Add alert blocks in markdown (#29121)

- Follows https://github.com/go-gitea/gitea/pull/21711
- Closes https://github.com/go-gitea/gitea/issues/28316

Implement GitHub's alert blocks markdown feature

Docs:
-
https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#alerts
- https://github.com/orgs/community/discussions/16925

### Before

![image](https://github.com/go-gitea/gitea/assets/20454870/14f7b02a-5de5-4fd0-8437-a055dadb31f2)

### After

![image](https://github.com/go-gitea/gitea/assets/20454870/ed06a869-e545-42f1-bf25-4ba20b1be196)

## :warning: BREAKING :warning:

The old syntax no longer works

How to migrate:

If you used
```md
> **Note** My note
```

Switch to
```md
> [!NOTE]
> My note
```

---------

Signed-off-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Giteabot <teabot@gitea.io>

4 files changed, 97 insertions, 31 deletions

diff --git a/modules/markup/markdown/ast.go b/modules/markup/markdown/ast.go
index 3e6e291ab2..77ce5cb359 100644
--- a/modules/markup/markdown/ast.go
+++ b/modules/markup/markdown/ast.go
@@ -182,12 +182,7 @@ func IsColorPreview(node ast.Node) bool {
 	return ok
 }
 
-const (
-	AttentionNote    string = "Note"
-	AttentionWarning string = "Warning"
-)
-
-// Attention is an inline for a color preview
+// Attention is an inline for an attention
 type Attention struct {
 	ast.BaseInline
 	AttentionType string
diff --git a/modules/markup/markdown/goldmark.go b/modules/markup/markdown/goldmark.go
index 178e3d2fdd..36ce6397f4 100644
--- a/modules/markup/markdown/goldmark.go
+++ b/modules/markup/markdown/goldmark.go
@@ -53,7 +53,6 @@ func (g *ASTTransformer) Transform(node *ast.Document, reader text.Reader, pc pa
 		}
 	}
 
-	attentionMarkedBlockquotes := make(container.Set[*ast.Blockquote])
 	_ = ast.Walk(node, func(n ast.Node, entering bool) (ast.WalkStatus, error) {
 		if !entering {
 			return ast.WalkContinue, nil
@@ -197,18 +196,55 @@ func (g *ASTTransformer) Transform(node *ast.Document, reader text.Reader, pc pa
 			if css.ColorHandler(strings.ToLower(string(colorContent))) {
 				v.AppendChild(v, NewColorPreview(colorContent))
 			}
-		case *ast.Emphasis:
-			// check if inside blockquote for attention, expected hierarchy is
-			// Emphasis < Paragraph < Blockquote
-			blockquote, isInBlockquote := n.Parent().Parent().(*ast.Blockquote)
-			if isInBlockquote && !attentionMarkedBlockquotes.Contains(blockquote) {
-				fullText := string(n.Text(reader.Source()))
-				if fullText == AttentionNote || fullText == AttentionWarning {
-					v.SetAttributeString("class", []byte("attention-"+strings.ToLower(fullText)))
-					v.Parent().InsertBefore(v.Parent(), v, NewAttention(fullText))
-					attentionMarkedBlockquotes.Add(blockquote)
-				}
+		case *ast.Blockquote:
+			// We only want attention blockquotes when the AST looks like:
+			// Text: "["
+			// Text: "!TYPE"
+			// Text(SoftLineBreak): "]"
+
+			// grab these nodes and make sure we adhere to the attention blockquote structure
+			firstParagraph := v.FirstChild()
+			if firstParagraph.ChildCount() < 3 {
+				return ast.WalkContinue, nil
+			}
+			firstTextNode, ok := firstParagraph.FirstChild().(*ast.Text)
+			if !ok || string(firstTextNode.Segment.Value(reader.Source())) != "[" {
+				return ast.WalkContinue, nil
+			}
+			secondTextNode, ok := firstTextNode.NextSibling().(*ast.Text)
+			if !ok || !attentionTypeRE.MatchString(string(secondTextNode.Segment.Value(reader.Source()))) {
+				return ast.WalkContinue, nil
 			}
+			thirdTextNode, ok := secondTextNode.NextSibling().(*ast.Text)
+			if !ok || string(thirdTextNode.Segment.Value(reader.Source())) != "]" {
+				return ast.WalkContinue, nil
+			}
+
+			// grab attention type from markdown source
+			attentionType := strings.ToLower(strings.TrimPrefix(string(secondTextNode.Segment.Value(reader.Source())), "!"))
+
+			// color the blockquote
+			v.SetAttributeString("class", []byte("gt-py-3 attention attention-"+attentionType))
+
+			// create an emphasis to make it bold
+			emphasis := ast.NewEmphasis(2)
+			emphasis.SetAttributeString("class", []byte("attention-"+attentionType))
+			firstParagraph.InsertBefore(firstParagraph, firstTextNode, emphasis)
+
+			// capitalize first letter
+			attentionText := ast.NewString([]byte(strings.ToUpper(string(attentionType[0])) + attentionType[1:]))
+
+			// replace the ![TYPE] with icon+Type
+			emphasis.AppendChild(emphasis, attentionText)
+			for i := 0; i < 2; i++ {
+				lineBreak := ast.NewText()
+				lineBreak.SetSoftLineBreak(true)
+				firstParagraph.InsertAfter(firstParagraph, emphasis, lineBreak)
+			}
+			firstParagraph.InsertBefore(firstParagraph, emphasis, NewAttention(attentionType))
+			firstParagraph.RemoveChild(firstParagraph, firstTextNode)
+			firstParagraph.RemoveChild(firstParagraph, secondTextNode)
+			firstParagraph.RemoveChild(firstParagraph, thirdTextNode)
 		}
 		return ast.WalkContinue, nil
 	})
@@ -339,17 +375,23 @@ func (r *HTMLRenderer) renderCodeSpan(w util.BufWriter, source []byte, n ast.Nod
 // renderAttention renders a quote marked with i.e. "> **Note**" or "> **Warning**" with a corresponding svg
 func (r *HTMLRenderer) renderAttention(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
 	if entering {
-		_, _ = w.WriteString(`<span class="attention-icon attention-`)
+		_, _ = w.WriteString(`<span class="gt-mr-2 gt-vm attention-`)
 		n := node.(*Attention)
 		_, _ = w.WriteString(strings.ToLower(n.AttentionType))
 		_, _ = w.WriteString(`">`)
 
 		var octiconType string
 		switch n.AttentionType {
-		case AttentionNote:
+		case "note":
 			octiconType = "info"
-		case AttentionWarning:
+		case "tip":
+			octiconType = "light-bulb"
+		case "important":
+			octiconType = "report"
+		case "warning":
 			octiconType = "alert"
+		case "caution":
+			octiconType = "stop"
 		}
 		_, _ = w.WriteString(string(svg.RenderHTML("octicon-" + octiconType)))
 	} else {
@@ -417,7 +459,10 @@ func (r *HTMLRenderer) renderSummary(w util.BufWriter, source []byte, node ast.N
 	return ast.WalkContinue, nil
 }
 
-var validNameRE = regexp.MustCompile("^[a-z ]+$")
+var (
+	validNameRE     = regexp.MustCompile("^[a-z ]+$")
+	attentionTypeRE = regexp.MustCompile("^!(NOTE|TIP|IMPORTANT|WARNING|CAUTION)$")
+)
 
 func (r *HTMLRenderer) renderIcon(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
 	if !entering {
diff --git a/modules/markup/sanitizer.go b/modules/markup/sanitizer.go
index 992e85b989..ffc33c3b8e 100644
--- a/modules/markup/sanitizer.go
+++ b/modules/markup/sanitizer.go
@@ -64,9 +64,10 @@ func createDefaultPolicy() *bluemonday.Policy {
 	policy.AllowAttrs("class").Matching(regexp.MustCompile(`^color-preview$`)).OnElements("span")
 
 	// For attention
+	policy.AllowAttrs("class").Matching(regexp.MustCompile(`^gt-py-3 attention attention-\w+$`)).OnElements("blockquote")
 	policy.AllowAttrs("class").Matching(regexp.MustCompile(`^attention-\w+$`)).OnElements("strong")
-	policy.AllowAttrs("class").Matching(regexp.MustCompile(`^attention-icon attention-\w+$`)).OnElements("span", "strong")
-	policy.AllowAttrs("class").Matching(regexp.MustCompile(`^svg octicon-\w+$`)).OnElements("svg")
+	policy.AllowAttrs("class").Matching(regexp.MustCompile(`^gt-mr-2 gt-vm attention-\w+$`)).OnElements("span", "strong")
+	policy.AllowAttrs("class").Matching(regexp.MustCompile(`^svg octicon-(\w|-)+$`)).OnElements("svg")
 	policy.AllowAttrs("viewBox", "width", "height", "aria-hidden").OnElements("svg")
 	policy.AllowAttrs("fill-rule", "d").OnElements("path")
 
diff --git a/web_src/css/base.css b/web_src/css/base.css
index 198e87c0e2..ea32aac6f7 100644
--- a/web_src/css/base.css
+++ b/web_src/css/base.css
@@ -1268,20 +1268,45 @@ img.ui.avatar,
   border-radius: var(--border-radius);
 }
 
-.attention-icon {
-  vertical-align: text-top;
+.attention {
+  color: var(--color-text) !important;
 }
 
-.attention-note {
-  font-weight: unset;
-  color: var(--color-info-text);
+blockquote.attention-note {
+  border-left-color: var(--color-blue-dark-1);
+}
+strong.attention-note, span.attention-note {
+  color: var(--color-blue-dark-1);
+}
+
+blockquote.attention-tip {
+  border-left-color: var(--color-success-text);
+}
+strong.attention-tip, span.attention-tip {
+  color: var(--color-success-text);
 }
 
-.attention-warning {
-  font-weight: unset;
+blockquote.attention-important {
+  border-left-color: var(--color-violet-dark-1);
+}
+strong.attention-important, span.attention-important {
+  color: var(--color-violet-dark-1);
+}
+
+blockquote.attention-warning {
+  border-left-color: var(--color-warning-text);
+}
+strong.attention-warning, span.attention-warning {
   color: var(--color-warning-text);
 }
 
+blockquote.attention-caution {
+  border-left-color: var(--color-red-dark-1);
+}
+strong.attention-caution, span.attention-caution {
+  color: var(--color-red-dark-1);
+}
+
 .center:not(.popup) {
   text-align: center;
 }