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Syscall and Sysvar for last restart slot

Summary​

Create a new syscall and sysvar that can be used to get the last restart slot.

A new Sysvar : SysvarLastRestartS1ot1111111111111111111111

Method signature for syscall: fn sol_get_last_restart_slot() -> Slot

Motivation​

In Solana, when the cluster cannot reach consensus, it is currently restarted using ledger-tool with a hard fork on a slot. All participating nodes need to restart their validator specifying the hard fork slot. Once all nodes participating in the restart effort exceed 80% of the total stakes, the restart is successful, and the cluster continues from the hard fork slot. So hard fork is a tool to intentionally fork off the nodes not participating in the restart effort.

Program developers may find it useful to know that a cluster restart has recently occurred. This information can help them prevent arbitrage and liquidation caused by outdated oracle price account states. However, the cluster's restart process takes several hours; in that time, the world can change, causing asset prices to fluctuate. As the cluster updates the state of all accounts, malicious actors could take advantage of the delay by conducting trades using outdated prices, anticipating that they will soon catch up to the actual prices of real-world assets. Knowing that cluster restart has been done recently, programs can manage these cases more appropriately.

Alternatives Considered​

We need to have the value of the last restart slot while executing the program. We cannot use an account because then it should be updated just after the restart is successful, which will add complexity. The best way is to create a new syscall and sysvar to get this information during the execution of a transaction, which will help us get the last restart slot. We prefer syscall over sysvar because the newly created sysvar has to be passed in the instruction this takes up transaction accounts space. In the long-term Solana evolution, we plan to get rid of syscalls and make everything a sysvar, so we have decided to implement both a syscall and a sysvar to address short and long-term evolution.

New Terminology​

None

Detailed Design​

The following criteria should be met to choose a restart slot.

  • Should be of type unsigned int 64 bits.
  • Should be indicative of when the cluster was restarted
  • Should be monotonically increasing
  • Should be less than equal to the current slot
  • Should be synchronized in the whole cluster
  • Can also be indicative of a slow block
  • The first restart slot should be 0

In Solana client hard fork variable satisfies nearly all the conditions except it is not indicative of slow block, it is a good candidate.

The syscall and sysvar will be available in architectures eBPF and SBF. For now, this functionality is left undefined in program runtime v2.

Feature Gate​

This feature should be protected by the feature gate this is because this feature can create differences in consensus. Solana core developers should create a new keypair and use it to enable this feature on the cluster.

Upon activation, the functionality described below becomes available in the next epoch.

Creation of a new sysvar​

Similar to the clock or rent sysvar we can create a new sysvar for last restart slot. We have to implement a new class to store the required values, which then could be used by sysvar during execution.

Sysvar Id: SysvarLastRestartS1ot1111111111111111111111

Creation of a new syscall​

The implementation of this syscall is pretty straightforward. The syscall should have following signature:

fn sol_get_last_restart_slot() -> u64

Murmur3 hash is: 0xb532af38

Overview of changes for Solana client​

In Solana Labs validator client all the cluster restart-related data is already stored in the bank structure. For other validator clients, we have to get the last restart slot information and make it accessible to the runtime of the program. We have to set this data in invoke context executing the program. Then the syscall and sysvar can use invoke context to return the data to the executing program.

Compute budget for the syscall​

As this feature involves moving a u64 from the bank to the invoke context and returning it when syscall or sysvar is called.

So considering :

/// Cluster averaged compute unit to micro-sec conversion rate
COMPUTE_UNIT_TO_US_RATIO = 30;

We can set the maximum compute limit for the syscall to:

COMPUTE_UNITS_LIMIT = 2 * COMPUTE_UNIT_TO_US_RATIO

Impact​

Programs managing time-sensitive states may upgrade to use this syscall to better manage exceptional events like restarts. We expect this change to improve the robustness of Solana's financial markets as outlined above.

Programs will need to be recompiled and redeployed (upgraded) to adopt this feature. Interface changes (e.g. account inputs) are not required.

Security Considerations​

None

Backwards Compatibility​

The programs using the new syscall could not be used on Solana version which does not implement this feature. Existing programs that do not use this feature are not impacted at all. Feature gate should be used to enable this feature when the majority of the cluster is using the required version.