The Graph team asked us to audit a bugfix present in the Staking contract when an allocation is closed with a poi=0x0.
The pull request that we have audited is the PR#459 at commit 168c05b87346e3c0acfbe4abb40ae21d8b39b5e4 and the audited file is:
contracts/staking/Staking.sol
Indexers can get rewards for indexing subgraphs, and these rewards are minted when an indexer closes an allocation referencing a non-trivial proof of indexing. These allocations are closed by calling one of many functions within the Staking contract that call as a subroutine the _closeAllocation function. A valid call to _closeAllocation with a non-trivial proof of indexing will update the snapshots since they are used to calculate the minted rewards.
An indexer is able to opt-out of rewards when closing out an allocation by passing in _poi set to 0x0. But in this case, the takeRewards method of the RewardsManager is not called, and thus the snapshots are not updated before unallocating tokens. This makes it to where the RewardsManager‘s calculations of various values will use non updated snapshots.
The Graph team made a simple fix addressing this issue by having the _closeAllocation function call the _updateRewards function in the case the proof of indexing is trivial. As a subroutine of this _updateRewards function is a call to the rewardsManager‘s onSubgraphAllocation function. This onSubgraphAllocation function is the subroutine responsible for updating the rewards snapshot within the takeRewards function. So now with this fix, this subroutine is called in all cases, and thus the snapshots are always updated.
We are satisfied by the solution proposed, and we are glad that The Graph team is being proactive in hardening their contracts against such bugs. We must note that the PR in question is still not merged; we assume that The Graph team will merge it as it is and that no other bugs are introduced in eventual changes.
The code has been audited by two auditors during the course of one day.
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We found this bugfix to be straightforward and clean, and we detected no issues worth mentioning.