Body encoding cannot be NIL

Start with the IMAP FETCH handling that produces BODYSTRUCTURE, using the logged FETCH 2:1 request and response to reproduce the NIL body-encoding case. Trace how the body encoding is selected and verify the response conforms to the issue's stated requirement that it cannot be NIL.

Describe the bug

According to this comment, maddy seems to send NIL body encoding to FETCH response (within BODYSTRUCTURE) whereas it cannot be null.

Steps to reproduce

NC

Log files

maddy  | 2025-01-25T10:30:52.572441139Z [debug] tls.loader.acme/handshake: choosing certificate {"identifier":"imap.services.domain.tld","num_choices":1}                   
maddy  | 2025-01-25T10:30:52.572507024Z [debug] tls.loader.acme/handshake: default certificate selection results        {"hash":"ad2fea10f731303445f1be751303e1e2dcfb39760b252
43baa899121e9b6aa02","identifier":"imap.services.domain.tld","issuer_key":"acme-v02.api.letsencrypt.org-directory","managed":true,"subjects":["imap.services.domain.tld"]}
maddy  | 2025-01-25T10:30:52.572521371Z [debug] tls.loader.acme/handshake: matched certificate in cache {"expiration":"2025-04-19T14:50:02.000","hash":"ad2fea10f731303445f1be
751303e1e2dcfb39760b25243baa899121e9b6aa02","managed":true,"remote_ip":"192.168.1.2","remote_port":"38888","subjects":["imap.services.domain.tld"]}                         
maddy  | 2025-01-25T10:30:52.575758785Z imap: * OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR CHILDREN UNSELECT MOVE IDLE APPENDLIMIT AUTH=PLAIN COMPRESS] IMAP4rev1 Service Ready
maddy  | 2025-01-25T10:30:52.575972430Z imap: 0.0.wnJF090B AUTHENTICATE PLAIN AHByaW50ZXIAZ2MzJTNpRkZGSkR3UmRMNGg0OGI=                                                        
maddy  | 2025-01-25T10:30:52.803061876Z imap: 0.0.wnJF090B OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR CHILDREN UNSELECT MOVE IDLE APPENDLIMIT I18NLEVEL=1 SORT THREAD=ORDEREDSU
BJECT COMPRESS NAMESPACE] AUTHENTICATE completed                                                                                                                              
maddy  | 2025-01-25T10:30:52.803515145Z imap: 0.1.lgySQXmV SELECT INBOX                                                                                                       
maddy  | 2025-01-25T10:30:52.803906957Z [debug] imapsql: initialized uidMap for selected mailbox: 2 [8 9]                                                                     
maddy  | 2025-01-25T10:30:52.804090726Z imap: * FLAGS (\Seen \Answered \Flagged \Deleted \Draft old)                                                                          
maddy  | 2025-01-25T10:30:52.804132715Z imap: * OK [PERMANENTFLAGS (\Seen \Answered \Flagged \Deleted \Draft \* old)] Flags permitted.                                        
maddy  | 2025-01-25T10:30:52.804191286Z imap: * OK [UNSEEN 2] Message 2 is first unseen                                                                                       
maddy  | 2025-01-25T10:30:52.804287068Z imap: * OK [UIDVALIDITY 1437467386] UIDs valid                                                                                        
maddy  | 2025-01-25T10:30:52.804301274Z imap: * 2 EXISTS                                                                                                                      
maddy  | 2025-01-25T10:30:52.804324398Z imap: * 0 RECENT                                                                                                                      
maddy  | 2025-01-25T10:30:52.804340288Z imap: * OK [UIDNEXT 10] Predicted next UID                                                                                            
maddy  | 2025-01-25T10:30:52.804395293Z imap: 0.1.lgySQXmV OK [READ-WRITE] SELECT completed                                                                                   
maddy  | 2025-01-25T10:30:52.804660335Z imap: 0.2.kxqbTv0z FETCH 2:1 (UID FLAGS ENVELOPE BODYSTRUCTURE)                                                                       
maddy  | 2025-01-25T10:30:52.804985040Z [debug] imapsql: resolved false 8:9 to 8:9                                                                                            
maddy  | 2025-01-25T10:30:52.805530003Z [debug] imapsql: scanMessages: scanned msgId=8 (seq 1) [UID FLAGS ENVELOPE BODYSTRUCTURE]                                             
maddy  | 2025-01-25T10:30:52.805637647Z [debug] imapsql: scanMessages: scanned msgId=9 (seq 2) [UID FLAGS ENVELOPE BODYSTRUCTURE]                                             
maddy  | 2025-01-25T10:30:52.805692511Z imap: * 1 FETCH (UID 8 FLAGS (old \Seen) ENVELOPE ("Fri, 24 Jan 2025 18:10:41 +0100" "this is a test" ((NIL NIL "printer" "services.ca
sona.rocks")) ((NIL NIL "printer" "services.domain.tld")) ((NIL NIL "printer" "services.domain.tld")) ((NIL NIL "printer" "services.domain.tld")) NIL NIL NIL "<ggzxrvlr
b43i4xfot7bws2qntl6j5lxwe4a6eqzdseplqlylgo@ct6wl36ufvlu>") BODYSTRUCTURE ("text" "plain" ("charset" "us-ascii") NIL NIL NIL 7 1 NIL ("inline" ()) NIL NIL))                   
maddy  | 2025-01-25T10:30:52.805801507Z imap: * 2 FETCH (UID 9 FLAGS (old) ENVELOPE ("Fri, 24 Jan 2025 18:15:31 +0100" "bla" ((NIL NIL "printer" "services.domain.tld")) ((N
IL NIL "printer" "services.domain.tld")) ((NIL NIL "printer" "services.domain.tld")) ((NIL NIL "printer" "services.domain.tld")) NIL NIL NIL "<du46gib7rpdxuv4d5mb3sa25c
anezopr6cxdqe2wdqpjeuqkhr@rzdxpuppmuws>") BODYSTRUCTURE ("text" "plain" ("charset" "us-ascii") NIL NIL NIL 10 1 NIL ("inline" ()) NIL NIL))                                   
maddy  | 2025-01-25T10:30:52.805922988Z imap: 0.2.kxqbTv0z OK FETCH completed

Configuration file

NC

Environment information

NC

@petrm feel free to add incomplete or missing information information