seqmap: sync seqNum to uid after expunge

This patch updates the seqNums after an Expunge operation. When an
expunge operation occurs, the seqNum of the deleted message is reported.
The Imap spec [0] states that an immediate decrement of all seqnums greater
than the deleted occurs, even before the next reporting of an expunge
update.

[0]: https://datatracker.ietf.org/doc/html/rfc3501#section-7.4.1

Fixes: https://todo.sr.ht/~rjarry/aerc/61
Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>
Signed-off-by: Robin Jarry <robin@jarry.cc>

worker/imap/seqmap.go

@@ -1,6 +1,8 @@
 package imap
 
-import "sync"
+import (
+	"sync"
+)
 
 type SeqMap struct {
 	lock sync.Mutex
@@ -35,7 +37,17 @@ func (s *SeqMap) Pop(seqnum uint32) (uint32, bool) {
 	s.lock.Lock()
 	uid, found := s.m[seqnum]
 	if found {
-		delete(s.m, seqnum)
+		m := make(map[uint32]uint32)
+		for s, u := range s.m {
+			if s > seqnum {
+				// All sequence numbers greater than the removed one must be decremented by one
+				// https://datatracker.ietf.org/doc/html/rfc3501#section-7.4.1
+				m[s-1] = u
+			} else if s < seqnum {
+				m[s] = u
+			}
+		}
+		s.m = m
 	}
 	s.lock.Unlock()
 	return uid, found

worker/imap/seqmap_test.go

@@ -39,12 +39,15 @@ func TestSeqMap(t *testing.T) {
 	assert.Equal(found, true)
 	assert.Equal(seqmap.Size(), 2)
 
+	// Repop the same seqnum should work because of the syncing
 	_, found = seqmap.Pop(1)
-	assert.Equal(found, false)
-	assert.Equal(seqmap.Size(), 2)
+	assert.Equal(found, true)
+	assert.Equal(seqmap.Size(), 1)
 
+	// sync means we already have a 1. This is replacing that UID so the size
+	// shouldn't increase
 	seqmap.Put(1, 7331)
-	assert.Equal(seqmap.Size(), 3)
+	assert.Equal(seqmap.Size(), 1)
 
 	seqmap.Clear()
 	assert.Equal(seqmap.Size(), 0)