ClassCastException while retrieving mail from IMAP

Checklist

• I have used the search function to see if someone else has already submitted the same bug report.
• I will describe the problem with as much detail as possible.

App version

6.400

Where did you get the app from?

F-Droid

Android version

10

Device model

No response

Steps to reproduce

1. Try to fetch new messages from IMAP server

Expected behavior

Message fetched OK

Actual behavior

java.lang.ClassCastException: com.fsck.k9.mail.store.imap.ImapList cannot be cast to java.lang.String

Fetching messages stops at this position without any error message.

Still reproduces in version 6.510.

Logs

04-02 00:22:02.035 17431 17452 V RealImapConnection: conn164086162>>> 28 UID FETCH 19619,19618,19617,19616 (UID FLAGS INTERNALDATE RFC822.SIZE BODY.PEEK[HEADER.FIELDS (date subject from content-type to cc reply-to message-id references in-reply-to list-unsubscribe X-K9mail-Identity Chat-Version)])
04-02 00:22:02.095 17431 17452 V RealImapConnection: conn164086162<<<#null# [5327, FETCH, [UID, 19616, FLAGS, [\Seen, encrypted, system_hamon], INTERNALDATE, 29-Mar-2023 18:44:42 +0000, RFC822.SIZE, 84581, BODY, [HEADER.FIELDS, [Chat-Version, list-unsubscribe, in-reply-to, references, message-id, reply-to, content-type, X-K9mail-Identity, subject, cc, to, from, date]], Date: Wed, 29 Mar 2023 21:44:07 +0300 (MSK)
04-02 00:22:02.095 17431 17452 V RealImapConnection: From: addr1@example1.org
04-02 00:22:02.095 17431 17452 V RealImapConnection: To: addr2@example2.ru
04-02 00:22:02.095 17431 17452 V RealImapConnection: Message-ID: <141234567.2591681234567890.JavaMail.tech_user@rmcp-monolith-6c6bcfffff-7ffff>
04-02 00:22:02.095 17431 17452 V RealImapConnection: Subject: subject
04-02 00:22:02.095 17431 17452 V RealImapConnection: Content-Type: multipart/mixed; 
04-02 00:22:02.095 17431 17452 V RealImapConnection: 	boundary="----=_Part_364_1114187583.1680115447937"
04-02 00:22:02.095 17431 17452 V RealImapConnection: 
04-02 00:22:02.095 17431 17452 V RealImapConnection: ]]
...
04-02 00:22:02.137 17431 17452 D ImapSync: SYNC: Synced unsynced messages for folder INBOX
04-02 00:22:02.138 17431 17452 D ImapSync: SYNC: Have 4 large messages and 0 small messages out of 4 unsynced messages
04-02 00:22:02.138 17431 17452 D ImapSync: SYNC: Fetching 0 small messages for folder INBOX
04-02 00:22:02.138 17431 17452 D ImapSync: SYNC: Done fetching small messages for folder INBOX
04-02 00:22:02.139 17431 17452 D ImapSync: SYNC: Fetching large messages for folder INBOX
04-02 00:22:02.141 17431 17452 V RealImapConnection: conn164086162>>> 29 UID FETCH 19616,19617,19618,19619 (UID BODYSTRUCTURE)
04-02 00:22:02.197 17431 17452 V RealImapConnection: conn164086162<<<#null# [5327, FETCH, [UID, 19616, BODYSTRUCTURE, [[related, [boundary, ----=_Part_365_528345150.1680115447937], NIL, NIL, NIL], [application, pdf, [name, account_details_*5222.pdf], NIL, NIL, base64, 81284, NIL, [attachment, [filename, account_details_*5222.pdf]], NIL, NIL], mixed, [boundary, ----=_Part_364_1114187583.1680115447937], NIL, NIL, NIL]]]
04-02 00:22:02.198 17431 17452 E ImapSync: synchronizeMailbox
04-02 00:22:02.198 17431 17452 E ImapSync: java.lang.ClassCastException: com.fsck.k9.mail.store.imap.ImapList cannot be cast to java.lang.String
04-02 00:22:02.198 17431 17452 E ImapSync: 	at com.fsck.k9.mail.store.imap.ImapList.getString(ImapList.java:38)
04-02 00:22:02.198 17431 17452 E ImapSync: 	at com.fsck.k9.mail.store.imap.RealImapFolder.parseBodyStructure(RealImapFolder.kt:861)
04-02 00:22:02.198 17431 17452 E ImapSync: 	at com.fsck.k9.mail.store.imap.RealImapFolder.parseBodyStructure(RealImapFolder.kt:828)
04-02 00:22:02.198 17431 17452 E ImapSync: 	at com.fsck.k9.mail.store.imap.RealImapFolder.handleFetchResponse(RealImapFolder.kt:746)
04-02 00:22:02.198 17431 17452 E ImapSync: 	at com.fsck.k9.mail.store.imap.RealImapFolder.fetch(RealImapFolder.kt:597)
04-02 00:22:02.198 17431 17452 E ImapSync: 	at com.fsck.k9.backend.imap.ImapSync.downloadLargeMessages(ImapSync.kt:561)
04-02 00:22:02.198 17431 17452 E ImapSync: 	at com.fsck.k9.backend.imap.ImapSync.downloadMessages(ImapSync.kt:367)
04-02 00:22:02.198 17431 17452 E ImapSync: 	at com.fsck.k9.backend.imap.ImapSync.synchronizeMailboxSynchronous(ImapSync.kt:198)
04-02 00:22:02.198 17431 17452 E ImapSync: 	at com.fsck.k9.backend.imap.ImapSync.sync(ImapSync.kt:34)
04-02 00:22:02.198 17431 17452 E ImapSync: 	at com.fsck.k9.backend.imap.ImapBackend.sync(ImapBackend.kt:55)
04-02 00:22:02.198 17431 17452 E ImapSync: 	at com.fsck.k9.controller.MessagingController.syncFolder(MessagingController.java:616)
04-02 00:22:02.198 17431 17452 E ImapSync: 	at com.fsck.k9.controller.MessagingController.synchronizeMailboxSynchronous(MessagingController.java:556)
04-02 00:22:02.198 17431 17452 E ImapSync: 	at com.fsck.k9.controller.MessagingController.lambda$synchronizeMailbox$3(MessagingController.java:528)
04-02 00:22:02.198 17431 17452 E ImapSync: 	at com.fsck.k9.controller.MessagingController.$r8$lambda$ar_6eNiqt1c67rfo1nCyGoIX3Sg(Unknown Source:0)
04-02 00:22:02.198 17431 17452 E ImapSync: 	at com.fsck.k9.controller.MessagingController$$ExternalSyntheticLambda20.run(Unknown Source:10)
04-02 00:22:02.198 17431 17452 E ImapSync: 	at com.fsck.k9.controller.MessagingController.runInBackground(MessagingController.java:222)
04-02 00:22:02.198 17431 17452 E ImapSync: 	at com.fsck.k9.controller.MessagingController.access$000(MessagingController.java:108)
04-02 00:22:02.198 17431 17452 E ImapSync: 	at com.fsck.k9.controller.MessagingController$1.run(MessagingController.java:163)
04-02 00:22:02.198 17431 17452 E ImapSync: 	at java.lang.Thread.run(Thread.java:919)

Start in mail/src/main/java/com/fsck/k9/mail/store/imap/RealImapFolder.kt at parseBodyStructure, especially the calls around line 861, and inspect ImapList.getString() in ImapList.java. Use the logged BODYSTRUCTURE response to reproduce the cast failure. Done means the affected IMAP message can be fetched without a ClassCastException and synchronization continues.