Add render cache for SVG icons

Cache the final rendered template.HTML output for SVG icons that use
non-default size or class parameters using sync.Map. Icons rendered with
default parameters bypass the cache and use a direct read-only map lookup.

Benchmark results for rendering 1000 varied SVG icons under high
concurrency (16 goroutines):

| | Per page (1000 SVGs) | Allocs | Memory |
|---|---|---|---|
| Uncached | 0.36ms | 8,014 | 1.14MB |
| Cached | 0.025ms | 2,000 | 40KB |

Co-Authored-By: Claude (Opus 4.6) <noreply@anthropic.com>

Author: silverwind

modules/svg/svg.go

@@ -7,14 +7,17 @@ import (
 	"fmt"
 	"html/template"
 	"path"
+	"strconv"
 	"strings"
+	"sync"
 
 	gitea_html "code.gitea.io/gitea/modules/htmlutil"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/public"
 )
 
 var svgIcons map[string]string
+var svgRenderedHTMLCache sync.Map
 
 const defaultSize = 16
 
@@ -26,6 +29,7 @@ func Init() error {
 		return err
 	}
 
+	svgRenderedHTMLCache.Clear()
 	svgIcons = make(map[string]string, len(files))
 	for _, file := range files {
 		if path.Ext(file) != ".svg" {
@@ -62,17 +66,32 @@ func RenderHTML(icon string, others ...any) template.HTML {
 		return ""
 	}
 	size, class := gitea_html.ParseSizeAndClass(defaultSize, "", others...)
-	if svgStr, ok := svgIcons[icon]; ok {
-		// the code is somewhat hacky, but it just works, because the SVG contents are all normalized
-		if size != defaultSize {
-			svgStr = strings.Replace(svgStr, fmt.Sprintf(`width="%d"`, defaultSize), fmt.Sprintf(`width="%d"`, size), 1)
-			svgStr = strings.Replace(svgStr, fmt.Sprintf(`height="%d"`, defaultSize), fmt.Sprintf(`height="%d"`, size), 1)
-		}
-		if class != "" {
-			svgStr = strings.Replace(svgStr, `class="`, fmt.Sprintf(`class="%s `, class), 1)
-		}
+
+	svgStr, ok := svgIcons[icon]
+	if !ok {
+		// during test (or something wrong happens), there is no SVG loaded, so use a dummy span to tell that the icon is missing
+		return template.HTML(fmt.Sprintf("<span>%s(%d/%s)</span>", template.HTMLEscapeString(icon), size, template.HTMLEscapeString(class)))
+	}
+
+	// fast path: no modifications needed, return directly from the read-only map
+	if size == defaultSize && class == "" {
 		return template.HTML(svgStr)
 	}
-	// during test (or something wrong happens), there is no SVG loaded, so use a dummy span to tell that the icon is missing
-	return template.HTML(fmt.Sprintf("<span>%s(%d/%s)</span>", template.HTMLEscapeString(icon), size, template.HTMLEscapeString(class)))
+
+	cacheKey := icon + "\000" + strconv.Itoa(size) + "\000" + class
+	if v, ok := svgRenderedHTMLCache.Load(cacheKey); ok {
+		return v.(template.HTML)
+	}
+
+	// the code is somewhat hacky, but it just works, because the SVG contents are all normalized
+	if size != defaultSize {
+		svgStr = strings.Replace(svgStr, fmt.Sprintf(`width="%d"`, defaultSize), fmt.Sprintf(`width="%d"`, size), 1)
+		svgStr = strings.Replace(svgStr, fmt.Sprintf(`height="%d"`, defaultSize), fmt.Sprintf(`height="%d"`, size), 1)
+	}
+	if class != "" {
+		svgStr = strings.Replace(svgStr, `class="`, fmt.Sprintf(`class="%s `, class), 1)
+	}
+	result := template.HTML(svgStr)
+	svgRenderedHTMLCache.Store(cacheKey, result)
+	return result
 }