imap: fix data race on seqMap array

There are concurrent threads that are accessing and modifying
IMAPWorker.seqMap (the mapping of sequence numbers to message UIDs).
This can lead to crashes when trying to add and remove a message ID.

panic: runtime error: index out of range [391] with length 390

goroutine 1834 [running]:
git.sr.ht/~rjarry/aerc/logging.PanicHandler()
	logging/panic-logger.go:47 +0x6de
panic({0xa41760, 0xc0019b3290})
	/usr/lib/golang/src/runtime/panic.go:838 +0x207
git.sr.ht/~rjarry/aerc/worker/imap.(*IMAPWorker).handleFetchMessages.func1()
	worker/imap/fetch.go:214 +0x185
created by git.sr.ht/~rjarry/aerc/worker/imap.(*IMAPWorker).handleFetchMessages
	worker/imap/fetch.go:209 +0x12b

Use a map which makes more sense than a simple array for random access
operations. Also, it allows better typing for the key values. Protect
the map with a mutex. Add internal API to access the map. Add basic unit
tests to ensure that concurrent access works.

Fixes: https://todo.sr.ht/~rjarry/aerc/49
Signed-off-by: Robin Jarry <robin@jarry.cc>
Acked-by: Moritz Poldrack <moritz@poldrack.dev>

worker/imap/flags.go

@@ -2,6 +2,7 @@ package imap
 
 import (
 	"fmt"
+
 	"github.com/emersion/go-imap"
 
 	"git.sr.ht/~rjarry/aerc/logging"
@@ -26,9 +27,11 @@ func (imapw *IMAPWorker) handleDeleteMessages(msg *types.DeleteMessages) {
 		defer logging.PanicHandler()
 
 		for seqNum := range ch {
-			i := seqNum - 1
-			deleted = append(deleted, imapw.seqMap[i])
-			imapw.seqMap = append(imapw.seqMap[:i], imapw.seqMap[i+1:]...)
+			if uid, found := imapw.seqMap.Pop(seqNum); !found {
+				imapw.worker.Logger.Printf("handleDeleteMessages unknown seqnum: %v", seqNum)
+			} else {
+				deleted = append(deleted, uid)
+			}
 		}
 		done <- nil
 	}()