Contract Reader Handle Empty Bytes (#15688)

In some cases, a contract call will return an empty result `0x`, which decodes to an empty set of bytes. The codec
can't decode this empty set except in specific cases where a slice of bytes is the expected result.

This commit adds a short-circuit to decoding an empty result where the return value is unaltered.

core/services/relay/evm/chain_reader.go

@@ -206,7 +206,11 @@ func (cr *chainReader) GetLatestValue(ctx context.Context, readName string, conf
 	ptrToValue, isValue := returnVal.(*values.Value)
 	if !isValue {
 		_, err = binding.GetLatestValueWithHeadData(ctx, common.HexToAddress(address), confidenceLevel, params, returnVal)
-		return err
+		if err != nil {
+			return err
+		}
+
+		return nil
 	}
 
 	contractType, err := cr.CreateContractType(readName, false)

core/services/relay/evm/read/batch.go

@@ -241,32 +241,36 @@ func (c *defaultEvmBatchCaller) unpackBatchResults(
 			return nil, callErr
 		}
 
-		if err = c.codec.Decode(
-			ctx,
-			packedBytes,
-			call.ReturnVal,
-			codec.WrapItemType(call.ContractName, call.ReadName, false),
-		); err != nil {
-			if len(packedBytes) == 0 {
-				callErr := newErrorFromCall(
-					fmt.Errorf("%w: %w: %s", types.ErrInternal, errEmptyOutput, err.Error()),
-					call, block, batchReadType,
-				)
-
-				callErr.Result = &hexEncodedOutputs[idx]
-
-				results[idx].err = callErr
-			} else {
-				callErr := newErrorFromCall(
-					fmt.Errorf("%w: codec decode result: %s", types.ErrInvalidType, err.Error()),
-					call, block, batchReadType,
-				)
-
-				callErr.Result = &hexEncodedOutputs[idx]
-				results[idx].err = callErr
-			}
+		// the codec can't do anything with no bytes, so skip decoding and allow
+		// the result to be the empty struct or value
+		if len(packedBytes) > 0 {
+			if err = c.codec.Decode(
+				ctx,
+				packedBytes,
+				call.ReturnVal,
+				codec.WrapItemType(call.ContractName, call.ReadName, false),
+			); err != nil {
+				if len(packedBytes) == 0 {
+					callErr := newErrorFromCall(
+						fmt.Errorf("%w: %w: %s", types.ErrInternal, errEmptyOutput, err.Error()),
+						call, block, batchReadType,
+					)
+
+					callErr.Result = &hexEncodedOutputs[idx]
+
+					results[idx].err = callErr
+				} else {
+					callErr := newErrorFromCall(
+						fmt.Errorf("%w: codec decode result: %s", types.ErrInvalidType, err.Error()),
+						call, block, batchReadType,
+					)
+
+					callErr.Result = &hexEncodedOutputs[idx]
+					results[idx].err = callErr
+				}
 
-			continue
+				continue
+			}
 		}
 
 		results[idx].returnVal = call.ReturnVal

core/services/relay/evm/read/method.go

@@ -2,6 +2,7 @@ package read
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"math/big"
 	"sync"
@@ -22,6 +23,8 @@ import (
 	evmclient "github.com/smartcontractkit/chainlink/v2/core/chains/evm/client"
 )
 
+var ErrEmptyContractReturnValue = errors.New("the contract return value was empty")
+
 type MethodBinding struct {
 	// read-only properties
 	contractName string
@@ -173,6 +176,12 @@ func (b *MethodBinding) GetLatestValueWithHeadData(ctx context.Context, addr com
 		return nil, callErr
 	}
 
+	// there may be cases where the contract value has not been set and the RPC returns with a value of 0x
+	// which is a set of empty bytes. there is no need for the codec to run in this case.
+	if len(bytes) == 0 {
+		return block.ToChainAgnosticHead(), nil
+	}
+
 	if err = b.codec.Decode(ctx, bytes, returnVal, codec.WrapItemType(b.contractName, b.method, false)); err != nil {
 		callErr := newErrorFromCall(
 			fmt.Errorf("%w: decode return data: %s", commontypes.ErrInvalidType, err.Error()),