Questions regarding lookaheadEntry.timestamp == slotTimestamp check in proveIncorrectLookahead

[dantaik]

In the following code snippet:

// We pull the preconfer present at the required slot timestamp in the lookahead.
// If no preconfer is present for a slot, we simply use the 0-address to denote the preconfer.
address preconferInLookahead;
if (lookaheadEntry.timestamp == slotTimestamp && !lookaheadEntry.isFallback) {
    // The slot was dedicated to a specific preconfer
    preconferInLookahead = lookaheadEntry.preconfer;
} else {
    // The slot was empty and it was the next preconfer who was expected to preconf in advanced, OR
    // the slot was empty and the preconfer was expected to be the fallback preconfer for the epoch.
    // We still use the zero address because technically the slot itself was empty in the lookahead.
    preconferInLookahead = address(0);
}

Why lookaheadEntry.timestamp == slotTimestamp (I don't quite understand the comment in the else-statement). It seems preconferInLookahead will be zero unless this strict if-statement is true.

[AnshuJalan]

Why lookaheadEntry.timestamp == slotTimestamp

If the validator at a certain slot does not belong to a preconfer, the slot is considered to be empty and a preconfer from a future slot may start preconfing in advanced. So, lookaheadEntry.timestamp == slotTimestamp checks if the slot was actually dedicated to the preconfer and was not an empty slot where a future preconfer was preconfing in advanced.

We represent these empty slots in this function by stating that the 0-address was the preconfer for the slot.

It seems preconferInLookahead will be zero unless this strict if-statement is true.

Now that I see it again, the else statement is redundant and confusing, makes sense to remove it.

[AnshuJalan]

Fixed here: NethermindEth/Taiko-Preconf-AVS#180

[dantaik]

@AnshuJalan, could you please confirm if my understanding is correct here:

  struct LookaheadBufferEntry {
        // True if the preconfer is randomly selected
        bool isFallback;
        // Timestamp of the slot when the preconfer becomes the L1 validator
        uint40 timestamp;
        // Timestamp of the last slot with a valid preconfer
        uint40 prevTimestamp;
        // Address of the preconfer who is also the L1 validator
        // The preconfer has the right to propose a block in the range (prevTimestamp, timestamp]
        address preconfer;
    }

Even if two consecutive slots, 5th and 6th, have the same preconfer, we still need two separate entries for these blocks, representing (60, 72] and (72, 84], rather than a single entry for (60, 84], assuming beaconGenesis is 0.

[AnshuJalan]

Correct.
Some additional note on this:

• We need this because the entry also represents that the preconfer owned the validator who proposed that block. Thus, it was a "dedicated slot" of the preconfer and not a slot where he was preconfing in advanced. This is crucial for lookahead fault proofs.
• This is one place where I have been thinking of future optimisations. It should be rather simple, by introducing one extra field that represents the number of consecutive "dedicated slots" for the preconfer.