Only re-render the view(s) you're working on

Hugo already, in its server mode,  support partial rebuilds. To put it simply: If you change `about.md`, only that content page is read and processed, then Hugo does some processing (taxonomies etc.) and the full site is rendered.
This commit covers the rendering part: We now only re-render the pages you work on, i.e. the last n pages you watched in the browser (which obviously also includes the  page in the example above).

To be more specific: When you are running the hugo server in watch (aka. livereload) mode, and change a template or a content file, then we do a partial re-rendering of the following:

* The current content page (if it is a content change)
* The home page
* Up to the last 10 pages you visited on the site.

This should in most cases be enough, but if you navigate to something completely different, you may see stale content. Doing an edit will then refresh that page.

Note that this feature is enabled by default. To turn it off, run `hugo server --disableFastRender`.

Fixes #3962
See  #1643

2 files changed, 160 insertions, 0 deletions

diff --git a/common/types/evictingqueue.go b/common/types/evictingqueue.go
new file mode 100644
index 000000000..152dc4c41
--- /dev/null
+++ b/common/types/evictingqueue.go
@@ -0,0 +1,89 @@
+// Copyright 2017-present 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 types contains types shared between packages in Hugo.
+package types
+
+import (
+	"sync"
+)
+
+// EvictingStringQueue is a queue which automatically evicts elements from the head of
+// the queue when attempting to add new elements onto the queue and it is full.
+// This queue orders elements LIFO (last-in-first-out). It throws away duplicates.
+// Note: This queue currently does not contain any remove (poll etc.) methods.
+type EvictingStringQueue struct {
+	size int
+	vals []string
+	set  map[string]bool
+	mu   sync.Mutex
+}
+
+// NewEvictingStringQueue creates a new queue with the given size.
+func NewEvictingStringQueue(size int) *EvictingStringQueue {
+	return &EvictingStringQueue{size: size, set: make(map[string]bool)}
+}
+
+// Add adds a new string to the tail of the queue if it's not already there.
+func (q *EvictingStringQueue) Add(v string) {
+	q.mu.Lock()
+	if q.set[v] {
+		q.mu.Unlock()
+		return
+	}
+
+	if len(q.set) == q.size {
+		// Full
+		delete(q.set, q.vals[0])
+		q.vals = append(q.vals[:0], q.vals[1:]...)
+	}
+	q.set[v] = true
+	q.vals = append(q.vals, v)
+	q.mu.Unlock()
+}
+
+// Peek looks at the last element added to the queue.
+func (q *EvictingStringQueue) Peek() string {
+	q.mu.Lock()
+	l := len(q.vals)
+	if l == 0 {
+		q.mu.Unlock()
+		return ""
+	}
+	elem := q.vals[l-1]
+	q.mu.Unlock()
+	return elem
+}
+
+// PeekAll looks at all the elements in the queue, with the newest first.
+func (q *EvictingStringQueue) PeekAll() []string {
+	q.mu.Lock()
+	vals := make([]string, len(q.vals))
+	copy(vals, q.vals)
+	q.mu.Unlock()
+	for i, j := 0, len(vals)-1; i < j; i, j = i+1, j-1 {
+		vals[i], vals[j] = vals[j], vals[i]
+	}
+	return vals
+}
+
+// PeekAllSet returns PeekAll as a set.
+func (q *EvictingStringQueue) PeekAllSet() map[string]bool {
+	all := q.PeekAll()
+	set := make(map[string]bool)
+	for _, v := range all {
+		set[v] = true
+	}
+
+	return set
+}
diff --git a/common/types/evictingqueue_test.go b/common/types/evictingqueue_test.go
new file mode 100644
index 000000000..a33f1a344
--- /dev/null
+++ b/common/types/evictingqueue_test.go
@@ -0,0 +1,71 @@
+// Copyright 2017-present 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 types
+
+import (
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestEvictingStringQueue(t *testing.T) {
+	assert := require.New(t)
+
+	queue := NewEvictingStringQueue(3)
+
+	assert.Equal("", queue.Peek())
+	queue.Add("a")
+	queue.Add("b")
+	queue.Add("a")
+	assert.Equal("b", queue.Peek())
+	queue.Add("b")
+	assert.Equal("b", queue.Peek())
+
+	queue.Add("a")
+	queue.Add("b")
+
+	assert.Equal([]string{"b", "a"}, queue.PeekAll())
+	assert.Equal("b", queue.Peek())
+	queue.Add("c")
+	queue.Add("d")
+	// Overflowed, a should now be removed.
+	assert.Equal([]string{"d", "c", "b"}, queue.PeekAll())
+	assert.Len(queue.PeekAllSet(), 3)
+	assert.True(queue.PeekAllSet()["c"])
+}
+
+func TestEvictingStringQueueConcurrent(t *testing.T) {
+	var wg sync.WaitGroup
+	val := "someval"
+
+	queue := NewEvictingStringQueue(3)
+
+	for j := 0; j < 100; j++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			queue.Add(val)
+			v := queue.Peek()
+			if v != val {
+				t.Error("wrong val")
+			}
+			vals := queue.PeekAll()
+			if len(vals) != 1 || vals[0] != val {
+				t.Error("wrong val")
+			}
+		}()
+	}
+	wg.Wait()
+}