output: Speed up layout calculations

```
BenchmarkLayout-4     4883          497           -89.82%

benchmark             old allocs     new allocs     delta
BenchmarkLayout-4     18             1              -94.44%

benchmark             old bytes     new bytes     delta
BenchmarkLayout-4     1624          32            -98.03%
```

hugolib/page.go

@@ -190,6 +190,8 @@ type Page struct {
 	permalink    string
 	relPermalink string
 
+	layoutDescriptor output.LayoutDescriptor
+
 	scratch *Scratch
 
 	// It would be tempting to use the language set on the Site, but in they way we do
@@ -666,7 +668,7 @@ func (p *Page) layouts(layouts ...string) []string {
 	}
 
 	return p.s.layoutHandler.For(
-		p.createLayoutDescriptor(),
+		p.layoutDescriptor,
 		layoutOverride,
 		output.HTMLType)
 }
@@ -880,6 +882,7 @@ func (p *Page) initURLs() error {
 	p.permalink = p.s.permalink(rel)
 	rel = p.s.PathSpec.PrependBasePath(rel)
 	p.relPermalink = rel
+	p.layoutDescriptor = p.createLayoutDescriptor()
 	return nil
 }
 
@@ -1558,7 +1561,7 @@ func (p *Page) Hugo() *HugoInfo {
 func (p *Page) RSSlink() template.URL {
 	// TODO(bep) we cannot have two of these
 	// Remove in Hugo 0.20
-	helpers.Deprecated(".Page", "Use RSSlink", "RSSLink", true)
+	helpers.Deprecated(".Page", "RSSlink", "Use RSSLink", true)
 	return p.RSSLink
 }
 

hugolib/site.go

@@ -1656,7 +1656,7 @@ func (s *Site) kindFromSections(sections []string) string {
 }
 
 func (s *Site) layouts(p *PageOutput) []string {
-	return s.layoutHandler.For(p.createLayoutDescriptor(), "", p.outputFormat)
+	return s.layoutHandler.For(p.layoutDescriptor, "", p.outputFormat)
 }
 
 func (s *Site) preparePages() error {

output/layout.go

@@ -17,6 +17,7 @@ import (
 	"fmt"
 	"path"
 	"strings"
+	"sync"
 )
 
 // LayoutDescriptor describes how a layout should be chosen. This is
@@ -32,10 +33,19 @@ type LayoutDescriptor struct {
 // TODO(bep) output improve names
 type LayoutHandler struct {
 	hasTheme bool
+
+	mu    sync.RWMutex
+	cache map[layoutCacheKey][]string
+}
+
+type layoutCacheKey struct {
+	d              LayoutDescriptor
+	layoutOverride string
+	f              Format
 }
 
 func NewLayoutHandler(hasTheme bool) *LayoutHandler {
-	return &LayoutHandler{hasTheme: hasTheme}
+	return &LayoutHandler{hasTheme: hasTheme, cache: make(map[layoutCacheKey][]string)}
 }
 
 const (
@@ -62,6 +72,16 @@ indexes/indexes.NAME.SUFFIX indexes/indexes.SUFFIX
 )
 
 func (l *LayoutHandler) For(d LayoutDescriptor, layoutOverride string, f Format) []string {
+
+	// We will get lots of requests for the same layouts, so avoid recalculations.
+	key := layoutCacheKey{d, layoutOverride, f}
+	l.mu.RLock()
+	if cacheVal, found := l.cache[key]; found {
+		l.mu.RUnlock()
+		return cacheVal
+	}
+	l.mu.RUnlock()
+
 	var layouts []string
 
 	layout := d.Layout
@@ -110,6 +130,10 @@ func (l *LayoutHandler) For(d LayoutDescriptor, layoutOverride string, f Format)
 		return layoutsWithThemeLayouts
 	}
 
+	l.mu.Lock()
+	l.cache[key] = layouts
+	l.mu.Unlock()
+
 	return layouts
 }
 

output/layout_test.go

@@ -73,4 +73,15 @@ func TestLayout(t *testing.T) {
 			}
 		})
 	}
+
+}
+
+func BenchmarkLayout(b *testing.B) {
+	descriptor := LayoutDescriptor{Kind: "taxonomyTerm", Section: "categories"}
+	l := NewLayoutHandler(false)
+
+	for i := 0; i < b.N; i++ {
+		layouts := l.For(descriptor, "", HTMLType)
+		require.NotEmpty(b, layouts)
+	}
 }