Make the max queue size and give up time configurable

Today, the maximum number of items in the queue, as well as how long we
keep attempting to send each item, is hard-coded and not changed by end
users.

While they are totally adequate for chasquid's main use cases, it can
still be useful for some users to change them.

So this patch adds two new configuration options for those settings.
They're marked experimental for now, so we can adjust them if needed
after they get more exposure.

Thanks to Lewis Ross-Jones <lewis_r_j@hotmail.com> for suggesting this
improvement, and help with testing it.

internal/queue/queue.go

@@ -33,12 +33,6 @@ import (
 )
 
 const (
-	// Maximum size of the queue; we reject emails when we hit this.
-	maxQueueSize = 200
-
-	// Give up sending attempts after this duration.
-	giveUpAfter = 20 * time.Hour
-
 	// Prefix for item file names.
 	// This is for convenience, versioning, and to be able to tell them apart
 	// temporary files and other cruft.
@@ -83,12 +77,6 @@ func init() {
 
 // Queue that keeps mail waiting for delivery.
 type Queue struct {
-	// Items in the queue. Map of id -> Item.
-	q map[string]*Item
-
-	// Mutex protecting q.
-	mu sync.RWMutex
-
 	// Couriers to use to deliver mail.
 	localC  courier.Courier
 	remoteC courier.Courier
@@ -101,6 +89,18 @@ type Queue struct {
 
 	// Aliases resolver.
 	aliases *aliases.Resolver
+
+	// The maximum number of items in the queue.
+	MaxItems int
+
+	// Give up sending attempts after this long.
+	GiveUpAfter time.Duration
+
+	// Mutex protecting q.
+	mu sync.RWMutex
+
+	// Items in the queue. Map of id -> Item.
+	q map[string]*Item
 }
 
 // New creates a new Queue instance.
@@ -115,6 +115,16 @@ func New(path string, localDomains *set.String, aliases *aliases.Resolver,
 		localDomains: localDomains,
 		path:         path,
 		aliases:      aliases,
+
+		// We reject emails when we hit this.
+		// Note the actual default used in the daemon is set in the config. We
+		// put a non-zero value here just to be safe.
+		MaxItems: 100,
+
+		// We give up sending (and return a DSN) after this long.
+		// Note the actual default used in the daemon is set in the config. We
+		// put a non-zero value here just to be safe.
+		GiveUpAfter: 20 * time.Hour,
 	}
 	return q, err
 }
@@ -155,8 +165,8 @@ func (q *Queue) Put(tr *trace.Trace, from string, to []string, data []byte) (str
 	tr = tr.NewChild("Queue.Put", from)
 	defer tr.Finish()
 
-	if q.Len() >= maxQueueSize {
-		tr.Errorf("queue full")
+	if nItems := q.Len(); nItems >= q.MaxItems {
+		tr.Errorf("queue full (%d items)", nItems)
 		return "", errQueueFull
 	}
 	putCount.Add(1)
@@ -305,7 +315,7 @@ func (item *Item) SendLoop(q *Queue) {
 	defer tr.Finish()
 	tr.Printf("from %s", item.From)
 
-	for time.Since(item.CreatedAt) < giveUpAfter {
+	for time.Since(item.CreatedAt) < q.GiveUpAfter {
 		// Send to all recipients that are still pending.
 		var wg sync.WaitGroup
 		for _, rcpt := range item.Rcpt {