catchup: Provide more information to client when requested block not available

catchup/universalFetcher.go

@@ -19,9 +19,9 @@ package catchup
 import (
 	"context"
 	"encoding/binary"
-	"errors"
 	"fmt"
 	"net/http"
+	"strconv"
 	"time"
 
 	"github.com/algorand/go-deadlock"
@@ -173,6 +173,9 @@ func (w *wsFetcherClient) requestBlock(ctx context.Context, round basics.Round)
 	}
 
 	if errMsg, found := resp.Topics.GetValue(network.ErrorKey); found {
+		if latest, lfound := resp.Topics.GetValue(rpcs.LatestRoundKey); lfound {
+			return nil, noBlockForRoundError{round: round, latest: basics.Round(binary.BigEndian.Uint64(latest))}
+		}
 		return nil, makeErrWsFetcherRequestFailed(round, w.target.GetAddress(), string(errMsg))
 	}
 
@@ -195,7 +198,11 @@ func (w *wsFetcherClient) requestBlock(ctx context.Context, round basics.Round)
 // set max fetcher size to 10MB, this is enough to fit the block and certificate
 const fetcherMaxBlockBytes = 10 << 20
 
-var errNoBlockForRound = errors.New("No block available for given round")
+type noBlockForRoundError struct {
+	latest, round basics.Round
+}
+
+func (noBlockForRoundError) Error() string { return "no block available for given round" }
 
 // HTTPFetcher implements FetcherClient doing an HTTP GET of the block
 type HTTPFetcher struct {
@@ -239,7 +246,13 @@ func (hf *HTTPFetcher) getBlockBytes(ctx context.Context, r basics.Round) (data
 	case http.StatusOK:
 	case http.StatusNotFound: // server could not find a block with that round numbers.
 		response.Body.Close()
-		return nil, errNoBlockForRound
+		noBlockErr := noBlockForRoundError{round: r}
+		if latestBytes := response.Header.Get(rpcs.BlockResponseLatestRoundHeader); latestBytes != "" {
+			if latest, pErr := strconv.ParseUint(latestBytes, 10, 64); pErr == nil {
+				noBlockErr.latest = basics.Round(latest)
+			}
+		}
+		return nil, noBlockErr
 	default:
 		bodyBytes, err := rpcs.ResponseBytes(response, hf.log, fetcherMaxBlockBytes)
 		hf.log.Warnf("HTTPFetcher.getBlockBytes: response status code %d from '%s'. Response body '%s' ", response.StatusCode, blockURL, string(bodyBytes))

catchup/universalFetcher_test.go

@@ -74,7 +74,9 @@ func TestUGetBlockWs(t *testing.T) {
 	block, cert, duration, err = fetcher.fetchBlock(context.Background(), next+1, up)
 
 	require.Error(t, err)
-	require.Contains(t, err.Error(), "requested block is not available")
+	require.Error(t, noBlockForRoundError{}, err)
+	require.Equal(t, next+1, err.(noBlockForRoundError).round)
+	require.Equal(t, next, err.(noBlockForRoundError).latest)
 	require.Nil(t, block)
 	require.Nil(t, cert)
 	require.Equal(t, int64(duration), int64(0))
@@ -118,8 +120,10 @@ func TestUGetBlockHTTP(t *testing.T) {
 
 	block, cert, duration, err = fetcher.fetchBlock(context.Background(), next+1, net.GetPeers()[0])
 
-	require.Error(t, errNoBlockForRound, err)
-	require.Contains(t, err.Error(), "No block available for given round")
+	require.Error(t, noBlockForRoundError{}, err)
+	require.Equal(t, next+1, err.(noBlockForRoundError).round)
+	require.Equal(t, next, err.(noBlockForRoundError).latest)
+	require.Contains(t, err.Error(), "no block available for given round")
 	require.Nil(t, block)
 	require.Nil(t, cert)
 	require.Equal(t, int64(duration), int64(0))

rpcs/blockService.go

@@ -54,6 +54,9 @@ const blockResponseRetryAfter = "3"
 const blockServerMaxBodyLength = 512                                               // we don't really pass meaningful content here, so 512 bytes should be a safe limit
 const blockServerCatchupRequestBufferSize = 10
 
+// BlockResponseLatestRoundHeader is returned in the response header when the requested block is not available
+const BlockResponseLatestRoundHeader = "X-Latest-Round"
+
 // BlockServiceBlockPath is the path to register BlockService as a handler for when using gorilla/mux
 // e.g. .Handle(BlockServiceBlockPath, &ls)
 const BlockServiceBlockPath = "/v{version:[0-9.]+}/{genesisID}/block/{round:[0-9a-z]+}"
@@ -65,6 +68,7 @@ const (
 	BlockDataKey       = "blockData"       // Block-data topic-key in the response
 	CertDataKey        = "certData"        // Cert-data topic-key in the response
 	BlockAndCertValue  = "blockAndCert"    // block+cert request data (as the value of requestDataTypeKey)
+	LatestRoundKey     = "latest"
 )
 
 var errBlockServiceClosed = errors.New("block service is shutting down")
@@ -239,12 +243,13 @@ func (bs *BlockService) ServeHTTP(response http.ResponseWriter, request *http.Re
 	}
 	encodedBlockCert, err := bs.rawBlockBytes(basics.Round(round))
 	if err != nil {
-		switch err.(type) {
+		switch lerr := err.(type) {
 		case ledgercore.ErrNoEntry:
 			// entry cound not be found.
 			ok := bs.redirectRequest(round, response, request)
 			if !ok {
 				response.Header().Set("Cache-Control", blockResponseMissingBlockCacheControl)
+				response.Header().Set(BlockResponseLatestRoundHeader, fmt.Sprintf("%d", lerr.Latest))
 				response.WriteHeader(http.StatusNotFound)
 			}
 			return
@@ -456,8 +461,12 @@ func (bs *BlockService) rawBlockBytes(round basics.Round) ([]byte, error) {
 func topicBlockBytes(log logging.Logger, dataLedger LedgerForBlockService, round basics.Round, requestType string) (network.Topics, uint64) {
 	blk, cert, err := dataLedger.EncodedBlockCert(round)
 	if err != nil {
-		switch err.(type) {
+		switch lerr := err.(type) {
 		case ledgercore.ErrNoEntry:
+			return network.Topics{
+				network.MakeTopic(network.ErrorKey, []byte(blockNotAvailableErrMsg)),
+				network.MakeTopic(LatestRoundKey, binary.BigEndian.AppendUint64([]byte{}, uint64(lerr.Latest))),
+			}, 0
 		default:
 			log.Infof("BlockService topicBlockBytes: %s", err)
 		}