Add function to create general dagsync blockhook function (#144)

* Add function to create general dagsync blockhook function

A general blockhook function is commonly used by clients of this library when segmented syncs are enabled.

* Ad and entries deserialization functions

dagsync/option.go

@@ -310,3 +310,35 @@ func ScopedBlockHook(hook BlockHookFunc) SyncOption {
 		sc.blockHook = hook
 	}
 }
+
+// MakeGeneralBlockHook creates a block hook function that sets the next sync
+// action based on whether the specified advertisement has a previous
+// advertisement in the chain..
+//
+// Use this when segmented sync is enabled and no other blockhook is defined.
+//
+// The supplied prevAdCid function takes the CID of the current advertisement
+// and returns the CID of the previous advertisement in the chain. This would
+// typically be done my loading the specified advertisement from the
+// ipld.LinkSystem and getting the previous CID.
+func MakeGeneralBlockHook(prevAdCid func(adCid cid.Cid) (cid.Cid, error)) BlockHookFunc {
+	return func(_ peer.ID, adCid cid.Cid, actions SegmentSyncActions) {
+		// The only kind of block we should get by loading CIDs here should be
+		// Advertisement.
+		//
+		// Because:
+		//  - The default subscription selector only selects advertisements.
+		//  - Entries are synced with an explicit selector separate from
+		//    advertisement syncs and should use dagsync.ScopedBlockHook to
+		//    override this hook and decode chunks instead.
+		//
+		// Therefore, we only attempt to load advertisements here and signal
+		// failure if the load fails.
+		prevCid, err := prevAdCid(adCid)
+		if err != nil {
+			actions.FailSync(err)
+		} else {
+			actions.SetNextSyncCid(prevCid)
+		}
+	}
+}

ingest/schema/types.go

@@ -1,12 +1,21 @@
 package schema
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
+	"io"
 
+	"github.com/ipfs/go-cid"
 	"github.com/ipld/go-ipld-prime"
+	cidlink "github.com/ipld/go-ipld-prime/linking/cid"
+	"github.com/ipld/go-ipld-prime/multicodec"
 	"github.com/ipld/go-ipld-prime/node/bindnode"
 	"github.com/multiformats/go-multihash"
+
+	// Import so these codecs get registered.
+	_ "github.com/ipld/go-ipld-prime/codec/dagcbor"
+	_ "github.com/ipld/go-ipld-prime/codec/dagjson"
 )
 
 type (
@@ -82,6 +91,15 @@ func UnwrapAdvertisement(node ipld.Node) (*Advertisement, error) {
 	return ad, nil
 }
 
+// Return the Advertisement's previous CID, or cid.Undef if there is no
+// previous CID.
+func (a Advertisement) PreviousCid() cid.Cid {
+	if a.PreviousID == nil {
+		return cid.Undef
+	}
+	return a.PreviousID.(cidlink.Link).Cid
+}
+
 func (a Advertisement) Validate() error {
 	if len(a.ContextID) > MaxContextIDLen {
 		return errors.New("context id too long")
@@ -94,6 +112,49 @@ func (a Advertisement) Validate() error {
 	return nil
 }
 
+// BytesToAdvertisement deserializes an Advertisement from a buffer. It does
+// not check that the given CID matches the data, as this should have been done
+// when the data was acquired.
+func BytesToAdvertisement(adCid cid.Cid, data []byte) (Advertisement, error) {
+	adNode, err := decodeIPLDNode(adCid.Prefix().Codec, bytes.NewBuffer(data), AdvertisementPrototype)
+	if err != nil {
+		return Advertisement{}, err
+	}
+	ad, err := UnwrapAdvertisement(adNode)
+	if err != nil {
+		return Advertisement{}, err
+	}
+	return *ad, nil
+}
+
+// BytesToEntryChunk deserializes an EntryChunk from a buffer. It does not
+// check that the given CID matches the data, as this should have been done
+// when the data was acquired.
+func BytesToEntryChunk(entCid cid.Cid, data []byte) (EntryChunk, error) {
+	entNode, err := decodeIPLDNode(entCid.Prefix().Codec, bytes.NewBuffer(data), EntryChunkPrototype)
+	if err != nil {
+		return EntryChunk{}, err
+	}
+	ent, err := UnwrapEntryChunk(entNode)
+	if err != nil {
+		return EntryChunk{}, err
+	}
+	return *ent, nil
+}
+
+// decodeIPLDNode decodes an ipld.Node from bytes read from an io.Reader.
+func decodeIPLDNode(codec uint64, r io.Reader, prototype ipld.NodePrototype) (ipld.Node, error) {
+	nb := prototype.NewBuilder()
+	decoder, err := multicodec.LookupDecoder(codec)
+	if err != nil {
+		return nil, err
+	}
+	if err = decoder(nb, r); err != nil {
+		return nil, err
+	}
+	return nb.Build(), nil
+}
+
 // UnwrapEntryChunk unwraps the given node as an entry chunk.
 //
 // Note that the node is reassigned to EntryChunkPrototype if its prototype is different.