fix: Make getCurrentBaseFees smarter on timing

l1-contracts/test/fees/FeeRollup.t.sol

@@ -299,8 +299,6 @@ contract FeeRollupTest is FeeModelTestPoints, DecoderBase {
     Slot nextSlot = Slot.wrap(1);
     Epoch nextEpoch = Epoch.wrap(1);
 
-    // We need the fee at the time just before the first slot
-
     // Loop through all of the L1 metadata
     for (uint256 i = 0; i < l1Metadata.length; i++) {
       // Predict what the fee will be before we jump in time!

yarn-project/sequencer-client/src/global_variable_builder/global_builder.ts

@@ -51,7 +51,15 @@ export class GlobalVariableBuilder implements GlobalVariableBuilderInterface {
    * @returns Base fees for the expected next block
    */
   public async getCurrentBaseFees(): Promise<GasFees> {
-    const timestamp = BigInt((await this.publicClient.getBlock()).timestamp + BigInt(this.ethereumSlotDuration));
+    // Since this might be called in the middle of a slot where a block might have been published,
+    // we need to fetch the last block written, and estimate the earliest timestamp for the next block.
+    // The timestamp of that last block will act as a lower bound for the next block.
+
+    const lastBlock = await this.rollupContract.read.getBlock([await this.rollupContract.read.getPendingBlockNumber()]);
+    const earliestTimestamp = await this.rollupContract.read.getTimestampForSlot([lastBlock.slotNumber + 1n]);
+    const nextEthTimestamp = BigInt((await this.publicClient.getBlock()).timestamp + BigInt(this.ethereumSlotDuration));
+    const timestamp = earliestTimestamp > nextEthTimestamp ? earliestTimestamp : nextEthTimestamp;
+
     return new GasFees(Fr.ZERO, new Fr(await this.rollupContract.read.getManaBaseFeeAt([timestamp, true])));
   }
 
@@ -82,6 +90,7 @@ export class GlobalVariableBuilder implements GlobalVariableBuilderInterface {
     const slotFr = new Fr(slotNumber);
     const timestampFr = new Fr(timestamp);
 
+    // We can skip much of the logic in getCurrentBaseFees since it we already check that we are not within a slot elsewhere.
     const gasFees = new GasFees(Fr.ZERO, new Fr(await this.rollupContract.read.getManaBaseFeeAt([timestamp, true])));
 
     const globalVariables = new GlobalVariables(