improve(SpokePool): Prevent future deposit quoteTimestamps

contracts/SpokePool.sol

@@ -439,18 +439,14 @@ abstract contract SpokePool is
         require(amount <= MAX_TRANSFER_SIZE, "Amount too large");
         require(depositCounter[originToken] <= maxCount, "Above max count");
 
-        // This function assumes that L2 timing cannot be compared accurately and consistently to L1 timing. Therefore,
-        // block.timestamp is different from the L1 EVM's. Therefore, the quoteTimestamp must be within a configurable
-        // buffer of this contract's block time to allow for this variance.
-        // Note also that quoteTimestamp cannot be less than the buffer otherwise the following arithmetic can result
-        // in underflow. This isn't a problem as the deposit will revert, but the error might be unexpected for clients.
-
-        //slither-disable-next-line timestamp
-        require(
-            getCurrentTime() >= quoteTimestamp - depositQuoteTimeBuffer &&
-                getCurrentTime() <= quoteTimestamp + depositQuoteTimeBuffer,
-            "invalid quote time"
-        );
+        // Require that quoteTimestamp has a maximum age so that depositors pay an LP fee based on recent HubPool usage.
+        // It is assumed that cross-chain timestamps are normally loosely in-sync, but clock drift can occur. If the
+        // SpokePool time stalls or lags significantly, it is still possible to make deposits by setting quoteTimestamp
+        // within the configured buffer. The owner should pause deposits if this is undesirable. This will underflow if
+        // quoteTimestamp is more than depositQuoteTimeBuffer; this is safe but will throw an unintuitive error.
+
+        // slither-disable-next-line timestamp
+        require(getCurrentTime() - quoteTimestamp <= depositQuoteTimeBuffer, "invalid quoteTimestamp");
 
         // Increment count of deposits so that deposit ID for this spoke pool is unique.
         uint32 newDepositId = numberOfDeposits++;

test/SpokePool.Deposit.ts

@@ -288,7 +288,7 @@ describe("SpokePool Depositor Logic", async function () {
   });
 
   it("quoteTimestamp is out of range", async function () {
-    const revertReason = "invalid quote time";
+    const revertReason = "invalid quoteTimestamp";
     const quoteTimeBuffer = await spokePool.depositQuoteTimeBuffer();
 
     await expect(
@@ -301,11 +301,11 @@ describe("SpokePool Depositor Logic", async function () {
             amountToDeposit,
             destinationChainId,
             depositRelayerFeePct,
-            toBN(currentSpokePoolTime).add(quoteTimeBuffer + 1),
+            toBN(currentSpokePoolTime).add(1),
             maxUint256
           )
         )
-    ).to.be.revertedWith(revertReason);
+    ).to.be.reverted;
 
     await expect(
       spokePool
@@ -323,22 +323,25 @@ describe("SpokePool Depositor Logic", async function () {
         )
     ).to.be.revertedWith(revertReason);
 
-    // quoteTimestamp at max age.
-    await expect(
-      spokePool
-        .connect(depositor)
-        .deposit(
-          ...getDepositParams(
-            recipient.address,
-            erc20.address,
-            amountToDeposit,
-            destinationChainId,
-            depositRelayerFeePct,
-            currentSpokePoolTime.sub(quoteTimeBuffer),
-            maxUint256
+    // quoteTimestamp at the exact margins should succeed.
+    for (const offset of [0, quoteTimeBuffer]) {
+      await erc20.connect(depositor).approve(spokePool.address, amountToDeposit);
+      await expect(
+        spokePool
+          .connect(depositor)
+          .deposit(
+            ...getDepositParams(
+              recipient.address,
+              erc20.address,
+              amountToDeposit,
+              destinationChainId,
+              depositRelayerFeePct,
+              currentSpokePoolTime.sub(offset),
+              maxUint256
+            )
           )
-        )
-    ).to.emit(spokePool, "FundsDeposited");
+      ).to.emit(spokePool, "FundsDeposited");
+    }
   });
 
   it("maxCount is too low", async function () {