cmd/geth, core: add support for recording SHA3 preimages (#3543)

eth/api.go

@@ -560,3 +560,9 @@ func (api *PrivateDebugAPI) TraceTransaction(ctx context.Context, txHash common.
 	}
 	return nil, errors.New("database inconsistency")
 }
+
+// Preimage is a debug API function that returns the preimage for a sha3 hash, if known.
+func (api *PrivateDebugAPI) Preimage(ctx context.Context, hash common.Hash) (hexutil.Bytes, error) {
+	db := core.PreimageTable(api.eth.ChainDb())
+	return db.Get(hash.Bytes())
+}