Implement Header interface

[tzdybal]

Implement the Header interface:

// Header abstracts all methods required to perform header sync.
type Header interface {
	// New creates new instance of a header.
	New() Header
	// Hash returns hash of a header.
	Hash() Hash
	// Height returns the height of a header.
	Height() int64
	// LastHeader returns the hash of last header before this header (aka. previous header hash).
	LastHeader() Hash
	// Time returns time when header was created.
	Time() time.Time
	// IsRecent checks if header is recent against the given blockTime.
	IsRecent(duration time.Duration) bool
	// IsExpired checks if header is expired against trusting period.
	IsExpired() bool
	// VerifyAdjacent validates adjacent untrusted header against trusted header.
	VerifyAdjacent(Header) error
	// VerifyNonAdjacent validates non-adjacent untrusted header against trusted header.
	VerifyNonAdjacent(Header) error
	// Verify performs basic verification of untrusted header.
	Verify(Header) error
	// Validate performs basic validation to check for missed/incorrect fields.
	Validate() error

	encoding.BinaryMarshaler
	encoding.BinaryUnmarshaler
}

[gupadhyaya]

1. @tzdybal could you add some details as to why this is needed? I am missing the full context.
2. some functions in Header interface already exists, e.g., Hash() in types/hashing.go and MarshalBinary & UnmarshalBinary in types/serialization.go. we don't need to move them under the same file right? we can simply implement rest of the functions in e.g., header.go under types/header.go?
3. where can i read the specification for some of the functions like Verify, VerifyAdjacent, etc? or could you add some clarifying details as what they are expected to do?

[tzdybal]

1. @tzdybal could you add some details as to why this is needed? I am missing the full context.

This Header interface is a generic interface that will be used by https://github.com/celestiaorg/go-header/. This will be the common logic used to execute header sync in both celestia-node and rollmint light clients. Additionally we will need this interface to allow different headers implementations (for optimistic rollups, ZK rollups), and to enable users creating their own implementations.

2. some functions in Header interface already exists, e.g., `Hash()` in `types/hashing.go` and `MarshalBinary & UnmarshalBinary` in `types/serialization.go`. we don't need to move them under the same file right? we can simply implement rest of the functions in e.g., `header.go` under `types/header.go`?

Yes. We can simply implement the rest of the functions, without moving the existing ones. Note: I think that ValidateBasic can be simpy renamed to Validate.

3. where can i read the specification for some of the functions like `Verify`, `VerifyAdjacent`, etc? or could you add some clarifying details as what they are expected to do?

Unfortunately there is no documentation yet. The best you can get is to look at implementation in node - and at the same time, I'll work on creating godocs for all those methods.
https://github.com/celestiaorg/celestia-node/blob/da4f54bca1bfef86f53880ced569d37ffb4b8b84/header/verify.go#L36
https://github.com/celestiaorg/celestia-node/blob/da4f54bca1bfef86f53880ced569d37ffb4b8b84/header/verify.go#L51

[gupadhyaya]

@tzdybal

1. New should create empty Header or something similar to clone?
2. Header type has fileds Height and Time, are the interface methods necessary? Also it doesn't allow duplicate field and interface method

[gupadhyaya]

also, VerifyNonAdjacent requires Ensure that untrusted commit has enough of trusted commit's power. without the store, it is not possible to get access to ValidatorSet or Commits. any ideas on how to cover this? @tzdybal

[tzdybal]

1. New should create new instance of concrete type implementing the interface. Context: when deserializing we need actual object of given type, like:

var obj H
obj.UnmarshalBinary(data)

where H is generic type of header.Header interface. This won't work unfortunately, as default value for pointers is nil so deserialization will panic. And most probably interface will be implemented for pointer receiver.
The "hack" to mitigate this without using reflection, with construct like:

var empty H
obj := empty.New()
obj.UnmarshalBinary(data)

2. Yes, right now we have Height and Time as fields, but we need to refactor this. Because interfaces cannot define fields, and we need an universal method of accessing Height and Time in header library.

3. Sorry for that - I just copy-pasted the comment from Node implementation, and it makes no sense 🤦 This method definitely work without accessing store.

[gupadhyaya]

@tzdybal few more questions:

1. https://github.com/celestiaorg/go-header seems like not ready, should the rollmint repo add go-header as dependency and use Header interface definition? The Hash type seems to be missing.
2. Throughout rollmint [32]byte is used where as celestia-core uses tmbytes.HexBytes and the generic header interface planning to use a new type Hash. May be we should make these uniform across?
3. I am still confused about the New method. Should it not return an empty instance of https://github.com/celestiaorg/rollmint/blob/main/types/block.go#L10?
   I have a draft PR above and could you let me know if the direction i am taking is correct?

[tzdybal]

1. Yes - current go-header is out-of-date. After merging feat(header)!: introduce header.Header interface celestiaorg/celestia-node#1304, I'll extract the code from celestia-node to go-header (again), and we'll be able to add it as dependency for rollmint.
2. My PR replaces HexBytes in node, as it's completely unnecessary. For now, you can copy the hash definition, just like you copied Header interface. We'll refactor this when go-header is ready.
3. Yes it should return empty instance of our Header, as you mentioned 👍