adr: Peer Behaviour (tendermint#3539)

* [adr] ADR 037: Peer Behaviour inital draft
* Update docs/architecture/adr-037-peer-behaviour.md

Co-Authored-By: brapse <brapse@gmail.com>

* Update docs/architecture/adr-037-peer-behaviour.md
Co-Authored-By: brapse <brapse@gmail.com>

* [docs] adr-037 Better footnote styling
* [ADR] ADR-037 adjust Footnotes for github markdown
* [ADR] ADR-037 fix numbered list

docs/architecture/adr-037-peer-behaviour.md

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+# ADR 037: Peer Behaviour Interface
+
+## Changelog
+* 07-03-2019: Initial draft
+
+## Context
+
+The responsibility for signaling and acting upon peer behaviour lacks a single 
+owning component and is heavily coupled with the network stack[<sup>1</sup>](#references). Reactors
+maintain a reference to the `p2p.Switch` which they use to call 
+`switch.StopPeerForError(...)` when a peer misbehaves and 
+`switch.MarkAsGood(...)` when a peer contributes in some meaningful way. 
+While the switch handles `StopPeerForError` internally, the `MarkAsGood` 
+method delegates to another component, `p2p.AddrBook`. This scheme of delegation 
+across Switch obscures the responsibility for handling peer behaviour
+and ties up the reactors in a larger dependency graph when testing.
+
+## Decision
+
+Introduce a `PeerBehaviour` interface and concrete implementations which
+provide methods for reactors to signal peer behaviour without direct
+coupling `p2p.Switch`.  Introduce a ErrPeer to provide 
+concrete reasons for stopping peers.
+
+### Implementation Changes
+
+PeerBehaviour then becomes an interface for signaling peer errors as well
+as for marking peers as `good`.
+
+XXX: It might be better to pass p2p.ID instead of the whole peer but as
+a first draft maintain the underlying implementation as much as
+possible.
+
+```go
+type PeerBehaviour interface {
+    Errored(peer Peer, reason ErrPeer)
+    MarkPeerAsGood(peer Peer)
+}
+```
+
+Instead of signaling peers to stop with arbitrary reasons:
+`reason interface{}` 
+
+We introduce a concrete error type ErrPeer:
+```go
+type ErrPeer int
+
+const (
+    ErrPeerUnknown = iota
+    ErrPeerBadMessage
+    ErrPeerMessageOutofOrder
+    ...
+)
+```
+
+As a first iteration we provide a concrete implementation which wraps
+the switch:
+```go
+type SwitchedPeerBehaviour struct {
+    sw *Switch
+}
+
+func (spb *SwitchedPeerBehaviour) Errored(peer Peer, reason ErrPeer) {
+    spb.sw.StopPeerForError(peer, reason)
+}
+
+func (spb *SwitchedPeerBehaviour) MarkPeerAsGood(peer Peer) {
+    spb.sw.MarkPeerAsGood(peer)
+}
+
+func NewSwitchedPeerBehaviour(sw *Switch) *SwitchedPeerBehaviour {
+    return &SwitchedPeerBehaviour{
+        sw: sw,
+    }
+}
+```
+
+Reactors, which are often difficult to unit test[<sup>2</sup>](#references). could use an implementation which exposes the signals produced by the reactor in
+manufactured scenarios:
+
+```go
+type PeerErrors map[Peer][]ErrPeer
+type GoodPeers map[Peer]bool
+
+type StorePeerBehaviour struct {
+    pe PeerErrors
+    gp GoodPeers
+}
+
+func NewStorePeerBehaviour() *StorePeerBehaviour{
+    return &StorePeerBehaviour{
+        pe: make(PeerErrors),
+        gp: GoodPeers{},
+    }
+}
+
+func (spb StorePeerBehaviour) Errored(peer Peer, reason ErrPeer) {
+    if _, ok := spb.pe[peer]; !ok {
+        spb.pe[peer] = []ErrPeer{reason}
+    } else {
+        spb.pe[peer] = append(spb.pe[peer], reason)
+    }
+}
+
+func (mpb *StorePeerBehaviour) GetPeerErrors() PeerErrors {
+    return mpb.pe
+}
+
+func (spb *StorePeerBehaviour) MarkPeerAsGood(peer Peer) {
+    if _, ok := spb.gp[peer]; !ok {
+        spb.gp[peer] = true
+    }
+}
+
+func (spb *StorePeerBehaviour) GetGoodPeers() GoodPeers {
+    return spb.gp
+}
+```
+
+## Status
+
+Proposed
+
+## Consequences
+
+### Positive
+
+    * De-couple signaling from acting upon peer behaviour.
+    * Reduce the coupling of reactors and the Switch and the network
+      stack
+    * The responsibility of managing peer behaviour can be migrated to
+      a single component instead of split between the switch and the
+      address book.
+
+### Negative
+
+    * The first iteration will simply wrap the Switch and introduce a
+      level of indirection.
+
+### Neutral
+
+## References
+
+1. Issue [#2067](https://github.com/tendermint/tendermint/issues/2067): P2P Refactor
+2. PR: [#3506](https://github.com/tendermint/tendermint/pull/3506): ADR 036: Blockchain Reactor Refactor