[marketplace] add slot/request mapping, cancelled check

Add a requestId mapping to the Slot type. This allows the requestId to be obtained for a slot and the StorageRequest can be looked up from the id.

Add `isSlotCancelled` to check if the request that a slot belongs to has been cancelled (not enough slots filled before expiry).

contracts/Marketplace.sol

@@ -72,6 +72,7 @@ contract Marketplace is Collateral, Proofs {
     _submitProof(slotId, proof);
 
     slot.host = msg.sender;
+    slot.requestId = requestId;
     context.slotsFilled += 1;
     emit SlotFilled(requestId, slotIndex, slotId);
     if (context.slotsFilled == request.ask.slots) {
@@ -119,12 +120,12 @@ contract Marketplace is Collateral, Proofs {
     emit RequestCancelled(requestId);
   }
 
-  /// @notice Rseturn true if the request state is RequestState.Cancelled or if the request expiry time has elapsed and the request was never started.
+  /// @notice Return true if the request state is RequestState.Cancelled or if the request expiry time has elapsed and the request was never started.
   /// @dev Handles the case when a request may have been cancelled, but the client has not withdrawn its funds yet, and therefore the state has not yet been updated.
   /// @param requestId the id of the request
   /// @return true if request is cancelled
   function isCancelled(bytes32 requestId) public view returns (bool) {
-    RequestContext storage context = requestContexts[requestId];
+    RequestContext memory context = requestContexts[requestId];
     return
       context.state == RequestState.Cancelled ||
       (
@@ -133,6 +134,17 @@ contract Marketplace is Collateral, Proofs {
       );
   }
 
+  /// @notice Return true if the request state the slot belongs to is RequestState.Cancelled or if the request expiry time has elapsed and the request was never started.
+  /// @dev Handles the case when a request may have been cancelled, but the client has not withdrawn its funds yet, and therefore the state has not yet been updated.
+  /// @param slotId the id of the slot
+  /// @return true if request is cancelled
+  function isSlotCancelled(bytes32 slotId) public view returns (bool) {
+    Slot memory slot = slots[slotId];
+    require(slot.host != address(0), "Slot empty");
+    require(slot.requestId != 0, "Missing request id");
+    return isCancelled(slot.requestId);
+  }
+
   function _host(bytes32 slotId) internal view returns (address) {
     return slots[slotId].host;
   }
@@ -225,6 +237,7 @@ contract Marketplace is Collateral, Proofs {
   struct Slot {
     address host;
     bool hostPaid;
+    bytes32 requestId;
   }
 
   event StorageRequested(bytes32 requestId, Ask ask);

contracts/Storage.sol

@@ -65,7 +65,7 @@ contract Storage is Collateral, Marketplace {
   }
 
   function markProofAsMissing(bytes32 slotId, uint256 period) public {
-    require(!isCancelled(slotId), "Request was cancelled");
+    require(!isSlotCancelled(slotId), "Request was cancelled");
     _markProofAsMissing(slotId, period);
     if (_missed(slotId) % slashMisses == 0) {
       _slash(_host(slotId), slashPercentage);