seqmap: refactor seqmap to use slice instead of map

The imap worker's seqmap is represented as a map of sequence number to
UID. This presents a problem when expunging group of messages from the
mailbox: each individual expunge decrements the sequence numbers by 1
(for every sequence number greater than the expunged). This requires a
looping around the map to update the keys. The use of a map also
requires that both the sequence number and the UID of a message be known
in order to insert it into the map. This is only discovered by fetching
individual message body parts (flags, headers, etc), leaving the seqmap
to be empty until we have fetched information about each message. In
certain instances (if a mailbox has recently been loaded), all
information is loaded in memory and no new information is fetched -
leaving the seqmap empty and the UI out of sync with the worker.

Refactor the seqmap as a slice, so that any expunge automatically
decrements the rest of the sequences.

Use the results of FetchDirectoryContents or FetchDirectoryThreaded to
initialize the seqmap with all discovered UIDs. Sort the UIDs in
ascending order: IMAP specification requires that sequence numbers start
at 1 increase in order of ascending UID.

Add individual messages to the map if they come via a MessageUpdate and
have a sequence number larger than our slice.

Update seqmap tests with new logic.

Reference: https://datatracker.ietf.org/doc/html/rfc3501#section-2.3.1.2
Fixes: https://todo.sr.ht/~rjarry/aerc/69
Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>
Acked-by: Robin Jarry <robin@jarry.cc>

1 files changed, 42 insertions, 26 deletions

diff --git a/worker/imap/seqmap.go b/worker/imap/seqmap.go
index 093bbc52..3558fe48 100644
--- a/worker/imap/seqmap.go
+++ b/worker/imap/seqmap.go
@@ -1,13 +1,22 @@
 package imap
 
 import (
+	"sort"
 	"sync"
 )
 
 type SeqMap struct {
 	lock sync.Mutex
 	// map of IMAP sequence numbers to message UIDs
-	m map[uint32]uint32
+	m []uint32
+}
+
+// Initialize sets the initial seqmap of the mailbox
+func (s *SeqMap) Initialize(uids []uint32) {
+	s.lock.Lock()
+	s.m = uids
+	s.sort()
+	s.lock.Unlock()
 }
 
 func (s *SeqMap) Size() int {
@@ -17,44 +26,51 @@ func (s *SeqMap) Size() int {
 	return size
 }
 
+// Get returns the UID of the given seqnum
 func (s *SeqMap) Get(seqnum uint32) (uint32, bool) {
+	if int(seqnum) > s.Size() || seqnum < 1 {
+		return 0, false
+	}
 	s.lock.Lock()
-	uid, found := s.m[seqnum]
+	uid := s.m[seqnum-1]
 	s.lock.Unlock()
-	return uid, found
+	return uid, true
 }
 
-func (s *SeqMap) Put(seqnum, uid uint32) {
+// Put adds a UID to the slice. Put should only be used to add new messages
+// into the slice
+func (s *SeqMap) Put(uid uint32) {
 	s.lock.Lock()
-	if s.m == nil {
-		s.m = make(map[uint32]uint32)
+	for _, n := range s.m {
+		if n == uid {
+			// We already have this UID, don't insert it.
+			s.lock.Unlock()
+			return
+		}
 	}
-	s.m[seqnum] = uid
+	s.m = append(s.m, uid)
+	s.sort()
 	s.lock.Unlock()
 }
 
+// Pop removes seqnum from the SeqMap. seqnum must be a valid seqnum, ie
+// [1:size of mailbox]
 func (s *SeqMap) Pop(seqnum uint32) (uint32, bool) {
-	s.lock.Lock()
-	uid, found := s.m[seqnum]
-	if found {
-		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
+	if int(seqnum) > s.Size() || seqnum < 1 {
+		return 0, false
 	}
+	s.lock.Lock()
+	uid := s.m[seqnum-1]
+	s.m = append(s.m[:seqnum-1], s.m[seqnum:]...)
 	s.lock.Unlock()
-	return uid, found
+	return uid, true
 }
 
-func (s *SeqMap) Clear() {
-	s.lock.Lock()
-	s.m = make(map[uint32]uint32)
-	s.lock.Unlock()
+// sort sorts the slice in ascending UID order. See:
+// https://datatracker.ietf.org/doc/html/rfc3501#section-2.3.1.2
+func (s *SeqMap) sort() {
+	// Always be sure the SeqMap is sorted
+	sort.Slice(s.m, func(i, j int) bool {
+		return s.m[i] < s.m[j]
+	})
 }