improve training of junk filter

before, we used heuristics to decide when to train/untrain a message as junk or
nonjunk: the message had to be seen, be in certain mailboxes. then if a message
was marked as junk, it was junk. and otherwise it was nonjunk. this wasn't good
enough: you may want to keep some messages around as neither junk or nonjunk.
and that wasn't possible.

ideally, we would just look at the imap $Junk and $NotJunk flags. the problem
is that mail clients don't set these flags, or don't make it easy. thunderbird
can set the flags based on its own bayesian filter. it has a shortcut for
marking Junk and moving it to the junk folder (good), but the counterpart of
notjunk only marks a message as notjunk without showing in the UI that it was
marked as notjunk. there is also no "move and mark as notjunk" mechanism. e.g.
"archive" does not mark a message as notjunk. ios mail and mutt don't appear to
have any way to see or change the $Junk and $NotJunk flags.

what email clients do have is the ability to move messages to other
mailboxes/folders. so mox now has a mechanism that allows you to configure
mailboxes that automatically set $Junk or $NotJunk (or clear both) when a
message is moved/copied/delivered to that folder. e.g. a mailbox called junk or
spam or rejects marks its messags as junk. inbox, postmaster, dmarc, tlsrpt,
neutral* mark their messages as neither junk or notjunk. other folders mark
their messages as notjunk. e.g. list/*, archive. this functionality is
optional, but enabled with the quickstart and for new accounts.

also, mox now keeps track of the previous training of a message and will only
untrain/train if needed. before, there probably have been duplicate or missing
(un)trainings.

this also includes a new subcommand "retrain" to recreate the junkfilter for an
account. you should run it after updating to this version. and you should
probably also modify your account config to include the AutomaticJunkFlags.

imapserver/server.go

@@ -1951,11 +1951,13 @@ func (c *conn) cmdDelete(tag, cmd string, p *parser) {
 				_, err = qm.Delete()
 				xcheckf(err, "removing messages")
 
-				conf, _ := c.account.Conf()
-				if name != conf.RejectsMailbox {
-					err = c.account.Untrain(c.log, remove)
-					xcheckf(err, "untraining deleted messages")
+				// Mark messages as not needing training. Then retrain them, so that are untrained if they were.
+				for i := range remove {
+					remove[i].Junk = false
+					remove[i].Notjunk = false
 				}
+				err = c.account.RetrainMessages(c.log, tx, remove, true)
+				xcheckf(err, "untraining deleted messages")
 			}
 
 			err = tx.Delete(&store.Mailbox{ID: mb.ID})
@@ -2519,7 +2521,7 @@ func (c *conn) cmdAppend(tag, cmd string, p *parser) {
 				MsgPrefix:     msgPrefix,
 			}
 			isSent := name == "Sent"
-			c.account.DeliverX(c.log, tx, &msg, msgFile, true, isSent, true, true)
+			c.account.DeliverX(c.log, tx, &msg, msgFile, true, isSent, true)
 		})
 
 		// Fetch pending changes, possibly with new UIDs, so we can apply them before adding our own new UID.
@@ -2684,11 +2686,14 @@ func (c *conn) xexpunge(uidSet *numSet, missingMailboxOK bool) []store.Message {
 			_, err = qm.Delete()
 			xcheckf(err, "removing messages marked for deletion")
 
-			conf, _ := c.account.Conf()
-			if mb.Name != conf.RejectsMailbox {
-				err = c.account.Untrain(c.log, remove)
-				xcheckf(err, "untraining deleted messages")
+			// Mark removed messages as not needing training, then retrain them, so if they
+			// were trained, they get untrained.
+			for i := range remove {
+				remove[i].Junk = false
+				remove[i].Notjunk = false
 			}
+			err = c.account.RetrainMessages(c.log, tx, remove, true)
+			xcheckf(err, "untraining deleted messages")
 		})
 
 		// Broadcast changes to other connections. We may not have actually removed any
@@ -2920,6 +2925,9 @@ func (c *conn) cmdxCopy(isUID bool, tag, cmd string, p *parser) {
 			for _, m := range xmsgs {
 				msgs[m.UID] = m
 			}
+			nmsgs := make([]store.Message, len(xmsgs))
+
+			conf, _ := c.account.Conf()
 
 			// Insert new messages into database.
 			var origMsgIDs, newMsgIDs []int64
@@ -2934,9 +2942,12 @@ func (c *conn) cmdxCopy(isUID bool, tag, cmd string, p *parser) {
 				m.UID = uidFirst + store.UID(i)
 				m.MailboxID = mbDst.ID
 				m.MailboxOrigID = mbSrc.ID
+				m.TrainedJunk = nil
+				m.JunkFlagsForMailbox(mbDst.Name, conf)
 				err := tx.Insert(&m)
 				xcheckf(err, "inserting message")
 				msgs[uid] = m
+				nmsgs[i] = m
 				origUIDs = append(origUIDs, uid)
 				newUIDs = append(newUIDs, m.UID)
 				newMsgIDs = append(newMsgIDs, m.ID)
@@ -2964,11 +2975,8 @@ func (c *conn) cmdxCopy(isUID bool, tag, cmd string, p *parser) {
 				createdIDs = append(createdIDs, newMsgIDs[i])
 			}
 
-			conf, _ := c.account.Conf()
-			if mbDst.Name != conf.RejectsMailbox {
-				err = c.account.Train(c.log, xmsgs)
-				xcheckf(err, "train copied messages")
-			}
+			err = c.account.RetrainMessages(c.log, tx, nmsgs, false)
+			xcheckf(err, "train copied messages")
 		})
 
 		// Broadcast changes to other connections.
@@ -3087,6 +3095,7 @@ func (c *conn) cmdxMove(isUID bool, tag, cmd string, p *parser) {
 				xserverErrorf("uid and message mismatch")
 			}
 
+			conf, _ := c.account.Conf()
 			for i := range msgs {
 				m := &msgs[i]
 				if m.UID != uids[i] {
@@ -3094,11 +3103,15 @@ func (c *conn) cmdxMove(isUID bool, tag, cmd string, p *parser) {
 				}
 				m.MailboxID = mbDst.ID
 				m.UID = uidnext
+				m.JunkFlagsForMailbox(mbDst.Name, conf)
 				uidnext++
 				err := tx.Update(m)
 				xcheckf(err, "updating moved message in database")
 			}
 
+			err = c.account.RetrainMessages(c.log, tx, msgs, false)
+			xcheckf(err, "retraining messages after move")
+
 			// Prepare broadcast changes to other connections.
 			changes = make([]store.Change, 0, 1+len(msgs))
 			changes = append(changes, store.ChangeRemoveUIDs{MailboxID: c.mailboxID, UIDs: uids})
@@ -3172,12 +3185,10 @@ func (c *conn) cmdxStore(isUID bool, tag, cmd string, p *parser) {
 	updates := store.FlagsQuerySet(mask, flags)
 
 	var updated []store.Message
-	var oflags []store.Flags
 
 	c.account.WithWLock(func() {
-		var mb store.Mailbox
 		c.xdbwrite(func(tx *bstore.Tx) {
-			mb = c.xmailboxID(tx, c.mailboxID) // Validate.
+			c.xmailboxID(tx, c.mailboxID) // Validate.
 
 			uidargs := c.xnumSetCondition(isUID, nums)
 
@@ -3188,11 +3199,6 @@ func (c *conn) cmdxStore(isUID bool, tag, cmd string, p *parser) {
 			q := bstore.QueryTx[store.Message](tx)
 			q.FilterNonzero(store.Message{MailboxID: c.mailboxID})
 			q.FilterEqual("UID", uidargs...)
-			q.FilterFn(func(m store.Message) bool {
-				// We use this filter just to get the pre-update flags...
-				oflags = append(oflags, m.Flags)
-				return true
-			})
 			if len(updates) == 0 {
 				var err error
 				updated, err = q.List()
@@ -3202,29 +3208,10 @@ func (c *conn) cmdxStore(isUID bool, tag, cmd string, p *parser) {
 				_, err := q.UpdateFields(updates)
 				xcheckf(err, "updating flags")
 			}
-		})
 
-		conf, _ := c.account.Conf()
-		if mb.Name != conf.RejectsMailbox {
-			jf, _, err := c.account.OpenJunkFilter(c.log)
-			if err == nil {
-				defer func() {
-					if jf != nil {
-						err := jf.Close()
-						c.xsanity(err, "closing junkfilter")
-					}
-				}()
-				for i, m := range updated {
-					err := c.account.Retrain(c.log, jf, oflags[i], m)
-					xcheckf(err, "retraining message")
-				}
-				err = jf.Close()
-				jf = nil
-				xcheckf(err, "closing junkfilter")
-			} else if !errors.Is(err, store.ErrNoJunkFilter) {
-				xcheckf(err, "open junk filter for retraining")
-			}
-		}
+			err := c.account.RetrainMessages(c.log, tx, updated, false)
+			xcheckf(err, "training messages")
+		})
 
 		// Broadcast changes to other connections.
 		changes := make([]store.Change, len(updated))