60 lines
2.0 KiB
Markdown
60 lines
2.0 KiB
Markdown
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# Snapshot and Revert in Ethermint
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EVM uses state-reverting exceptions to handle errors. Such an exception will undo all changes made to the state in the current call (and all its sub-calls), and the caller could handle the error and don't propagate. We need to implement the `Snapshot` and `RevertToSnapshot` apis in `StateDB` interfaces to support this feature.
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[go-ethereum implementation](https://github.com/ethereum/go-ethereum/blob/master/core/state/journal.go#L39) manages transient states in memory, and uses a list of journal logs to record all the state modification operations done so far, snapshot is an index in the log list, and to revert to a snapshot it just undo the journal logs after the snapshot index in reversed order.
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Ethermint uses cosmos-sdk's storage api to manage states, fortunately the storage api supports creating cached overlays, it works like this:
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```golang
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// create a cached overlay storage on top of ctx storage.
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overlayCtx, commit := ctx.CacheContext()
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// Modify states using the overlayed storage
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err := doCall(overlayCtx)
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if err != nil {
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return err
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}
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// commit will write the dirty states into the underlying storage
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commit()
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// Now, just drop the overlayCtx and keep using ctx
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```
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And it can be used in a nested way, like this:
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```golang
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overlayCtx1, commit1 := ctx.CacheContext()
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doCall1(overlayCtx1)
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{
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overlayCtx2, commit2 := overlayCtx1.CacheContext()
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doCall2(overlayCtx2)
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commit2()
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}
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commit1()
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```
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With this feature, we can use a stake of overlayed contexts to implement nested `Snapshot` and `RevertToSnapshot` calls.
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```golang
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type cachedContext struct {
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ctx sdk.Context
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commit func()
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}
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var contextStack []cachedContext
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func Snapshot() int {
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ctx, commit := contextStack.Top().CacheContext()
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contextStack.Push(cachedContext{ctx, commit})
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return len(contextStack) - 1
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}
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func RevertToSnapshot(int snapshot) {
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contextStack = contextStack[:snapshot]
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}
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func Commit() {
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for i := len(contextStack) - 1; i >= 0; i-- {
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contextStack[i].commit()
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}
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contextStack = {}
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}
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```
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