Update processor to query queues based on priority

processor.go

@@ -7,6 +7,7 @@ package asynq
 import (
 	"fmt"
 	"log"
+	"math/rand"
 	"sync"
 	"time"
 
@@ -109,12 +110,7 @@ func (p *processor) start() {
 // exec pulls a task out of the queue and starts a worker goroutine to
 // process the task.
 func (p *processor) exec() {
-	// TODO(hibiken): Randomize the order to avoid starving low priority queues
+	qnames := p.queues()
-	var qnames []string
-	for q := range p.queueConfig {
-		qnames = append(qnames, q)
-	}
-
 	msg, err := p.rdb.Dequeue(qnames...)
 	if err == rdb.ErrNoProcessableTask {
 		// queues are empty, this is a normal behavior.
@@ -209,6 +205,21 @@ func (p *processor) kill(msg *base.TaskMessage, e error) {
 	}
 }
 
+// queues returns a list of queues to query. Order of the list
+// is based roughly on the priority of each queue, but randomizes
+// it to avoid starving low priority queues.
+func (p *processor) queues() []string {
+	var names []string
+	for qname, priority := range p.queueConfig {
+		for i := 0; i < int(priority); i++ {
+			names = append(names, qname)
+		}
+	}
+	r := rand.New(rand.NewSource(time.Now().UnixNano()))
+	r.Shuffle(len(names), func(i, j int) { names[i], names[j] = names[j], names[i] })
+	return uniq(names, len(p.queueConfig))
+}
+
 // perform calls the handler with the given task.
 // If the call returns without panic, it simply returns the value,
 // otherwise, it recovers from panic and returns an error.
@@ -220,3 +231,20 @@ func perform(h Handler, task *Task) (err error) {
 	}()
 	return h.ProcessTask(task)
 }
+
+// uniq dedupes elements and returns a slice of unique names of length l.
+// Order of the output slice is based on the input list.
+func uniq(names []string, l int) []string {
+	var res []string
+	seen := make(map[string]struct{})
+	for _, s := range names {
+		if _, ok := seen[s]; !ok {
+			seen[s] = struct{}{}
+			res = append(res, s)
+		}
+		if len(res) == l {
+			break
+		}
+	}
+	return res
+}

processor_test.go

@@ -6,6 +6,7 @@ package asynq
 
 import (
 	"fmt"
+	"sort"
 	"sync"
 	"testing"
 	"time"
@@ -178,6 +179,43 @@ func TestProcessorRetry(t *testing.T) {
 	}
 }
 
+func TestProcessorQueues(t *testing.T) {
+	sortOpt := cmp.Transformer("SortStrings", func(in []string) []string {
+		out := append([]string(nil), in...) // Copy input to avoid mutating it
+		sort.Strings(out)
+		return out
+	})
+
+	tests := []struct {
+		queueCfg map[string]uint
+		want     []string
+	}{
+		{
+			queueCfg: map[string]uint{
+				"high":    6,
+				"default": 3,
+				"low":     1,
+			},
+			want: []string{"high", "default", "low"},
+		},
+		{
+			queueCfg: map[string]uint{
+				"default": 1,
+			},
+			want: []string{"default"},
+		},
+	}
+
+	for _, tc := range tests {
+		p := newProcessor(nil, 10, tc.queueCfg, defaultDelayFunc)
+		got := p.queues()
+		if diff := cmp.Diff(tc.want, got, sortOpt); diff != "" {
+			t.Errorf("with queue config: %v\n(*processor).queues() = %v, want %v\n(-want,+got):\n%s",
+				tc.queueCfg, got, tc.want, diff)
+		}
+	}
+}
+
 func TestPerform(t *testing.T) {
 	tests := []struct {
 		desc    string