use a sentinel to confirm output happened

contracts/Challenge.sol

@@ -110,7 +110,9 @@ contract Challenge {
     // confirm the finalSystemHash asserts the state you claim (in $t0-$t7) and the machine is stopped
     // you must load these proofs into MIPS before calling this
     // we disagree at the end
-    require(mem.ReadBytes32(finalSystemState, 0x30000800) == assertionRoot, "you are claiming a different state root in machine");
+
+    require(mem.ReadMemory(finalSystemState, 0x30000800) == 0x1337f00d, "state is not outputted");
+    require(mem.ReadBytes32(finalSystemState, 0x30000804) == assertionRoot, "you are claiming a different state root in machine");
     require(mem.ReadMemory(finalSystemState, 0xC0000080) == 0x5EAD0000, "machine is not stopped in final state (PC == 0x5EAD0000)");
 
     return newChallengeTrusted(startState, finalSystemState, stepCount);

minigeth/oracle/embedded_mips.go

@@ -57,10 +57,12 @@ func Halt() {
 }
 
 func Output(output common.Hash, receipts common.Hash) {
-	ret := byteAt(0x30000800, 0x20)
+	ret := byteAt(0x30000804, 0x20)
 	copy(ret, output.Bytes())
-	rret := byteAt(0x30000820, 0x20)
+	rret := byteAt(0x30000824, 0x20)
 	copy(rret, receipts.Bytes())
+	magic := byteAt(0x30000800, 4)
+	copy(magic, []byte{0x13, 0x37, 0xf0, 0x0d})
 	Halt()
 }
 

mipigeth/run.py

@@ -295,8 +295,11 @@ except unicorn.UcError:
 if not died_well:
   raise Exception("program exitted early")
 
+magic = struct.unpack(">I", mu.mem_read(0x30000800, 4))[0]
+assert magic == 0x1337f00d
+
 real_hash = binascii.hexlify(inputs[-0x40:-0x20])
-compare_hash = binascii.hexlify(mu.mem_read(0x30000800, 0x20))
+compare_hash = binascii.hexlify(mu.mem_read(0x30000804, 0x20))
 print("compare", real_hash, "to computed", compare_hash)
 
 if real_hash != compare_hash: