Fixing PVM Consensus Failures: Date.now() Bug

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Fixing PVM Consensus Failures: Date.now() Bug

Hey guys! Let's dive into a critical bug that's been causing some serious headaches in the PVM (Poker Virtual Machine). We're talking about a situation where the PVM is using Date.now() instead of the blockchain's timestamp, which is leading to consensus failures when multiple validators are running. This is a big deal, so let's break it down.

The Bug: Date.now() vs. Blockchain Timestamp

The heart of the problem lies in how the PVM handles action timestamps. Instead of relying on the blockchain's official timestamp, the PVM has been using Date.now(). For those unfamiliar, Date.now() grabs the current time from the local system. The issue? Validators run on different machines, and these machines inevitably have slightly different clocks. Even tiny discrepancies can create chaos in a distributed system like a blockchain.

Imagine a poker game where each validator has its own slightly different version of time. When processing actions, like a player joining a game or placing a bet, the PVM generates timestamps using its local clock. This leads to discrepancies in the game state across different validators. This, in turn, affects the AppHashes, unique identifiers for the application's state, causing them to diverge. And when the AppHashes don't match, consensus fails. Nodes reject each other's blocks, and the whole system grinds to a halt. It's like trying to agree on the score of a game when everyone has a slightly different view of when the plays happened. It's a recipe for disagreement and instability, which is precisely what we are trying to avoid in a blockchain. This is especially problematic in the context of a game since every fraction of a second can influence the game.

The Fallout of Time Discrepancies

The use of Date.now() as opposed to the deterministic blockTimestamp has several critical consequences:

  • Different Game States: Because of clock differences, each validator ends up with a unique version of the game. For example, the order of events could change from one validator to the next.
  • AppHash Mismatch: The AppHash is a cryptographic hash of the application state. Different timestamps will cause differences in the hash, leading to blocks being rejected.
  • Consensus Failure: When validators disagree on the state of the blockchain, they cannot reach consensus, making the chain unusable.

Evidence: The Logs Don't Lie

The evidence is pretty clear, and it comes straight from the logs. Take a look at these two timestamps from two different validators processing the same action – a player joining a game. Notice how each of the timestamps, 1764976545458 and 1764976482956, are different even though they're processing the same transaction. This difference is enough to cause significant problems. The variation shows the time difference from using Date.now()

node1:

timestamp: 1764976545458

texashodl:

timestamp: 1764976482956

These timestamps should be identical. These are the symptoms of using Date.now() and not the deterministic blockTimestamp and indicate that the clocks are off and the PVM is not synchronizing correctly. It's like having two referees at a game, and each one has a slightly different understanding of when time starts and stops. You cannot get consensus on when the play occurred.

The Fix: Leveraging the Blockchain's Timestamp

The good news? The solution is already built into the system! The blockchain is already passing a deterministic blockTimestamp parameter. The fix involves ensuring the PVM uses this parameter instead of relying on the local system time. Specifically, it needs to use params[8] from the JSON-RPC request. This parameter is the Cosmos block time which is passed from the blockchain.

Let's break down how this works from the code. The blockchain is passing a blockTimestamp parameter (index 8) within the JSON-RPC parameters array. It looks something like this:

// From pokerchain/x/poker/keeper/msg_server_perform_action.go
blockTimestamp := sdkCtx.BlockTime().UnixMilli() // Milliseconds since epoch

request := JSONRPCRequest {
 Method: "perform_action",
 Params: []interface{} {
 playerId, // from
 gameId, // to
 action, // action
 value, // value
 actionIndex, // index
 gameStateJson, // gameStateJson
 gameOptionsJson, // gameOptionsJson
 seatData, // data
 blockTimestamp, // timestamp (Cosmos block time for deterministic gameplay)
 },
 ...
}

This shows that the blockchain is doing its part. The critical step is to have the PVM access this blockTimestamp. The sdkCtx.BlockTime().UnixMilli() provides the timestamp in milliseconds since the epoch, ensuring a consistent and reliable timestamp for all actions. The perform_action message server in the go code contains the proper method to determine and pass the correct blockTimestamp.

Impact and Workarounds: What's at Stake?

The impact of this bug is critical. Any game action, whether it's joining, leaving, or performing any other action, will cause a consensus failure. This means the game essentially becomes unusable in a multi-validator environment. There is a workaround but is definitely not recommended for production:

  • Single Validator Only: You could, in theory, run a single validator. However, this defeats the whole purpose of a decentralized, fault-tolerant system. It is a risky option, as it introduces a single point of failure.

Finding and Fixing the Problem: Where to Look

To fix this, you'll need to hunt down every instance of Date.now() in the action processing code. Replace these with the blockTimestamp parameter passed by the blockchain (params[8]).

Here are the files to check for this issue:

  • Look for uses of Date.now() in action processing code and replace with the passed timestamp parameter.

Make sure to test thoroughly after making the changes to ensure everything works as expected.

Why This Matters

This fix is crucial for the stability and reliability of the PVM. By using the blockchain's timestamp, we ensure that all validators agree on the order and timing of events, which is essential for maintaining consensus. It's the only way to build a robust, decentralized poker game that can withstand the test of time and maintain the integrity of the game. Making sure we use the correct timestamp solves the consensus failures and keeps the game running smoothly.