use bitswap sessions when fetching messages, and cancel them

1. Explicitly use a session when fetching messages for a block. Technically,
GetBlocks would use an internal session so we'd only end up with two, but one
per block is even better.
2. Actually cancel sessions after a timeout (threading through the context).

NOTE: We should seriously consider having a single session for all blocks, but
we can do that in a followup as it may have unintended consequences (e.g., leaks).

chain/sub/incoming.go

@@ -13,6 +13,7 @@ import (
 	"github.com/filecoin-project/specs-actors/actors/util/adt"
 	lru "github.com/hashicorp/golang-lru"
 	blocks "github.com/ipfs/go-block-format"
+	"github.com/ipfs/go-blockservice"
 	bserv "github.com/ipfs/go-blockservice"
 	"github.com/ipfs/go-cid"
 	cbor "github.com/ipfs/go-ipld-cbor"
@@ -44,6 +45,10 @@ var ErrSoftFailure = errors.New("soft validation failure")
 var ErrInsufficientPower = errors.New("incoming block's miner does not have minimum power")
 
 func HandleIncomingBlocks(ctx context.Context, bsub *pubsub.Subscription, s *chain.Syncer, bserv bserv.BlockService, cmgr connmgr.ConnManager) {
+	// Timeout after (block time + propagation delay). This is useless at
+	// this point.
+	timeout := time.Duration(build.BlockDelaySecs+build.PropagationDelaySecs) * time.Second
+
 	for {
 		msg, err := bsub.Next(ctx)
 		if err != nil {
@@ -64,15 +69,22 @@ func HandleIncomingBlocks(ctx context.Context, bsub *pubsub.Subscription, s *cha
 		src := msg.GetFrom()
 
 		go func() {
+			ctx, cancel := context.WithTimeout(ctx, timeout)
+			defer cancel()
+
+			// NOTE: we could also share a single session between
+			// all requests but that may have other consequences.
+			ses := blockservice.NewSession(ctx, bserv)
+
 			start := build.Clock.Now()
 			log.Debug("about to fetch messages for block from pubsub")
-			bmsgs, err := FetchMessagesByCids(context.TODO(), bserv, blk.BlsMessages)
+			bmsgs, err := FetchMessagesByCids(ctx, ses, blk.BlsMessages)
 			if err != nil {
 				log.Errorf("failed to fetch all bls messages for block received over pubusb: %s; source: %s", err, src)
 				return
 			}
 
-			smsgs, err := FetchSignedMessagesByCids(context.TODO(), bserv, blk.SecpkMessages)
+			smsgs, err := FetchSignedMessagesByCids(ctx, ses, blk.SecpkMessages)
 			if err != nil {
 				log.Errorf("failed to fetch all secpk messages for block received over pubusb: %s; source: %s", err, src)
 				return
@@ -97,7 +109,7 @@ func HandleIncomingBlocks(ctx context.Context, bsub *pubsub.Subscription, s *cha
 
 func FetchMessagesByCids(
 	ctx context.Context,
-	bserv bserv.BlockService,
+	bserv bserv.BlockGetter,
 	cids []cid.Cid,
 ) ([]*types.Message, error) {
 	out := make([]*types.Message, len(cids))
@@ -126,7 +138,7 @@ func FetchMessagesByCids(
 // FIXME: Duplicate of above.
 func FetchSignedMessagesByCids(
 	ctx context.Context,
-	bserv bserv.BlockService,
+	bserv bserv.BlockGetter,
 	cids []cid.Cid,
 ) ([]*types.SignedMessage, error) {
 	out := make([]*types.SignedMessage, len(cids))
@@ -156,12 +168,12 @@ func FetchSignedMessagesByCids(
 //  blocks we did not request.
 func fetchCids(
 	ctx context.Context,
-	bserv bserv.BlockService,
+	bserv bserv.BlockGetter,
 	cids []cid.Cid,
 	cb func(int, blocks.Block) error,
 ) error {
 	// FIXME: Why don't we use the context here?
-	fetchedBlocks := bserv.GetBlocks(context.TODO(), cids)
+	fetchedBlocks := bserv.GetBlocks(ctx, cids)
 
 	cidIndex := make(map[cid.Cid]int)
 	for i, c := range cids {