feat(graphsync): give missing blocks a named error (#227)

Missing block error should have inspectable values

graphsync.go

@@ -3,6 +3,7 @@ package graphsync
 import (
 	"context"
 	"errors"
+	"fmt"
 
 	"github.com/ipfs/go-cid"
 	"github.com/ipld/go-ipld-prime"
@@ -94,6 +95,17 @@ func (e RequestNotFoundErr) Error() string {
 	return "request not found"
 }
 
+// RemoteMissingBlockErr indicates that the remote peer was missing a block
+// in the selector requested. It is a non-terminal error in the error stream
+// for a request and does NOT cause a request to fail completely
+type RemoteMissingBlockErr struct {
+	Link ipld.Link
+}
+
+func (e RemoteMissingBlockErr) Error() string {
+	return fmt.Sprintf("remote peer is missing block: %s", e.Link.String())
+}
+
 var (
 	// ErrExtensionAlreadyRegistered means a user extension can be registered only once
 	ErrExtensionAlreadyRegistered = errors.New("extension already registered")

requestmanager/asyncloader/responsecache/responsecache.go

@@ -1,7 +1,6 @@
 package responsecache
 
 import (
-	"fmt"
 	"sync"
 
 	blocks "github.com/ipfs/go-block-format"
@@ -59,7 +58,7 @@ func (rc *ResponseCache) AttemptLoad(requestID graphsync.RequestID, link ipld.Li
 	rc.responseCacheLk.Lock()
 	defer rc.responseCacheLk.Unlock()
 	if rc.linkTracker.IsKnownMissingLink(requestID, link) {
-		return nil, fmt.Errorf("remote peer is missing block: %s", link.String())
+		return nil, graphsync.RemoteMissingBlockErr{Link: link}
 	}
 	data, _ := rc.unverifiedBlockStore.VerifyBlock(link, linkContext)
 	return data, nil