Legal holds, step 3: deleted items in a held account are kept

Every way of deleting mail (JMAP, IMAP EXPUNGE, POP3, mailbox removal,
Trash emptying) and Sieve scripts, events, contacts and files now asks
how the account's deletions are kept: a hold keeps them with no expiry
(archivedUntil 9999-12-31), even with undelete off; otherwise undelete's
period applies as before (LH-4).

A hold's date range decides by the item's own date (LH-3). Mail is noted
as held at deletion and settled when it's archived, once its received
date is known; outside the range it gets undelete's deadline or isn't
kept. Events go by their start, with a day's slack for time zones;
recurring events, contacts, files and scripts are held whole.

A groupware item's note now stays until its archive succeeds, and a
failure retries the task instead of being logged and lost (LH-5).

crates/features/src/hold/mod.rs

@@ -26,6 +26,85 @@ use store::{
 };
 use trc::AddContext;
 
+/// The deadline a held archived item carries: the last second of 9999. It
+/// never passes, so every expiry check keeps the item without knowing about
+/// holds (LH-4, LH-5); releasing a hold gives it a real deadline (LH-10).
+pub const HELD_UNTIL: u64 = 253_402_300_799;
+
+/// Whether an archived item's deadline marks it as held. Anything past the
+/// year 9000 counts, so a deadline computed from a hold a moment earlier or
+/// later still reads as held.
+pub fn is_held_until(until: u64) -> bool {
+    until >= 221_845_392_000
+}
+
+/// A day, in seconds: the slack either side of a range for an event's start,
+/// whose time zone isn't known here.
+const DAY: u64 = 86_400;
+
+/// How an account's deleted items are kept: its holds' ranges, and the
+/// undelete period for whatever no hold covers (LH-3, LH-4).
+#[derive(Debug, Clone, Default, PartialEq, Eq)]
+pub struct Keeping {
+    /// `archiveDeletedItemsFor`, in seconds, if undelete is on.
+    pub retention: Option<u64>,
+    /// Each active hold's range on this account; `(None, None)` is a whole
+    /// account. Empty when nothing holds it.
+    pub ranges: Vec<(Option<u64>, Option<u64>)>,
+}
+
+impl Keeping {
+    pub fn new(retention: Option<u64>, holds: &[Hold]) -> Keeping {
+        Keeping {
+            retention,
+            ranges: holds.iter().map(|h| (h.from, h.to)).collect(),
+        }
+    }
+
+    /// Whether any hold reaches the account at all.
+    pub fn is_held(&self) -> bool {
+        !self.ranges.is_empty()
+    }
+
+    /// Whether deleted items need noting: something may keep them.
+    pub fn keeps_anything(&self) -> bool {
+        self.is_held() || self.retention.is_some()
+    }
+
+    /// Whether a hold covers an item dated `date`. No date means the item is
+    /// held whole, whatever the range (LH-3).
+    pub fn covers(&self, date: Option<u64>) -> bool {
+        self.ranges.iter().any(|(from, to)| match date {
+            None => true,
+            Some(at) => {
+                from.is_none_or(|from| at >= from) && to.is_none_or(|to| at <= to)
+            }
+        })
+    }
+
+    /// Like `covers`, for an event's start: a day of slack either side, since
+    /// its time zone isn't known here.
+    pub fn covers_event(&self, start: Option<u64>) -> bool {
+        self.ranges.iter().any(|(from, to)| match start {
+            None => true,
+            Some(at) => {
+                from.is_none_or(|from| at + DAY >= from)
+                    && to.is_none_or(|to| at <= to.saturating_add(DAY))
+            }
+        })
+    }
+
+    /// Until when an item deleted at `now` is kept: held, the undelete
+    /// period, or not at all.
+    pub fn until(&self, now: u64, held: bool) -> Option<u64> {
+        if held {
+            Some(HELD_UNTIL)
+        } else {
+            self.retention.map(|retention| now + retention)
+        }
+    }
+}
+
 const FEATURE: u8 = b'H';
 const KIND_HOLD: u8 = b'h';
 
@@ -511,6 +590,27 @@ mod tests {
         assert!(held.scope.covers(&member) && !held.scope.covers(&moved));
     }
 
+    #[test]
+    fn keeping_deleted_items() {
+        let whole = Keeping::new(None, &[hold(accounts(&[2]), None, None)]);
+        assert!(whole.covers(Some(5)) && whole.covers(None));
+        assert_eq!(whole.until(100, whole.covers(Some(5))), Some(HELD_UNTIL));
+        assert!(is_held_until(whole.until(100, true).unwrap()));
+
+        // LH-3: a range holds only what's inside it; outside, undelete's rules
+        let ranged = Keeping::new(Some(30), &[hold(accounts(&[2]), Some(1_000), Some(2_000))]);
+        assert!(ranged.covers(Some(1_500)) && !ranged.covers(Some(2_500)));
+        assert!(ranged.covers(None), "contacts, files and scripts are held whole");
+        assert_eq!(ranged.until(100, ranged.covers(Some(2_500))), Some(130));
+        assert!(ranged.covers_event(Some(2_000 + 3_600)), "a day of slack for an event");
+
+        // Neither held nor undelete: nothing is kept
+        let none = Keeping::new(None, &[]);
+        assert!(!none.keeps_anything());
+        assert_eq!(none.until(100, false), None);
+        assert!(!is_held_until(100 + 30 * 365 * 86_400));
+    }
+
     #[test]
     fn stored_as_json() {
         let current = hold(accounts(&[2]), Some(100), None);