IterNextCostFlat not charged

[alpe]

Summary of Bug

Iterator in gaskv only charges gas for existing entries.

Version

v0.42.9 and current master

Steps to Reproduce

bank.GetBalances costs no gas for an address without tokens.

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• Not duplicate issue
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[ethanfrey]

Creating an iterator does a db read (pre-loading data). Which is why you can call Value() Key() and Valid() before Next(). If we only charge on Next, this means we post-pay for the data previously read.

Meaning, I can read the first value of an iterator for free if I never use Next.

I would propose a charge upon creation (based on value loaded) and charging after next for the "next" value.

[amaury1093]

Meaning, I can read the first value of an iterator for free if I never use Next.

That's actually incorrect. See:

cosmos-sdk/store/gaskv/store.go

Lines 76 to 78 in ed5d165

	// Iterator implements the KVStore interface. It returns an iterator which
	// incurs a flat gas cost for seeking to the first key/value pair and a variable
	// gas cost based on the current value's length if the iterator is valid.

The improvement here proposed by @alpe would be to charge a flat cost even if there are no entries (both upon creation and on .Next()), which I support.

[ethanfrey]

Okay, you are right, the first seek is charged... but only if it is valid

cosmos-sdk/store/gaskv/store.go

Lines 110 to 112 in ed5d165

	if gi.Valid() {
	gi.(*gasIterator).consumeSeekGas()
	}

Meaning you can run as many seeks as you want for empty ranges with no gas cost. This is especially bad for smart contracts. And each search to find the first value also consumes a fair bit of I/O to read it.

Honestly, I would just replace those lines with simply:

gi.(*gasIterator).consumeSeekGas()