession creates a new session that allows for
+// controlled exchange of wantlists to decrease the bandwidth overhead.
+// If the current exchange is a SessionExchange, a new exchange
+// session will be created. Otherwise, the current exchange will be used
+// directly.
 func NewSession(ctx context.Context, bs BlockService) *Session {
-	exchange := bs.Exchange()
-	if bswap, ok := exchange.(*bitswap.Bitswap); ok {
-		ses := bswap.NewSession(ctx)
+	exch := bs.Exchange()
+	if sessEx, ok := exch.(exchange.SessionExchange); ok {
+		ses := sessEx.NewSession(ctx)
 		return &Session{
 			ses: ses,
 			bs:  bs.Blockstore(),
 		}
 	}
 	return &Session{
-		ses: exchange,
+		ses: exch,
 		bs:  bs.Blockstore(),
 	}
 }