frontend: Set up opensearchdescription aka Tab To Search

This used to work on godoc.org, let's set it up for pkg.go.dev too.

golang/go#58133

internal/frontend/server.go

@@ -193,6 +193,7 @@ Disallow: /fetch/*
 Sitemap: https://pkg.go.dev/sitemap/index.xml
 `))
 	}))
+	handle("/opensearch.xml", s.opensearchHandler())
 	s.installDebugHandlers(handle)
 }
 
@@ -466,6 +467,21 @@ func (s *Server) aboutHandler() http.HandlerFunc {
 	})
 }
 
+func (s *Server) opensearchHandler() http.HandlerFunc {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/opensearchdescription+xml")
+		http.ServeContent(w, r, "", time.Time{}, strings.NewReader(`<?xml version="1.0"?>
+<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/">
+    <InputEncoding>UTF-8</InputEncoding>
+    <ShortName>Go Packages</ShortName>
+    <Description>Go Packages: Go Package Documentation</Description>
+    <Url type="text/html" method="get" template="https://` + r.Host + `/search?q={searchTerms}"/>
+    <Url type="application/opensearchdescription+xml" rel="self" template="https://` + r.Host + `/opensearch.xml"/>
+</OpenSearchDescription>
+`))
+	})
+}
+
 // newBasePage returns a base page for the given request and title.
 func (s *Server) newBasePage(r *http.Request, title string) basePage {
 	q := rawSearchQuery(r)

static/frontend/frontend.tmpl

@@ -30,6 +30,7 @@
     <link rel="shortcut icon" href="/static/shared/icon/favicon.ico">
     {{block "canonical" .}}{{end}}
     <link href="/static/frontend/frontend.min.css?version={{.AppVersionLabel}}" rel="stylesheet">
+    <link type="application/opensearchdescription+xml" rel="search" href="/opensearch.xml?version={{.AppVersionLabel}}" title="Go Packages" />
     {{block "title" .}}
       <title>Go Packages</title>
     {{end}}