// Copyright 2020 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

package snap

import (
	"bytes"
	"fmt"
	"time"

	"github.com/ethereum/go-ethereum/common"
	"github.com/ethereum/go-ethereum/core"
	"github.com/ethereum/go-ethereum/core/state"
	"github.com/ethereum/go-ethereum/light"
	"github.com/ethereum/go-ethereum/log"
	"github.com/ethereum/go-ethereum/metrics"
	"github.com/ethereum/go-ethereum/p2p"
	"github.com/ethereum/go-ethereum/p2p/enode"
	"github.com/ethereum/go-ethereum/p2p/enr"
	"github.com/ethereum/go-ethereum/rlp"
	"github.com/ethereum/go-ethereum/trie"
)

const (
	// softResponseLimit is the target maximum size of replies to data retrievals.
	softResponseLimit = 2 * 1024 * 1024

	// maxCodeLookups is the maximum number of bytecodes to serve. This number is
	// there to limit the number of disk lookups.
	maxCodeLookups = 1024

	// stateLookupSlack defines the ratio by how much a state response can exceed
	// the requested limit in order to try and avoid breaking up contracts into
	// multiple packages and proving them.
	stateLookupSlack = 0.1

	// maxTrieNodeLookups is the maximum number of state trie nodes to serve. This
	// number is there to limit the number of disk lookups.
	maxTrieNodeLookups = 1024

	// maxTrieNodeTimeSpent is the maximum time we should spend on looking up trie nodes.
	// If we spend too much time, then it's a fairly high chance of timing out
	// at the remote side, which means all the work is in vain.
	maxTrieNodeTimeSpent = 5 * time.Second
)

// Handler is a callback to invoke from an outside runner after the boilerplate
// exchanges have passed.
type Handler func(peer *Peer) error

// Backend defines the data retrieval methods to serve remote requests and the
// callback methods to invoke on remote deliveries.
type Backend interface {
	// Chain retrieves the blockchain object to serve data.
	Chain() *core.BlockChain

	// RunPeer is invoked when a peer joins on the `eth` protocol. The handler
	// should do any peer maintenance work, handshakes and validations. If all
	// is passed, control should be given back to the `handler` to process the
	// inbound messages going forward.
	RunPeer(peer *Peer, handler Handler) error

	// PeerInfo retrieves all known `snap` information about a peer.
	PeerInfo(id enode.ID) interface{}

	// Handle is a callback to be invoked when a data packet is received from
	// the remote peer. Only packets not consumed by the protocol handler will
	// be forwarded to the backend.
	Handle(peer *Peer, packet Packet) error
}

// MakeProtocols constructs the P2P protocol definitions for `snap`.
func MakeProtocols(backend Backend, dnsdisc enode.Iterator) []p2p.Protocol {
	// Filter the discovery iterator for nodes advertising snap support.
	dnsdisc = enode.Filter(dnsdisc, func(n *enode.Node) bool {
		var snap enrEntry
		return n.Load(&snap) == nil
	})

	protocols := make([]p2p.Protocol, len(ProtocolVersions))
	for i, version := range ProtocolVersions {
		version := version // Closure

		protocols[i] = p2p.Protocol{
			Name:    ProtocolName,
			Version: version,
			Length:  protocolLengths[version],
			Run: func(p *p2p.Peer, rw p2p.MsgReadWriter) error {
				return backend.RunPeer(newPeer(version, p, rw), func(peer *Peer) error {
					return handle(backend, peer)
				})
			},
			NodeInfo: func() interface{} {
				return nodeInfo(backend.Chain())
			},
			PeerInfo: func(id enode.ID) interface{} {
				return backend.PeerInfo(id)
			},
			Attributes:     []enr.Entry{&enrEntry{}},
			DialCandidates: dnsdisc,
		}
	}
	return protocols
}

// handle is the callback invoked to manage the life cycle of a `snap` peer.
// When this function terminates, the peer is disconnected.
func handle(backend Backend, peer *Peer) error {
	for {
		if err := handleMessage(backend, peer); err != nil {
			peer.Log().Debug("Message handling failed in `snap`", "err", err)
			return err
		}
	}
}

// handleMessage is invoked whenever an inbound message is received from a
// remote peer on the `snap` protocol. The remote connection is torn down upon
// returning any error.
func handleMessage(backend Backend, peer *Peer) error {
	// Read the next message from the remote peer, and ensure it's fully consumed
	msg, err := peer.rw.ReadMsg()
	if err != nil {
		return err
	}
	if msg.Size > maxMessageSize {
		return fmt.Errorf("%w: %v > %v", errMsgTooLarge, msg.Size, maxMessageSize)
	}
	defer msg.Discard()
	start := time.Now()
	// Track the emount of time it takes to serve the request and run the handler
	if metrics.Enabled {
		h := fmt.Sprintf("%s/%s/%d/%#02x", p2p.HandleHistName, ProtocolName, peer.Version(), msg.Code)
		defer func(start time.Time) {
			sampler := func() metrics.Sample {
				return metrics.ResettingSample(
					metrics.NewExpDecaySample(1028, 0.015),
				)
			}
			metrics.GetOrRegisterHistogramLazy(h, nil, sampler).Update(time.Since(start).Microseconds())
		}(start)
	}
	// Handle the message depending on its contents
	switch {
	case msg.Code == GetAccountRangeMsg:
		// Decode the account retrieval request
		var req GetAccountRangePacket
		if err := msg.Decode(&req); err != nil {
			return fmt.Errorf("%w: message %v: %v", errDecode, msg, err)
		}
		if req.Bytes > softResponseLimit {
			req.Bytes = softResponseLimit
		}
		// Retrieve the requested state and bail out if non existent
		tr, err := trie.New(req.Root, backend.Chain().StateCache().TrieDB())
		if err != nil {
			return p2p.Send(peer.rw, AccountRangeMsg, &AccountRangePacket{ID: req.ID})
		}
		it, err := backend.Chain().Snapshots().AccountIterator(req.Root, req.Origin)
		if err != nil {
			return p2p.Send(peer.rw, AccountRangeMsg, &AccountRangePacket{ID: req.ID})
		}
		// Iterate over the requested range and pile accounts up
		var (
			accounts []*AccountData
			size     uint64
			last     common.Hash
		)
		for it.Next() && size < req.Bytes {
			hash, account := it.Hash(), common.CopyBytes(it.Account())

			// Track the returned interval for the Merkle proofs
			last = hash

			// Assemble the reply item
			size += uint64(common.HashLength + len(account))
			accounts = append(accounts, &AccountData{
				Hash: hash,
				Body: account,
			})
			// If we've exceeded the request threshold, abort
			if bytes.Compare(hash[:], req.Limit[:]) >= 0 {
				break
			}
		}
		it.Release()

		// Generate the Merkle proofs for the first and last account
		proof := light.NewNodeSet()
		if err := tr.Prove(req.Origin[:], 0, proof); err != nil {
			log.Warn("Failed to prove account range", "origin", req.Origin, "err", err)
			return p2p.Send(peer.rw, AccountRangeMsg, &AccountRangePacket{ID: req.ID})
		}
		if last != (common.Hash{}) {
			if err := tr.Prove(last[:], 0, proof); err != nil {
				log.Warn("Failed to prove account range", "last", last, "err", err)
				return p2p.Send(peer.rw, AccountRangeMsg, &AccountRangePacket{ID: req.ID})
			}
		}
		var proofs [][]byte
		for _, blob := range proof.NodeList() {
			proofs = append(proofs, blob)
		}
		// Send back anything accumulated
		return p2p.Send(peer.rw, AccountRangeMsg, &AccountRangePacket{
			ID:       req.ID,
			Accounts: accounts,
			Proof:    proofs,
		})

	case msg.Code == AccountRangeMsg:
		// A range of accounts arrived to one of our previous requests
		res := new(AccountRangePacket)
		if err := msg.Decode(res); err != nil {
			return fmt.Errorf("%w: message %v: %v", errDecode, msg, err)
		}
		// Ensure the range is monotonically increasing
		for i := 1; i < len(res.Accounts); i++ {
			if bytes.Compare(res.Accounts[i-1].Hash[:], res.Accounts[i].Hash[:]) >= 0 {
				return fmt.Errorf("accounts not monotonically increasing: #%d [%x] vs #%d [%x]", i-1, res.Accounts[i-1].Hash[:], i, res.Accounts[i].Hash[:])
			}
		}
		requestTracker.Fulfil(peer.id, peer.version, AccountRangeMsg, res.ID)

		return backend.Handle(peer, res)

	case msg.Code == GetStorageRangesMsg:
		// Decode the storage retrieval request
		var req GetStorageRangesPacket
		if err := msg.Decode(&req); err != nil {
			return fmt.Errorf("%w: message %v: %v", errDecode, msg, err)
		}
		if req.Bytes > softResponseLimit {
			req.Bytes = softResponseLimit
		}
		// TODO(karalabe): Do we want to enforce > 0 accounts and 1 account if origin is set?
		// TODO(karalabe):   - Logging locally is not ideal as remote faulst annoy the local user
		// TODO(karalabe):   - Dropping the remote peer is less flexible wrt client bugs (slow is better than non-functional)

		// Calculate the hard limit at which to abort, even if mid storage trie
		hardLimit := uint64(float64(req.Bytes) * (1 + stateLookupSlack))

		// Retrieve storage ranges until the packet limit is reached
		var (
			slots  [][]*StorageData
			proofs [][]byte
			size   uint64
		)
		for _, account := range req.Accounts {
			// If we've exceeded the requested data limit, abort without opening
			// a new storage range (that we'd need to prove due to exceeded size)
			if size >= req.Bytes {
				break
			}
			// The first account might start from a different origin and end sooner
			var origin common.Hash
			if len(req.Origin) > 0 {
				origin, req.Origin = common.BytesToHash(req.Origin), nil
			}
			var limit = common.HexToHash("0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff")
			if len(req.Limit) > 0 {
				limit, req.Limit = common.BytesToHash(req.Limit), nil
			}
			// Retrieve the requested state and bail out if non existent
			it, err := backend.Chain().Snapshots().StorageIterator(req.Root, account, origin)
			if err != nil {
				return p2p.Send(peer.rw, StorageRangesMsg, &StorageRangesPacket{ID: req.ID})
			}
			// Iterate over the requested range and pile slots up
			var (
				storage []*StorageData
				last    common.Hash
				abort   bool
			)
			for it.Next() {
				if size >= hardLimit {
					abort = true
					break
				}
				hash, slot := it.Hash(), common.CopyBytes(it.Slot())

				// Track the returned interval for the Merkle proofs
				last = hash

				// Assemble the reply item
				size += uint64(common.HashLength + len(slot))
				storage = append(storage, &StorageData{
					Hash: hash,
					Body: slot,
				})
				// If we've exceeded the request threshold, abort
				if bytes.Compare(hash[:], limit[:]) >= 0 {
					break
				}
			}
			slots = append(slots, storage)
			it.Release()

			// Generate the Merkle proofs for the first and last storage slot, but
			// only if the response was capped. If the entire storage trie included
			// in the response, no need for any proofs.
			if origin != (common.Hash{}) || abort {
				// Request started at a non-zero hash or was capped prematurely, add
				// the endpoint Merkle proofs
				accTrie, err := trie.New(req.Root, backend.Chain().StateCache().TrieDB())
				if err != nil {
					return p2p.Send(peer.rw, StorageRangesMsg, &StorageRangesPacket{ID: req.ID})
				}
				var acc state.Account
				if err := rlp.DecodeBytes(accTrie.Get(account[:]), &acc); err != nil {
					return p2p.Send(peer.rw, StorageRangesMsg, &StorageRangesPacket{ID: req.ID})
				}
				stTrie, err := trie.New(acc.Root, backend.Chain().StateCache().TrieDB())
				if err != nil {
					return p2p.Send(peer.rw, StorageRangesMsg, &StorageRangesPacket{ID: req.ID})
				}
				proof := light.NewNodeSet()
				if err := stTrie.Prove(origin[:], 0, proof); err != nil {
					log.Warn("Failed to prove storage range", "origin", req.Origin, "err", err)
					return p2p.Send(peer.rw, StorageRangesMsg, &StorageRangesPacket{ID: req.ID})
				}
				if last != (common.Hash{}) {
					if err := stTrie.Prove(last[:], 0, proof); err != nil {
						log.Warn("Failed to prove storage range", "last", last, "err", err)
						return p2p.Send(peer.rw, StorageRangesMsg, &StorageRangesPacket{ID: req.ID})
					}
				}
				for _, blob := range proof.NodeList() {
					proofs = append(proofs, blob)
				}
				// Proof terminates the reply as proofs are only added if a node
				// refuses to serve more data (exception when a contract fetch is
				// finishing, but that's that).
				break
			}
		}
		// Send back anything accumulated
		return p2p.Send(peer.rw, StorageRangesMsg, &StorageRangesPacket{
			ID:    req.ID,
			Slots: slots,
			Proof: proofs,
		})

	case msg.Code == StorageRangesMsg:
		// A range of storage slots arrived to one of our previous requests
		res := new(StorageRangesPacket)
		if err := msg.Decode(res); err != nil {
			return fmt.Errorf("%w: message %v: %v", errDecode, msg, err)
		}
		// Ensure the ranges are monotonically increasing
		for i, slots := range res.Slots {
			for j := 1; j < len(slots); j++ {
				if bytes.Compare(slots[j-1].Hash[:], slots[j].Hash[:]) >= 0 {
					return fmt.Errorf("storage slots not monotonically increasing for account #%d: #%d [%x] vs #%d [%x]", i, j-1, slots[j-1].Hash[:], j, slots[j].Hash[:])
				}
			}
		}
		requestTracker.Fulfil(peer.id, peer.version, StorageRangesMsg, res.ID)

		return backend.Handle(peer, res)

	case msg.Code == GetByteCodesMsg:
		// Decode bytecode retrieval request
		var req GetByteCodesPacket
		if err := msg.Decode(&req); err != nil {
			return fmt.Errorf("%w: message %v: %v", errDecode, msg, err)
		}
		if req.Bytes > softResponseLimit {
			req.Bytes = softResponseLimit
		}
		if len(req.Hashes) > maxCodeLookups {
			req.Hashes = req.Hashes[:maxCodeLookups]
		}
		// Retrieve bytecodes until the packet size limit is reached
		var (
			codes [][]byte
			bytes uint64
		)
		for _, hash := range req.Hashes {
			if hash == emptyCode {
				// Peers should not request the empty code, but if they do, at
				// least sent them back a correct response without db lookups
				codes = append(codes, []byte{})
			} else if blob, err := backend.Chain().ContractCode(hash); err == nil {
				codes = append(codes, blob)
				bytes += uint64(len(blob))
			}
			if bytes > req.Bytes {
				break
			}
		}
		// Send back anything accumulated
		return p2p.Send(peer.rw, ByteCodesMsg, &ByteCodesPacket{
			ID:    req.ID,
			Codes: codes,
		})

	case msg.Code == ByteCodesMsg:
		// A batch of byte codes arrived to one of our previous requests
		res := new(ByteCodesPacket)
		if err := msg.Decode(res); err != nil {
			return fmt.Errorf("%w: message %v: %v", errDecode, msg, err)
		}
		requestTracker.Fulfil(peer.id, peer.version, ByteCodesMsg, res.ID)

		return backend.Handle(peer, res)

	case msg.Code == GetTrieNodesMsg:
		// Decode trie node retrieval request
		var req GetTrieNodesPacket
		if err := msg.Decode(&req); err != nil {
			return fmt.Errorf("%w: message %v: %v", errDecode, msg, err)
		}
		if req.Bytes > softResponseLimit {
			req.Bytes = softResponseLimit
		}
		// Make sure we have the state associated with the request
		triedb := backend.Chain().StateCache().TrieDB()

		accTrie, err := trie.NewSecure(req.Root, triedb)
		if err != nil {
			// We don't have the requested state available, bail out
			return p2p.Send(peer.rw, TrieNodesMsg, &TrieNodesPacket{ID: req.ID})
		}
		snap := backend.Chain().Snapshots().Snapshot(req.Root)
		if snap == nil {
			// We don't have the requested state snapshotted yet, bail out.
			// In reality we could still serve using the account and storage
			// tries only, but let's protect the node a bit while it's doing
			// snapshot generation.
			return p2p.Send(peer.rw, TrieNodesMsg, &TrieNodesPacket{ID: req.ID})
		}
		// Retrieve trie nodes until the packet size limit is reached
		var (
			nodes [][]byte
			bytes uint64
			loads int // Trie hash expansions to cound database reads
		)
		for _, pathset := range req.Paths {
			switch len(pathset) {
			case 0:
				// Ensure we penalize invalid requests
				return fmt.Errorf("%w: zero-item pathset requested", errBadRequest)

			case 1:
				// If we're only retrieving an account trie node, fetch it directly
				blob, resolved, err := accTrie.TryGetNode(pathset[0])
				loads += resolved // always account database reads, even for failures
				if err != nil {
					break
				}
				nodes = append(nodes, blob)
				bytes += uint64(len(blob))

			default:
				// Storage slots requested, open the storage trie and retrieve from there
				account, err := snap.Account(common.BytesToHash(pathset[0]))
				loads++ // always account database reads, even for failures
				if err != nil {
					break
				}
				stTrie, err := trie.NewSecure(common.BytesToHash(account.Root), triedb)
				loads++ // always account database reads, even for failures
				if err != nil {
					break
				}
				for _, path := range pathset[1:] {
					blob, resolved, err := stTrie.TryGetNode(path)
					loads += resolved // always account database reads, even for failures
					if err != nil {
						break
					}
					nodes = append(nodes, blob)
					bytes += uint64(len(blob))

					// Sanity check limits to avoid DoS on the store trie loads
					if bytes > req.Bytes || loads > maxTrieNodeLookups || time.Since(start) > maxTrieNodeTimeSpent {
						break
					}
				}
			}
			// Abort request processing if we've exceeded our limits
			if bytes > req.Bytes || loads > maxTrieNodeLookups || time.Since(start) > maxTrieNodeTimeSpent {
				break
			}
		}
		// Send back anything accumulated
		return p2p.Send(peer.rw, TrieNodesMsg, &TrieNodesPacket{
			ID:    req.ID,
			Nodes: nodes,
		})

	case msg.Code == TrieNodesMsg:
		// A batch of trie nodes arrived to one of our previous requests
		res := new(TrieNodesPacket)
		if err := msg.Decode(res); err != nil {
			return fmt.Errorf("%w: message %v: %v", errDecode, msg, err)
		}
		requestTracker.Fulfil(peer.id, peer.version, TrieNodesMsg, res.ID)

		return backend.Handle(peer, res)

	default:
		return fmt.Errorf("%w: %v", errInvalidMsgCode, msg.Code)
	}
}

// NodeInfo represents a short summary of the `snap` sub-protocol metadata
// known about the host peer.
type NodeInfo struct{}

// nodeInfo retrieves some `snap` protocol metadata about the running host node.
func nodeInfo(chain *core.BlockChain) *NodeInfo {
	return &NodeInfo{}
}
