common/para: Add parallel task executor helper

Usage of this will come later.

common/para/para.go

@@ -0,0 +1,73 @@
+// Copyright 2019 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package para implements parallel execution helpers.
+package para
+
+import (
+	"context"
+
+	"golang.org/x/sync/errgroup"
+)
+
+// Workers configures a task executor with the most number of tasks to be executed in parallel.
+type Workers struct {
+	sem chan struct{}
+}
+
+// Runner wraps the lifecycle methods of a new task set.
+//
+// Run wil block until a worker is available or the context is cancelled,
+// and then run the given func in a new goroutine.
+// Wait will wait for all the running goroutines to finish.
+type Runner interface {
+	Run(func() error)
+	Wait() error
+}
+
+type errGroupRunner struct {
+	*errgroup.Group
+	w *Workers
+	ctx  context.Context
+}
+
+func (g *errGroupRunner) Run(fn func() error) {
+	select {
+	case g.w.sem <- struct{}{}:
+	case <-g.ctx.Done():
+		return
+	}
+
+	g.Go(func() error {
+		err := fn()
+		<-g.w.sem
+		return err
+	})
+}
+
+// New creates a new Workers with the given number of workers.
+func New(numWorkers int) *Workers {
+	return &Workers{
+		sem: make(chan struct{}, numWorkers),
+	}
+}
+
+// Start starts a new Runner.
+func (w *Workers) Start(ctx context.Context) (Runner, context.Context) {
+	g, ctx := errgroup.WithContext(ctx)
+	return &errGroupRunner{
+		Group: g,
+		ctx:   ctx,
+		w:  w,
+	}, ctx
+}