After queueing a message in the web api's, prevent context cancelation from completing message changes

Adding to the queue is done in a transaction, the queue db file is mox-global.
Appending the message to the Sent folder, removing it from Drafts, marking the
original message as answered/forwarded, is done in a separate database
transaction that gets the ctx passed in. If the ctx was canceled in between,
the queueing was finished, but the rest wasn't completed.

Reported by mteege, thanks!

Author: mjl-

webapisrv/server.go

@@ -1048,6 +1048,9 @@ func (s server) Send(ctx context.Context, req webapi.SendRequest) (resp webapi.S
 	xcheckf(err, "adding messages to the delivery queue")
 	metricSubmission.WithLabelValues("ok").Inc()
 
+	// Message has been added to the queue. Ensure we finish the work.
+	ctx = context.WithoutCancel(ctx)
+
 	if req.SaveSent {
 		// Append message to Sent mailbox and mark original messages as answered/forwarded.
 		acc.WithRLock(func() {

webmail/api.go

@@ -1005,6 +1005,10 @@ func (w Webmail) MessageSubmit(ctx context.Context, m SubmitMessage) {
 
 	var modseq store.ModSeq // Only set if needed.
 
+	// We have committed to sending the message. We want to follow through
+	// with appending to Sent and removing the draft message.
+	ctx = context.WithoutCancel(ctx)
+
 	// Append message to Sent mailbox, mark original messages as answered/forwarded,
 	// remove any draft message.
 	acc.WithWLock(func() {