go/worker/client: Better handle latest round queries with verification

When a query is requesting to be executed against the latest round and
the runtime reports a consensus verifier error, use an earlier round
instead as the latest round may not yet be verifiable by the light
client as it needs to wait for the validator signatures.

.changelog/5123.bugfix.md

@@ -0,0 +1,6 @@
+go/worker/client: Better handle latest round queries with verification
+
+When a query is requesting to be executed against the latest round and
+the runtime reports a consensus verifier error, use an earlier round
+instead as the latest round may not yet be verifiable by the light
+client as it needs to wait for the validator signatures.

go/runtime/host/protocol/errors.go

@@ -0,0 +1,12 @@
+package protocol
+
+import (
+	"github.com/oasisprotocol/oasis-core/go/common/errors"
+)
+
+// ModuleVerifierName is the name of the consensus verifier module inside the runtime.
+const ModuleVerifierName = "verifier"
+
+// ErrVerifierVerificationFailed is the error returned when consensus verifier fails to verify the
+// passed consensus light block.
+var ErrVerifierVerificationFailed = errors.New(ModuleVerifierName, 2, "verifier: light block verification failed")

go/worker/client/committee/node.go

@@ -11,6 +11,7 @@ import (
 
 	cmnBackoff "github.com/oasisprotocol/oasis-core/go/common/backoff"
 	"github.com/oasisprotocol/oasis-core/go/common/crypto/hash"
+	"github.com/oasisprotocol/oasis-core/go/common/errors"
 	"github.com/oasisprotocol/oasis-core/go/common/logging"
 	roothash "github.com/oasisprotocol/oasis-core/go/roothash/api"
 	"github.com/oasisprotocol/oasis-core/go/roothash/api/block"
@@ -149,22 +150,52 @@ func (n *Node) Query(ctx context.Context, round uint64, method string, args []by
 	}
 	maxMessages := dsc.Executor.MaxMessages
 
-	annBlk, err := n.commonNode.Runtime.History().GetAnnotatedBlock(ctx, round)
-	if err != nil {
-		return nil, fmt.Errorf("client: failed to fetch annotated block from history: %w", err)
-	}
+	var resolvedRound uint64
+	queryFn := func(round uint64) ([]byte, error) {
+		annBlk, err := n.commonNode.Runtime.History().GetAnnotatedBlock(ctx, round)
+		if err != nil {
+			return nil, fmt.Errorf("client: failed to fetch annotated block from history: %w", err)
+		}
+		resolvedRound = annBlk.Block.Header.Round
 
-	// Get consensus light block for state after executing block at given height.
-	lb, err := n.commonNode.Consensus.GetLightBlockForState(ctx, annBlk.Height)
-	if err != nil {
-		return nil, fmt.Errorf("client: failed to get light block at height %d: %w", annBlk.Height, err)
-	}
-	epoch, err := n.commonNode.Consensus.Beacon().GetEpoch(ctx, annBlk.Height)
-	if err != nil {
-		return nil, fmt.Errorf("client: failed to get epoch at height %d: %w", annBlk.Height, err)
+		// Get consensus light block for state after executing block at given height.
+		lb, err := n.commonNode.Consensus.GetLightBlockForState(ctx, annBlk.Height)
+		if err != nil {
+			return nil, fmt.Errorf("client: failed to get light block at height %d: %w", annBlk.Height, err)
+		}
+		epoch, err := n.commonNode.Consensus.Beacon().GetEpoch(ctx, annBlk.Height)
+		if err != nil {
+			return nil, fmt.Errorf("client: failed to get epoch at height %d: %w", annBlk.Height, err)
+		}
+
+		return hrt.Query(ctx, annBlk.Block, lb, epoch, maxMessages, method, args)
 	}
 
-	return hrt.Query(ctx, annBlk.Block, lb, epoch, maxMessages, method, args)
+	data, err := queryFn(round)
+	if errors.Is(err, protocol.ErrVerifierVerificationFailed) {
+		// The query failed due to the runtime's consensus verifier failing to verify the given
+		// header. We assume that this is because a finalized header is not yet available for the
+		// given round.
+		switch round {
+		case api.RoundLatest:
+			// Since we are allowed to decide what we see as the latest round, use an earlier one.
+			n.logger.Debug("runtime's consensus verifier reports failure, retrying",
+				"method", method,
+				"target_round", resolvedRound,
+			)
+
+			data, err = queryFn(resolvedRound - 1)
+		default:
+			// A specific round was given so this query is not yet possible.
+			n.logger.Debug("runtime's consensus verifier reports failure",
+				"method", method,
+				"target_round", round,
+			)
+
+			return nil, roothash.ErrNotFound
+		}
+	}
+	return data, err
 }
 
 func (n *Node) checkBlock(ctx context.Context, blk *block.Block, pending map[hash.Hash]*pendingTx) error {