fix delay with propagating mailbox changes to other imap (idle) connections

when broadcasting a change, we would try to send the changes on a channel,
non-blocking. if we couldn't send (because there was no pending blocked
receive), we would wait until the potential receiver would explicitly request
the changes. however, the imap idle handler would not explicitly request the
changes, but do a receive on the changes channel. since there was no pending
blocked send on the channel, that receive would block. only when another event
would come in, would both the pending and the new changes be sent.

we now use a channel only for signaling there are pending changes. the channel
is buffered, so when broadcasting we can just set the signal by a non-blocking
send and continue with the next listener. the receiver will get the buffered
signal. it can then get the changes directly, but lock-protected.

found when looking at a missing/delayed new message notification in thunderbird
when two messages arrive immediately after each other. this doesn't fix that
problem though: it seems thunderbird just ignores imap untagged "exists"
messages (indicating a new message arrived) during the "uid fetch" command that
it issued after notifications from an "idle" command.

http/import.go

@@ -802,9 +802,7 @@ func importMessages(ctx context.Context, log *mlog.Log, token string, acc *store
 		jf = nil
 	}
 
-	comm := store.RegisterComm(acc)
-	defer comm.Unregister()
-	comm.Broadcast(changes)
+	store.BroadcastChanges(acc, changes)
 	acc.Unlock()
 	err = acc.Close()
 	log.Check(err, "closing account after import")

imapserver/server.go

@@ -2660,8 +2660,8 @@ wait:
 			xcheckf(le.err, "get line")
 			line = le.line
 			break wait
-		case changes := <-c.comm.Changes:
-			c.applyChanges(changes, false)
+		case <-c.comm.Pending:
+			c.applyChanges(c.comm.Get(), false)
 			c.xflush()
 		case <-mox.Shutdown.Done():
 			// ../rfc/9051:5375

import.go

@@ -390,9 +390,7 @@ func importctl(ctx context.Context, ctl *ctl, mbox bool) {
 		ctl.log.Info("delivered messages through import", mlog.Field("count", len(deliveredIDs)))
 		deliveredIDs = nil
 
-		comm := store.RegisterComm(a)
-		defer comm.Unregister()
-		comm.Broadcast(changes)
+		store.BroadcastChanges(a, changes)
 	})
 
 	err = a.Close()

store/account.go

@@ -1039,9 +1039,7 @@ func (a *Account) DeliverMailbox(log *mlog.Log, mailbox string, m *Message, msgF
 	}
 
 	changes = append(changes, ChangeAddUID{m.MailboxID, m.UID, m.Flags, m.Keywords})
-	comm := RegisterComm(a)
-	defer comm.Unregister()
-	comm.Broadcast(changes)
+	BroadcastChanges(a, changes)
 	return nil
 }
 
@@ -1104,9 +1102,7 @@ func (a *Account) TidyRejectsMailbox(log *mlog.Log, rejectsMailbox string) (hasS
 		return false, err
 	}
 
-	comm := RegisterComm(a)
-	defer comm.Unregister()
-	comm.Broadcast(changes)
+	BroadcastChanges(a, changes)
 
 	return hasSpace, nil
 }
@@ -1198,9 +1194,7 @@ func (a *Account) RejectsRemove(log *mlog.Log, rejectsMailbox, messageID string)
 		return err
 	}
 
-	comm := RegisterComm(a)
-	defer comm.Unregister()
-	comm.Broadcast(changes)
+	BroadcastChanges(a, changes)
 
 	return nil
 }

store/state.go

@@ -1,6 +1,7 @@
 package store
 
 import (
+	"sync"
 	"sync/atomic"
 )
 
@@ -8,12 +9,13 @@ var (
 	register   = make(chan *Comm)
 	unregister = make(chan *Comm)
 	broadcast  = make(chan changeReq)
-	get        = make(chan *Comm)
 )
 
 type changeReq struct {
-	comm    *Comm
+	acc     *Account
+	comm    *Comm // Can be nil.
 	changes []Change
+	done    chan struct{}
 }
 
 type UID uint32 // IMAP UID.
@@ -72,7 +74,7 @@ var switchboardBusy atomic.Bool
 
 // Switchboard distributes changes to accounts to interested listeners. See Comm and Change.
 func Switchboard() chan struct{} {
-	regs := map[*Account]map[*Comm][]Change{}
+	regs := map[*Account]map[*Comm]struct{}{}
 	done := make(chan struct{})
 
 	if !switchboardBusy.CompareAndSwap(false, true) {
@@ -84,32 +86,36 @@ func Switchboard() chan struct{} {
 			select {
 			case c := <-register:
 				if _, ok := regs[c.acc]; !ok {
-					regs[c.acc] = map[*Comm][]Change{}
+					regs[c.acc] = map[*Comm]struct{}{}
 				}
-				regs[c.acc][c] = nil
+				regs[c.acc][c] = struct{}{}
+
 			case c := <-unregister:
 				delete(regs[c.acc], c)
 				if len(regs[c.acc]) == 0 {
 					delete(regs, c.acc)
 				}
+
 			case chReq := <-broadcast:
-				acc := chReq.comm.acc
-				for c, changes := range regs[acc] {
-					// Do not send the broadcaster back their own changes.
+				acc := chReq.acc
+				for c := range regs[acc] {
+					// Do not send the broadcaster back their own changes. chReq.comm is nil if not
+					// originating from a comm, so won't match in that case.
 					if c == chReq.comm {
 						continue
 					}
-					regs[acc][c] = append(changes, chReq.changes...)
+
+					c.Lock()
+					c.changes = append(c.changes, chReq.changes...)
+					c.Unlock()
+
 					select {
-					case c.Changes <- regs[acc][c]:
-						regs[acc][c] = nil
+					case c.Pending <- struct{}{}:
 					default:
 					}
 				}
-				chReq.comm.r <- struct{}{}
-			case c := <-get:
-				c.Changes <- regs[c.acc][c]
-				regs[c.acc][c] = nil
+				chReq.done <- struct{}{}
+
 			case <-done:
 				if !switchboardBusy.CompareAndSwap(true, false) {
 					panic("switchboard already unregistered?")
@@ -124,14 +130,20 @@ func Switchboard() chan struct{} {
 // Comm handles communication with the goroutine that maintains the
 // account/mailbox/message state.
 type Comm struct {
-	Changes chan []Change // Receives block until changes come in, e.g. for IMAP IDLE.
+	Pending chan struct{} // Receives block until changes come in, e.g. for IMAP IDLE.
+
 	acc *Account
-	r       chan struct{}
+
+	sync.Mutex
+	changes []Change
 }
 
 // Register starts a Comm for the account. Unregister must be called.
 func RegisterComm(acc *Account) *Comm {
-	c := &Comm{make(chan []Change), acc, make(chan struct{})}
+	c := &Comm{
+		Pending: make(chan struct{}, 1), // Bufferend so Switchboard can just do a non-blocking send.
+		acc:     acc,
+	}
 	register <- c
 	return c
 }
@@ -146,14 +158,27 @@ func (c *Comm) Broadcast(ch []Change) {
 	if len(ch) == 0 {
 		return
 	}
-	broadcast <- changeReq{c, ch}
-	<-c.r
+	done := make(chan struct{}, 1)
+	broadcast <- changeReq{c.acc, c, ch, done}
+	<-done
 }
 
-// Get retrieves pending changes. If no changes are pending a nil or empty list
+// Get retrieves all pending changes. If no changes are pending a nil or empty list
 // is returned.
 func (c *Comm) Get() []Change {
-	get <- c
-	changes := <-c.Changes
-	return changes
+	c.Lock()
+	defer c.Unlock()
+	l := c.changes
+	c.changes = nil
+	return l
+}
+
+// BroadcastChanges ensures changes are sent to all listeners on the accoount.
+func BroadcastChanges(acc *Account, ch []Change) {
+	if len(ch) == 0 {
+		return
+	}
+	done := make(chan struct{}, 1)
+	broadcast <- changeReq{acc, nil, ch, done}
+	<-done
 }