feat(17-03): sitemap plugin tracer, /sitemap.xml byte-identical to PHP

- golem15.sitemap plugin: golem15_sitemap_definitions migration, Definition
  model (Fillable theme and data) with tolerant PHP-shaped item decoding and
  PHP toArray item encoding
- Url::to and buildUrl port, libxml-format serializer, generator for url and
  registry items
- GET /sitemap.xml serves the definition with theme default (404 without one,
  empty body without items)
- tests/php/gen-goldens.php and the tracer golden; TestSitemapGolden and the
  Postgres TestSitemapTracer

classes/serialize.go

@@ -0,0 +1,105 @@
+package classes
+
+import "strings"
+
+// The sitemap document is written by hand in the exact format PHP's
+// DOMDocument::saveXML (libxml2) produced for the PHP plugin: the XML
+// declaration and the stylesheet processing instruction each followed by a
+// newline, everything else on one line, an empty urlset self-closed,
+// xhtml:link elements self-closed without a space. The standard library's
+// XML encoder escapes and closes elements differently, so the writer is a
+// strings.Builder.
+
+// The urlset opening tag's attributes, in the order PHP set them.
+const urlsetAttrs = ` xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"` +
+	` xmlns:xhtml="http://www.w3.org/1999/xhtml"` +
+	` xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"` +
+	` xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd"`
+
+// textEscaper is libxml's escaping of text nodes: &, <, > and carriage
+// return; quotes, tabs, newlines and UTF-8 stay literal.
+var textEscaper = strings.NewReplacer("&", "&amp;", "<", "&lt;", ">", "&gt;", "\r", "&#13;")
+
+// attrEscaper is libxml's escaping of attribute values: also the double
+// quote, tab and newline; the single quote stays literal.
+var attrEscaper = strings.NewReplacer(
+	"&", "&amp;", "<", "&lt;", ">", "&gt;", `"`, "&quot;",
+	"\r", "&#13;", "\t", "&#9;", "\n", "&#10;",
+)
+
+// EscapeText escapes s as libxml writes a text node.
+func EscapeText(s string) string { return textEscaper.Replace(s) }
+
+// EscapeAttr escapes s as libxml writes an attribute value.
+func EscapeAttr(s string) string { return attrEscaper.Replace(s) }
+
+// Alternate is one hreflang link of a sitemap URL.
+type Alternate struct {
+	Hreflang string
+	Href     string
+}
+
+// URLEntry is one url element: Loc, an optional Lastmod, Changefreq and
+// Priority as they are written, the alternates (written only when there are
+// more than one) and Extra, pre-escaped child XML that AddItem listeners
+// appended, written before the closing tag.
+type URLEntry struct {
+	Loc        string
+	Lastmod    string
+	Changefreq string
+	Priority   string
+	Alternates []Alternate
+	Extra      []string
+}
+
+// writeURL appends one url element.
+func writeURL(b *strings.Builder, e URLEntry) {
+	b.WriteString("<url><loc>")
+	b.WriteString(EscapeText(e.Loc))
+	b.WriteString("</loc>")
+	if e.Lastmod != "" {
+		b.WriteString("<lastmod>")
+		b.WriteString(EscapeText(e.Lastmod))
+		b.WriteString("</lastmod>")
+	}
+	b.WriteString("<changefreq>")
+	b.WriteString(EscapeText(e.Changefreq))
+	b.WriteString("</changefreq><priority>")
+	b.WriteString(EscapeText(e.Priority))
+	b.WriteString("</priority>")
+	if len(e.Alternates) > 1 {
+		for _, a := range e.Alternates {
+			b.WriteString(`<xhtml:link rel="alternate" hreflang="`)
+			b.WriteString(EscapeAttr(a.Hreflang))
+			b.WriteString(`" href="`)
+			b.WriteString(EscapeAttr(a.Href))
+			b.WriteString(`"/>`)
+		}
+	}
+	for _, x := range e.Extra {
+		b.WriteString(x)
+	}
+	b.WriteString("</url>")
+}
+
+// Serialize writes the sitemap document: the XML declaration, the
+// stylesheet processing instruction pointing at stylesheet (written as it
+// is, like libxml writes PI data) and the urlset with entries.
+func Serialize(stylesheet string, entries []URLEntry) string {
+	var b strings.Builder
+	b.WriteString("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n")
+	b.WriteString(`<?xml-stylesheet type="text/xsl" href="`)
+	b.WriteString(stylesheet)
+	b.WriteString("\"?>\n<urlset")
+	b.WriteString(urlsetAttrs)
+	if len(entries) == 0 {
+		b.WriteString("/>\n")
+		return b.String()
+	}
+	b.WriteString(">")
+	for _, e := range entries {
+		writeURL(&b, e)
+	}
+	b.WriteString("</urlset>\n")
+	return b.String()
+}