fsjobqueue: factor reflect.Select out of Dequeue()

Hopefully this make it easier to read.

internal/jobqueue/fsjobqueue/fsjobqueue.go

@@ -14,6 +14,7 @@ package fsjobqueue
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"reflect"
 	"sort"
@@ -150,36 +151,12 @@ func (q *fsJobQueue) Enqueue(jobType string, args interface{}, dependencies []uu
 }
 
 func (q *fsJobQueue) Dequeue(ctx context.Context, jobTypes []string, args interface{}) (uuid.UUID, error) {
-	// Return early if the context is already canceled.
-	if err := ctx.Err(); err != nil {
+
+	id, err := selectUUIDChannel(ctx, q.pendingChannels(jobTypes))
+	if err != nil {
 		return uuid.Nil, err
 	}
 
-	// Use reflect.Select(), because the `select` statement cannot operate
-	// on an unknown amount of channels.
-	cases := []reflect.SelectCase{
-		{
-			Dir:  reflect.SelectRecv,
-			Chan: reflect.ValueOf(ctx.Done()),
-		},
-	}
-	for _, jt := range jobTypes {
-		cases = append(cases, reflect.SelectCase{
-			Dir:  reflect.SelectRecv,
-			Chan: reflect.ValueOf(q.pendingChannel(jt)),
-		})
-	}
-
-	chosen, value, recvOK := reflect.Select(cases)
-	if chosen == 0 && !recvOK {
-		return uuid.Nil, ctx.Err()
-	}
-	// pending channels are never closed
-	if !recvOK {
-		panic("pending channel was closed unexpectedly")
-	}
-	id := value.Interface().(uuid.UUID)
-
 	j, err := q.readJob(id)
 	if err != nil {
 		return uuid.Nil, err
@@ -312,6 +289,25 @@ func (q *fsJobQueue) pendingChannel(jobType string) chan uuid.UUID {
 	return c
 }
 
+// Same as pendingChannel(), but for multiple job types.
+func (q *fsJobQueue) pendingChannels(jobTypes []string) []chan uuid.UUID {
+	q.pendingMutex.Lock()
+	defer q.pendingMutex.Unlock()
+
+	chans := make([]chan uuid.UUID, len(jobTypes))
+
+	for i, jt := range jobTypes {
+		c, exists := q.pending[jt]
+		if !exists {
+			c = make(chan uuid.UUID, 100)
+			q.pending[jt] = c
+		}
+		chans[i] = c
+	}
+
+	return chans
+}
+
 // Sorts and removes duplicates from `ids`.
 func uniqueUUIDList(ids []uuid.UUID) []uuid.UUID {
 	s := map[uuid.UUID]bool{}
@@ -335,3 +331,34 @@ func uniqueUUIDList(ids []uuid.UUID) []uuid.UUID {
 
 	return l
 }
+
+// Select on a list of `chan uuid.UUID`s. Returns an error if one of the
+// channels is closed.
+//
+// Uses reflect.Select(), because the `select` statement cannot operate on an
+// unknown amount of channels.
+func selectUUIDChannel(ctx context.Context, chans []chan uuid.UUID) (uuid.UUID, error) {
+	cases := []reflect.SelectCase{
+		{
+			Dir:  reflect.SelectRecv,
+			Chan: reflect.ValueOf(ctx.Done()),
+		},
+	}
+	for _, c := range chans {
+		cases = append(cases, reflect.SelectCase{
+			Dir:  reflect.SelectRecv,
+			Chan: reflect.ValueOf(c),
+		})
+	}
+
+	chosen, value, recvOK := reflect.Select(cases)
+	if !recvOK {
+		if chosen == 0 {
+			return uuid.Nil, ctx.Err()
+		} else {
+			return uuid.Nil, errors.New("channel was closed unexpectedly")
+		}
+	}
+
+	return value.Interface().(uuid.UUID), nil
+}