feat: explicit check on enqueue with same priority (#563)

* feat: explicit check on enqueue with same priority

It is invalid to do so as it would break the framework. Add explicit check instead of rely on
the correct implementation of ExitGame contracts.

issue: #559

* fix: for PR comments

plasma_framework/contracts/src/framework/ExitGameController.sol

@@ -104,6 +104,7 @@ contract ExitGameController is ExitGameRegistry {
      *         priority queue to enforce the priority of exit during 'processExits'
      * @dev emits ExitQueued event, which can be used to back trace the priority inside the queue
      * @dev Caller of this function should add "pragma experimental ABIEncoderV2;" on top of file
+     * @dev Priority (exitableAt, txPos, exitId) must be unique per queue. Do not enqueue when the same priority is already in the queue.
      * @param vaultId Vault ID of the vault that stores exiting funds
      * @param token Token for the exit
      * @param exitableAt The earliest time a specified exit can be processed
@@ -133,6 +134,7 @@ contract ExitGameController is ExitGameRegistry {
         queue.insert(priority);
 
         bytes32 delegationKey = getDelegationKey(priority, vaultId, token);
+        require(address(delegations[delegationKey]) == address(0), "The same priority is already enqueued");
         delegations[delegationKey] = exitProcessor;
 
         emit ExitQueued(exitId, priority);

plasma_framework/test/src/framework/ExitGameController.test.js

@@ -134,7 +134,7 @@ contract('ExitGameController', () => {
             );
         });
 
-        it('can enqueue with the same exitable timestamp (priority) multiple times', async () => {
+        it('rejects when the same priority is already enqueued', async () => {
             await this.dummyExitGame.enqueue(
                 VAULT_ID,
                 this.dummyExit.token,
@@ -144,6 +144,23 @@ contract('ExitGameController', () => {
                 this.dummyExit.exitProcessor,
             );
 
+            await expectRevert(
+                this.dummyExitGame.enqueue(
+                    VAULT_ID,
+                    this.dummyExit.token,
+                    this.dummyExit.exitableAt,
+                    this.dummyExit.txPos,
+                    this.dummyExit.exitId,
+                    this.dummyExit.exitProcessor,
+                ),
+                'The same priority is already enqueued',
+            );
+        });
+
+        it('can enqueue with the exact same priority to different priority queue', async () => {
+            const vaultId2 = VAULT_ID + 1;
+            await this.controller.addExitQueue(vaultId2, this.dummyToken);
+
             await this.dummyExitGame.enqueue(
                 VAULT_ID,
                 this.dummyExit.token,
@@ -153,6 +170,47 @@ contract('ExitGameController', () => {
                 this.dummyExit.exitProcessor,
             );
 
+            await this.dummyExitGame.enqueue(
+                vaultId2,
+                this.dummyExit.token,
+                this.dummyExit.exitableAt,
+                this.dummyExit.txPos,
+                this.dummyExit.exitId,
+                this.dummyExit.exitProcessor,
+            );
+
+            const key = exitQueueKey(VAULT_ID, this.dummyToken);
+            const priorityQueueAddress = await this.controller.exitsQueues(key);
+            const priorityQueue = await PriorityQueue.at(priorityQueueAddress);
+            expect(await priorityQueue.currentSize()).to.be.bignumber.equal(new BN(1));
+
+            const key2 = exitQueueKey(vaultId2, this.dummyToken);
+            const priorityQueueAddress2 = await this.controller.exitsQueues(key2);
+            const priorityQueue2 = await PriorityQueue.at(priorityQueueAddress2);
+            expect(await priorityQueue2.currentSize()).to.be.bignumber.equal(new BN(1));
+        });
+
+        it('can enqueue with the same exitable timestamp and txPos but with different exitId multiple times to the same queue', async () => {
+            const exitId1 = 111111;
+            const exitId2 = 22222;
+            await this.dummyExitGame.enqueue(
+                VAULT_ID,
+                this.dummyExit.token,
+                this.dummyExit.exitableAt,
+                this.dummyExit.txPos,
+                exitId1,
+                this.dummyExit.exitProcessor,
+            );
+
+            await this.dummyExitGame.enqueue(
+                VAULT_ID,
+                this.dummyExit.token,
+                this.dummyExit.exitableAt,
+                this.dummyExit.txPos,
+                exitId2,
+                this.dummyExit.exitProcessor,
+            );
+
             const key = exitQueueKey(VAULT_ID, this.dummyToken);
             const priorityQueueAddress = await this.controller.exitsQueues(key);
             const priorityQueue = await PriorityQueue.at(priorityQueueAddress);