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fix(imap): always advance expected UIDNEXT to avoid skipping IDLE in …
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…a loop

Ensure the client does not busy loop
skipping IDLE if UIDNEXT of the mailbox is higher than
the last seen UID plus 1, e.g. if the message
with UID=UIDNEXT-1 was deleted before we fetched it.

We do not fallback to UIDNEXT=1 anymore
if the STATUS command cannot determine UIDNEXT.
There are no known IMAP servers with broken STATUS so far.
This allows to guarantee that select_with_uidvalidity()
sets UIDNEXT for the mailbox and use it in fetch_new_messages()
to ensure that UIDNEXT always advances even
when there are no messages to fetch.
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@link2xt
link2xt committed Nov 6, 2023
1 parent 5549733 commit 7b83bdd
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Showing 2 changed files with 66 additions and 55 deletions.
119 changes: 65 additions & 54 deletions src/imap.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -568,9 +568,14 @@ impl Imap {
Ok(())
}

/// Select a folder and take care of uidvalidity changes.
/// Also, when selecting a folder for the first time, sets the uid_next to the current
/// Selects a folder and takes care of UIDVALIDITY changes.
///
/// When selecting a folder for the first time, sets the uid_next to the current
/// mailbox.uid_next so that no old emails are fetched.
///
/// Makes sure that UIDNEXT is known for `selected_mailbox`
/// and errors out if UIDNEXT cannot be determined.
///
/// Returns Result<new_emails> (i.e. whether new emails arrived),
/// if in doubt, returns new_emails=true so emails are fetched.
pub(crate) async fn select_with_uidvalidity(
Expand All @@ -591,6 +596,37 @@ impl Imap {
let new_uid_validity = mailbox
.uid_validity
.with_context(|| format!("No UIDVALIDITY for folder {folder}"))?;
let new_uid_next = if let Some(uid_next) = mailbox.uid_next {
uid_next
} else {
warn!(
context,
"SELECT response for IMAP folder {folder:?} has no UIDNEXT, fall back to STATUS command."
);

// RFC 3501 says STATUS command SHOULD NOT be used
// on the currently selected mailbox because the same
// information can be obtained by other means,
// such as reading SELECT response.
//
// However, it also says that UIDNEXT is REQUIRED
// in the SELECT response and if we are here,
// it is actually not returned.
//
// In particular, Winmail Pro Mail Server 5.1.0616
// never returns UIDNEXT in SELECT response,
// but responds to "STATUS INBOX (UIDNEXT)" command.
let status = session
.inner
.status(folder, "(UIDNEXT)")
.await
.context("STATUS (UIDNEXT) error for {folder:?}")?;

status
.uid_next
.context("STATUS {folder} (UIDNEXT) did not return UIDNEXT")?
};
mailbox.uid_next = Some(new_uid_next);

let old_uid_validity = get_uidvalidity(context, folder)
.await
Expand All @@ -606,18 +642,16 @@ impl Imap {
// the caller tries to fetch new messages (we could of course run a SELECT command now, but trying to fetch
// new messages is only one command, just as a SELECT command)
true
} else if let Some(uid_next) = mailbox.uid_next {
if uid_next < old_uid_next {
} else {
if new_uid_next < old_uid_next {
warn!(
context,
"The server illegally decreased the uid_next of folder {folder:?} from {old_uid_next} to {uid_next} without changing validity ({new_uid_validity}), resyncing UIDs...",
"The server illegally decreased the uid_next of folder {folder:?} from {old_uid_next} to {new_uid_next} without changing validity ({new_uid_validity}), resyncing UIDs...",
);
set_uid_next(context, folder, uid_next).await?;
set_uid_next(context, folder, new_uid_next).await?;
context.schedule_resync().await?;
}
uid_next != old_uid_next // If uid_next changed, there are new emails
} else {
true // We have no uid_next and if in doubt, return true
new_uid_next != old_uid_next // If UIDNEXT changed, there are new emails
};
return Ok(new_emails);
}
Expand All @@ -627,43 +661,6 @@ impl Imap {

// ============== uid_validity has changed or is being set the first time. ==============

let new_uid_next = match mailbox.uid_next {
Some(uid_next) => uid_next,
None => {
warn!(
context,
"SELECT response for IMAP folder {folder:?} has no UIDNEXT, fall back to STATUS command."
);

// RFC 3501 says STATUS command SHOULD NOT be used
// on the currently selected mailbox because the same
// information can be obtained by other means,
// such as reading SELECT response.
//
// However, it also says that UIDNEXT is REQUIRED
// in the SELECT response and if we are here,
// it is actually not returned.
//
// In particular, Winmail Pro Mail Server 5.1.0616
// never returns UIDNEXT in SELECT response,
// but responds to "SELECT INBOX (UIDNEXT)" command.
let status = session
.inner
.status(folder, "(UIDNEXT)")
.await
.context("STATUS (UIDNEXT) error for {folder:?}")?;

if let Some(uid_next) = status.uid_next {
uid_next
} else {
warn!(context, "STATUS {folder} (UIDNEXT) did not return UIDNEXT");

// Set UIDNEXT to 1 as a last resort fallback.
1
}
}
};

set_uid_next(context, folder, new_uid_next).await?;
set_uidvalidity(context, folder, new_uid_validity).await?;

Expand Down Expand Up @@ -867,14 +864,28 @@ impl Imap {
uids_fetch_in_batch.push(uid);
}

// determine which uid_next to use to update to
// receive_imf() returns an `Err` value only on recoverable errors, otherwise it just logs an error.
// `largest_uid_processed` is the largest uid where receive_imf() did NOT return an error.

// So: Update the uid_next to the largest uid that did NOT recoverably fail. Not perfect because if there was
// another message afterwards that succeeded, we will not retry. The upside is that we will not retry an infinite amount of times.
let largest_uid_without_errors = max(largest_uid_fetched, largest_uid_skipped.unwrap_or(0));
let new_uid_next = largest_uid_without_errors + 1;
// Advance uid_next to the maximum of the largest known UID plus 1
// and mailbox UIDNEXT.
// Largest known UID is normally less than UIDNEXT,
// but a message may have arrived between determining UIDNEXT
// and executing the FETCH command.
let mailbox_uid_next = self
.session
.as_ref()
.context("No IMAP session")?
.selected_mailbox
.as_ref()
.with_context(|| format!("Expected {folder:?} to be selected"))?
.uid_next
.with_context(|| {
format!(
"Expected UIDNEXT to be determined for {folder:?} by select_with_uidvalidity"
)
})?;
let new_uid_next = max(
max(largest_uid_fetched, largest_uid_skipped.unwrap_or(0)) + 1,
mailbox_uid_next,
);

if new_uid_next > old_uid_next {
set_uid_next(context, folder, new_uid_next).await?;
Expand Down
2 changes: 1 addition & 1 deletion src/imap/idle.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -53,7 +53,7 @@ impl Session {
if uid_next > expected_uid_next {
info!(
context,
"Skipping IDLE because UIDNEXT indicates there are new messages."
"Skipping IDLE on {folder:?} because UIDNEXT {uid_next}>{expected_uid_next} indicates there are new messages."
);
return Ok((self, info));
}
Expand Down

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