v1: serialization of SELECT / FETCH flags allows for empty strings

[ptrcnull]

failed to select mailbox: in response-data: in flag: imapwire: expected atom, got " "

The error originally comes from alps commit 652ea9c7, using go-imap v2.0.0-alpha.6.
Tracing the connection reveals the following exchange:

T4 SELECT INBOX
* FLAGS (\Seen \Answered \Flagged \Deleted \Draft  $Label1 $Label4 $forwarded $label1 \Recent junk nonjunk yeet)

I assume the issue lies in the two spaces between \Draft and $Label1 - however, that's being generated by go-imap v1.2.2-0.20220928192137-6fac715be9cf, running with maddy commit d9920f07.

For the record, RFC 9051 specifies a parenthesized list to be "delimited by space", which might be interpreted ambiguously... but seeing how Thunderbird, Evolution or other mail clients have no issue parsing that response, I imagine it's been assumed that it's valid grammar

[ptrcnull]

which might be interpreted ambiguously

well, now i realised that same RFC has actual grammar rules in section 9, which say:

SP              = <Defined in RFC 5234> = %x20
flag-list       = "(" [flag *(SP flag)] ")"
mailbox-data    =  "FLAGS" SP flag-list / ...

which would mean that the serialization of flags is wrong, either in go-imap or maddy, and the other clients are just being overly flexible with parsing those

[ptrcnull]

aaand attaching a debugger to maddy reveals that it's indeed passing an empty string to both Flags and PermanentFlags:

(dlv) p mbox
*github.com/emersion/go-imap.MailboxStatus {
	Name: "INBOX",
	ReadOnly: false,
	Items: map[github.com/emersion/go-imap.StatusItem]interface {} [
		"MESSAGES": nil, 
		"RECENT": nil, 
		"UIDNEXT": nil, 
		"UIDVALIDITY": nil, 
		"UNSEEN": nil, 
	],
	ItemsLocker: sync.Mutex {state: 0, sema: 0},
	Flags: []string len: 14, cap: 20, [
		"\\Seen",
		"\\Answered",
		"\\Flagged",
		"\\Deleted",
		"\\Draft",
		"",
		"$Label1",
		"$Label4",
		"$forwarded",
		"$label1",
		"\\Recent",
		"junk",
		"nonjunk",
		"yeet",
	],
	PermanentFlags: []string len: 15, cap: 24, [
		"\\Seen",
		"\\Answered",
		"\\Flagged",
		"\\Deleted",
		"\\Draft",
		"\\*",
		"",
		"$Label1",
		"$Label4",
		"$forwarded",
		"$label1",
		"\\Recent",
		"junk",
		"nonjunk",
		"yeet",
	],
	UnseenSeqNum: 10906,
	Messages: 11342,
	Recent: 2652,
	Unseen: 39,
	UidNext: 31161,
	UidValidity: 1527719296,
	AppendLimit: 0,}

while i think this should be a scenario that's being handled a little differently by this library, it's a maddy issue after all, so feel free to close it if there's no desire to fix this in some other way

[ptrcnull]

the same issue exists with Message.Flags - empty string gets serialized as an empty parenthesized list element, which later breaks parsing

[emersion]

FWIW, go-imap v2's server would error out in this situation:

go-imap/internal/imapwire/encoder.go

Line 201 in 38838c5

return len(s) > 0