Add Markdown diagrams and render hooks for code blocks

You can now create custom hook templates for code blocks, either one for all (`render-codeblock.html`) or for a given code language (e.g. `render-codeblock-go.html`).

We also used this new hook to add support for diagrams in Hugo:

* Goat (Go ASCII Tool) is built-in and enabled by default; just create a fenced code block with the language `goat` and start draw your Ascii diagrams.
* Another popular alternative for diagrams in Markdown, Mermaid (supported by GitHub), can also be implemented with a simple template. See the Hugo documentation for more information.

Updates #7765
Closes #9538
Fixes #9553
Fixes #8520
Fixes #6702
Fixes #9558

markup/goldmark/codeblocks/render.go

@@ -0,0 +1,159 @@
+// Copyright 2022 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 codeblocks
+
+import (
+	"bytes"
+	"fmt"
+
+	"github.com/gohugoio/hugo/markup/converter/hooks"
+	"github.com/gohugoio/hugo/markup/goldmark/internal/render"
+	"github.com/gohugoio/hugo/markup/internal/attributes"
+	"github.com/yuin/goldmark"
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/parser"
+	"github.com/yuin/goldmark/renderer"
+	"github.com/yuin/goldmark/text"
+	"github.com/yuin/goldmark/util"
+)
+
+type (
+	diagrams     struct{}
+	htmlRenderer struct{}
+)
+
+func New() goldmark.Extender {
+	return &diagrams{}
+}
+
+func (e *diagrams) Extend(m goldmark.Markdown) {
+	m.Parser().AddOptions(
+		parser.WithASTTransformers(
+			util.Prioritized(&Transformer{}, 100),
+		),
+	)
+	m.Renderer().AddOptions(renderer.WithNodeRenderers(
+		util.Prioritized(newHTMLRenderer(), 100),
+	))
+}
+
+func newHTMLRenderer() renderer.NodeRenderer {
+	r := &htmlRenderer{}
+	return r
+}
+
+func (r *htmlRenderer) RegisterFuncs(reg renderer.NodeRendererFuncRegisterer) {
+	reg.Register(KindCodeBlock, r.renderCodeBlock)
+}
+
+func (r *htmlRenderer) renderCodeBlock(w util.BufWriter, src []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	ctx := w.(*render.Context)
+
+	if entering {
+		return ast.WalkContinue, nil
+	}
+
+	n := node.(*codeBlock)
+	lang := string(n.b.Language(src))
+	ordinal := n.ordinal
+
+	var buff bytes.Buffer
+
+	l := n.b.Lines().Len()
+	for i := 0; i < l; i++ {
+		line := n.b.Lines().At(i)
+		buff.Write(line.Value(src))
+	}
+	text := buff.String()
+
+	var info []byte
+	if n.b.Info != nil {
+		info = n.b.Info.Segment.Value(src)
+	}
+	attrs := getAttributes(n.b, info)
+
+	v := ctx.RenderContext().GetRenderer(hooks.CodeBlockRendererType, lang)
+	if v == nil {
+		return ast.WalkStop, fmt.Errorf("no code renderer found for %q", lang)
+	}
+
+	cr := v.(hooks.CodeBlockRenderer)
+
+	err := cr.RenderCodeblock(
+		w,
+		codeBlockContext{
+			page:             ctx.DocumentContext().Document,
+			lang:             lang,
+			code:             text,
+			ordinal:          ordinal,
+			AttributesHolder: attributes.New(attrs, attributes.AttributesOwnerCodeBlock),
+		},
+	)
+
+	ctx.AddIdentity(cr)
+
+	return ast.WalkContinue, err
+}
+
+type codeBlockContext struct {
+	page    interface{}
+	lang    string
+	code    string
+	ordinal int
+	*attributes.AttributesHolder
+}
+
+func (c codeBlockContext) Page() interface{} {
+	return c.page
+}
+
+func (c codeBlockContext) Lang() string {
+	return c.lang
+}
+
+func (c codeBlockContext) Code() string {
+	return c.code
+}
+
+func (c codeBlockContext) Ordinal() int {
+	return c.ordinal
+}
+
+func getAttributes(node *ast.FencedCodeBlock, infostr []byte) []ast.Attribute {
+	if node.Attributes() != nil {
+		return node.Attributes()
+	}
+	if infostr != nil {
+		attrStartIdx := -1
+
+		for idx, char := range infostr {
+			if char == '{' {
+				attrStartIdx = idx
+				break
+			}
+		}
+
+		if attrStartIdx > 0 {
+			n := ast.NewTextBlock() // dummy node for storing attributes
+			attrStr := infostr[attrStartIdx:]
+			if attrs, hasAttr := parser.ParseAttributes(text.NewReader(attrStr)); hasAttr {
+				for _, attr := range attrs {
+					n.SetAttribute(attr.Name, attr.Value)
+				}
+				return n.Attributes()
+			}
+		}
+	}
+	return nil
+}