Implement EXTCODEHASH opcode for Constantinople

Closes ethereum#719

eth/vm/forks/constantinople/constants.py

@@ -0,0 +1 @@
+GAS_EXTCODEHASH_EIP1052 = 400

eth/vm/forks/constantinople/opcodes.py

@@ -13,8 +13,12 @@
 from eth.vm.forks.byzantium.opcodes import (
     BYZANTIUM_OPCODES
 )
+from eth.vm.forks.constantinople.constants import (
+    GAS_EXTCODEHASH_EIP1052
+)
 from eth.vm.logic import (
-    arithmetic
+    arithmetic,
+    context,
 )
 from eth.vm.opcode import (
     as_opcode
@@ -32,6 +36,11 @@
         mnemonic=mnemonics.SHR,
         gas_cost=constants.GAS_VERYLOW,
     ),
+    opcode_values.EXTCODEHASH: as_opcode(
+        logic_fn=context.extcodehash,
+        mnemonic=mnemonics.EXTCODEHASH,
+        gas_cost=GAS_EXTCODEHASH_EIP1052,
+    ),
 }
 
 CONSTANTINOPLE_OPCODES = merge(

eth/vm/logic/context.py

@@ -138,6 +138,13 @@ def extcodecopy(computation):
     computation.memory_write(mem_start_position, size, padded_code_bytes)
 
 
+def extcodehash(computation):
+    account = force_bytes_to_address(computation.stack_pop(type_hint=constants.BYTES))
+    code_hash = computation.state.account_db.get_code_hash(account)
+
+    computation.stack_push(code_hash)
+
+
 def returndatasize(computation):
     size = len(computation.return_data)
     computation.stack_push(size)

eth/vm/mnemonics.py

@@ -49,6 +49,7 @@
 GASPRICE = 'GASPRICE'
 EXTCODESIZE = 'EXTCODESIZE'
 EXTCODECOPY = 'EXTCODECOPY'
+EXTCODEHASH = 'EXTCODEHASH'
 RETURNDATASIZE = 'RETURNDATASIZE'
 RETURNDATACOPY = 'RETURNDATACOPY'
 #

eth/vm/opcode_values.py

@@ -55,6 +55,7 @@
 GASPRICE = 0x3a
 EXTCODESIZE = 0x3b
 EXTCODECOPY = 0x3c
+EXTCODEHASH = 0x3f
 RETURNDATASIZE = 0x3d
 RETURNDATACOPY = 0x3e