Merge pull request #73 from ethereum/blockhash

Add return code to get_block_hash_fn

bindings/go/evmc/host.c

@@ -97,8 +97,8 @@ static inline void go_exported_functions_type_checks()
     tx_context = getTxContext(context);
 
     evmc_get_block_hash_fn get_block_hash_fn = NULL;
-    get_block_hash_fn(uint256be, context, number);
-    getBlockHash(uint256be, context, number);
+    status = get_block_hash_fn(uint256be, context, number);
+    status = getBlockHash(uint256be, context, number);
 
     evmc_emit_log_fn emit_log_fn = NULL;
     emit_log_fn(context, address, data, size, uint256be, size);

bindings/go/evmc/host.go

@@ -79,7 +79,7 @@ type HostContext interface {
 	Selfdestruct(addr common.Address, beneficiary common.Address)
 	GetTxContext() (gasPrice common.Hash, origin common.Address, coinbase common.Address, number int64, timestamp int64,
 		gasLimit int64, difficulty common.Hash)
-	GetBlockHash(number int64) common.Hash
+	GetBlockHash(number int64) (common.Hash, error)
 	EmitLog(addr common.Address, topics []common.Hash, data []byte)
 	Call(kind CallKind,
 		destination common.Address, sender common.Address, value *big.Int, input []byte, gas int64, depth int,
@@ -182,11 +182,17 @@ func getTxContext(pCtx unsafe.Pointer) C.struct_evmc_tx_context {
 }
 
 //export getBlockHash
-func getBlockHash(pResult *C.struct_evmc_uint256be, pCtx unsafe.Pointer, number int64) {
+func getBlockHash(pResult *C.struct_evmc_uint256be, pCtx unsafe.Pointer, number int64) C.int {
 	idx := int((*C.struct_extended_context)(pCtx).index)
 	ctx := getHostContext(idx)
 
-	*pResult = evmcUint256be(ctx.GetBlockHash(number))
+	blockhash, err := ctx.GetBlockHash(number)
+	if err != nil {
+		return C.int(0)
+	}
+
+	*pResult = evmcUint256be(blockhash)
+	return C.int(1)
 }
 
 //export emitLog

examples/example_host.cpp

@@ -9,6 +9,13 @@
 
 #include <evmc/helpers.h>
 
+struct example_host_context : evmc_context
+{
+    example_host_context();
+
+    evmc_tx_context tx_context = {};
+};
+
 static evmc_uint256be balance(evmc_context* context, const evmc_address* address)
 {
     (void)context;
@@ -107,11 +114,17 @@ static evmc_tx_context get_tx_context(evmc_context* context)
     return result;
 }
 
-static void get_block_hash(evmc_uint256be* result, evmc_context* context, int64_t number)
+static int get_block_hash(evmc_uint256be* result, evmc_context* context, int64_t number)
 {
-    (void)result;
-    (void)context;
-    (void)number;
+    example_host_context* host = static_cast<example_host_context*>(context);
+    int64_t current_block_number = host->tx_context.block_number;
+
+    if (number >= current_block_number || number < current_block_number - 256)
+        return 0;
+
+    evmc_uint256be example_block_hash{};
+    *result = example_block_hash;
+    return 1;
 }
 
 static void emit_log(evmc_context* context,
@@ -134,10 +147,7 @@ static const evmc_host_interface interface = {
     copy_code,      selfdestruct, call,        get_tx_context, get_block_hash, emit_log,
 };
 
-struct example_host_context : evmc_context
-{
-    example_host_context() : evmc_context{&interface} {}
-};
+example_host_context::example_host_context() : evmc_context{&interface} {}
 
 extern "C" {
 

include/evmc/evmc.h

@@ -153,17 +153,19 @@ typedef struct evmc_tx_context (*evmc_get_tx_context_fn)(struct evmc_context* co
 /**
  * Get block hash callback function.
  *
- *  This callback function is used by an EVM to query the block hash of
- *  a given block.
+ *  This callback function is used by an VM to query the block hash of
+ *  a given block. If the requested block is not found, then an appropriate
+ *  result code is returned.
  *
- *  @param[out] result   The returned block hash value.
+ *  @param[out] result   The returned block hash value. Only written to
+ *                       if the return value is 1 (information is avialable).
  *  @param      context  The pointer to the Host execution context.
- *  @param      number   The block number. Must be a value between
- *                       (and including) 0 and 255.
+ *  @param      number   The block number.
+ *  @return              1 if the information is available, 0 otherwise.
  */
-typedef void (*evmc_get_block_hash_fn)(struct evmc_uint256be* result,
-                                       struct evmc_context* context,
-                                       int64_t number);
+typedef int (*evmc_get_block_hash_fn)(struct evmc_uint256be* result,
+                                      struct evmc_context* context,
+                                      int64_t number);
 
 /**
  * The execution status code.