inclusion: SubtreeWidth should define what happens when subtreeRootThreshold<=0 else a divide by zero panic on

[odeke-em]

Summary of Bug

Found by fuzzing in less than 2 minutes and even on CI/CD per https://github.com/celestiaorg/go-square/actions/runs/11515786009/job/32057128298?pr=119, we have a divide by zero at

go-square/inclusion/blob_share_commitment_rules.go

Line 84 in f0bf090

s := (shareCount / subtreeRootThreshold)

or simply

SubtreeWidth(2, 0)

Please define what happens when we have a subtreeRootThreshold=0

Version

f0bf090

Steps to Reproduce

Please see above.

Kindly cc-ing @cristaloleg @walldiss @Wondertan

For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged
• Contributor assigned/self-assigned

[evan-forbes]

the constant is consensus critical, so any tx or node that has a different value will have different blob share commitments and different data roots

therefore its safe to panic if a negative value is used, although I don't think we need to add if statements that check for this everywhere given its consensus critical

[evan-forbes]

alternatively to panicking, we could pass the app version to

go-square/inclusion/blob_share_commitment_rules.go

Lines 78 to 98 in f0bf090

	// SubTreeWidth returns the maximum number of leaves per subtree in the share
	// commitment over a given blob. The input should be the total number of shares
	// used by that blob. See ADR-013.
	func SubTreeWidth(shareCount, subtreeRootThreshold int) int {
	// Per ADR-013, we use a predetermined threshold to determine width of sub
	// trees used to create share commitments
	s := (shareCount / subtreeRootThreshold)
	// round up if the width is not an exact multiple of the threshold
	if shareCount%subtreeRootThreshold != 0 {
	s++
	}
	// use a power of two equal to or larger than the multiple of the subtree
	// root threshold
	s = RoundUpPowerOfTwo(s)
	// use the minimum of the subtree width and the min square size, this
	// gurarantees that a valid value is returned
	return min(s, BlobMinSquareSize(shareCount))
	}