Disallow finishing of contract that wasn't started

contracts/Storage.sol

@@ -10,7 +10,7 @@ contract Storage is Collateral, Marketplace, Proofs {
   uint256 public slashMisses;
   uint256 public slashPercentage;
 
-  mapping(bytes32 => bool) private finished;
+  mapping(bytes32 => ContractState) private contractState;
 
   constructor(
     IERC20 token,
@@ -33,14 +33,15 @@ contract Storage is Collateral, Marketplace, Proofs {
     Offer storage offer = _offer(id);
     require(msg.sender == offer.host, "Only host can call this function");
     require(_selectedOffer(offer.requestId) == id, "Offer was not selected");
+    contractState[id] = ContractState.started;
     Request storage request = _request(offer.requestId);
     _expectProofs(id, request.proofProbability, request.duration);
   }
 
   function finishContract(bytes32 id) public {
+    require(contractState[id] == ContractState.started, "Contract not started");
     require(block.timestamp > proofEnd(id), "Contract has not ended yet");
-    require(!finished[id], "Contract already finished");
-    finished[id] = true;
+    contractState[id] = ContractState.finished;
     Offer storage offer = _offer(id);
     require(token.transfer(offer.host, offer.price), "Payment failed");
   }
@@ -84,4 +85,10 @@ contract Storage is Collateral, Marketplace, Proofs {
       _slash(offer.host, slashPercentage);
     }
   }
+
+  enum ContractState {
+    none,
+    started,
+    finished
+  }
 }

test/Storage.test.js

@@ -84,7 +84,6 @@ describe("Storage", function () {
   describe("finishing the contract", function () {
     beforeEach(async function () {
       switchAccount(host)
-      await storage.startContract(id)
     })
 
     async function waitUntilEnd() {
@@ -93,12 +92,14 @@ describe("Storage", function () {
     }
 
     it("unlocks the host collateral", async function () {
+      await storage.startContract(id)
       await waitUntilEnd()
       await storage.finishContract(id)
       await expect(storage.withdraw()).not.to.be.reverted
     })
 
     it("pays the host", async function () {
+      await storage.startContract(id)
       await waitUntilEnd()
       const startBalance = await token.balanceOf(host.address)
       await storage.finishContract(id)
@@ -106,18 +107,31 @@ describe("Storage", function () {
       expect(endBalance - startBalance).to.equal(offer.price)
     })
 
+    it("is only allowed when the contract has started", async function () {
+      await expect(storage.finishContract(id)).to.be.revertedWith(
+        "Contract not started"
+      )
+    })
+
     it("is only allowed when end time has passed", async function () {
+      await storage.startContract(id)
       await expect(storage.finishContract(id)).to.be.revertedWith(
         "Contract has not ended yet"
       )
     })
 
     it("can only be done once", async function () {
+      await storage.startContract(id)
       await waitUntilEnd()
       await storage.finishContract(id)
-      await expect(storage.finishContract(id)).to.be.revertedWith(
-        "Contract already finished"
-      )
+      await expect(storage.finishContract(id)).to.be.reverted
+    })
+
+    it("can not be restarted", async function () {
+      await storage.startContract(id)
+      await waitUntilEnd()
+      await storage.finishContract(id)
+      await expect(storage.startContract(id)).to.be.reverted
     })
   })