fix #1688

* Add ACCEPT-tracking functionality (authorizing users to send DMs
  despite +R or other applicable restrictions)
* Sending a DM automatically accepts the recipient
* Add explicit ACCEPT command

irc/accept.go

@@ -0,0 +1,76 @@
+package irc
+
+import (
+	"sync"
+
+	"github.com/ergochat/ergo/irc/utils"
+)
+
+// tracks ACCEPT relationships, i.e., `accepter` is willing to receive DMs from
+// `accepted` despite some restriction (currently the only relevant restriction
+// is that `accepter` is +R and `accepted` is not logged in)
+
+type AcceptManager struct {
+	sync.RWMutex
+
+	// maps recipient -> whitelist of permitted senders:
+	// this is what we actually check
+	clientToAccepted map[*Client]utils.HashSet[*Client]
+	// this is the reverse mapping, it's needed so we can
+	// clean up the forward mapping during (*Client).destroy():
+	clientToAccepters map[*Client]utils.HashSet[*Client]
+}
+
+func (am *AcceptManager) Initialize() {
+	am.clientToAccepted = make(map[*Client]utils.HashSet[*Client])
+	am.clientToAccepters = make(map[*Client]utils.HashSet[*Client])
+}
+
+func (am *AcceptManager) MaySendTo(sender, recipient *Client) (result bool) {
+	am.RLock()
+	defer am.RUnlock()
+	return am.clientToAccepted[recipient].Has(sender)
+}
+
+func (am *AcceptManager) Accept(accepter, accepted *Client) {
+	am.Lock()
+	defer am.Unlock()
+
+	var m utils.HashSet[*Client]
+
+	m = am.clientToAccepted[accepter]
+	if m == nil {
+		m = make(utils.HashSet[*Client])
+		am.clientToAccepted[accepter] = m
+	}
+	m.Add(accepted)
+
+	m = am.clientToAccepters[accepted]
+	if m == nil {
+		m = make(utils.HashSet[*Client])
+		am.clientToAccepters[accepted] = m
+	}
+	m.Add(accepter)
+}
+
+func (am *AcceptManager) Unaccept(accepter, accepted *Client) {
+	am.Lock()
+	defer am.Unlock()
+
+	delete(am.clientToAccepted[accepter], accepted)
+	delete(am.clientToAccepters[accepted], accepter)
+}
+
+func (am *AcceptManager) Remove(client *Client) {
+	am.Lock()
+	defer am.Unlock()
+
+	for accepter := range am.clientToAccepters[client] {
+		delete(am.clientToAccepted[accepter], client)
+	}
+	for accepted := range am.clientToAccepted[client] {
+		delete(am.clientToAccepters[accepted], client)
+	}
+	delete(am.clientToAccepters, client)
+	delete(am.clientToAccepted, client)
+}