suggest long URL for new go links if peer exists

If you visit a non-existent go link, we render the home page and pre-
populate the "short" input with the name of the link, and autofocus the
"long" input so that you can simply paste a long URL and submit.

It is common (at least at Tailscale) to create go links that correspond
to the name of a device on the tailnet.  For example, go/who points to
http://who/.  With this change, when you visit a non-existent go link,
we check to see if a peer exists on the tailnet with that name, and if
so we suggest that as the long URL.

Signed-off-by: Will Norris <will@tailscale.com>

golink.go

@@ -257,6 +257,7 @@ type visitData struct {
 // homeData is the data used by the homeTmpl template.
 type homeData struct {
 	Short  string
+	Long   string
 	Clicks []visitData
 }
 
@@ -379,7 +380,7 @@ func serveHandler() http.Handler {
 	})
 }
 
-func serveHome(w http.ResponseWriter, short string) {
+func serveHome(w http.ResponseWriter, r *http.Request, short string) {
 	var clicks []visitData
 
 	stats.mu.Lock()
@@ -401,8 +402,23 @@ func serveHome(w http.ResponseWriter, short string) {
 		clicks = clicks[:200]
 	}
 
+	var long string
+	if short != "" && localClient != nil {
+		// if a peer exists with the short name, suggest it as the long URL
+		st, err := localClient.Status(r.Context())
+		if err == nil {
+			for _, p := range st.Peer {
+				if host, _, ok := strings.Cut(p.DNSName, "."); ok && host == short {
+					long = "http://" + host + "/"
+					break
+				}
+			}
+		}
+	}
+
 	homeTmpl.Execute(w, homeData{
 		Short:  short,
+		Long:   long,
 		Clicks: clicks,
 	})
 }
@@ -442,7 +458,7 @@ func serveGo(w http.ResponseWriter, r *http.Request) {
 	if r.URL.Path == "/" {
 		switch r.Method {
 		case "GET":
-			serveHome(w, "")
+			serveHome(w, r, "")
 		case "POST":
 			serveSave(w, r)
 		}
@@ -460,7 +476,7 @@ func serveGo(w http.ResponseWriter, r *http.Request) {
 	link, err := db.Load(short)
 	if errors.Is(err, fs.ErrNotExist) {
 		w.WriteHeader(http.StatusNotFound)
-		serveHome(w, short)
+		serveHome(w, r, short)
 		return
 	}
 	if err != nil {

tmpl/home.html

@@ -1,14 +1,17 @@
 {{ define "main" }}
     <h2 class="text-xl font-bold pb-2">Create a new link</h2>
 
+    {{ with .Long }}
+    <p class="">Did you mean <a class="text-blue-600 hover:underline" href="{{.}}">{{.}}</a> ? Create a go link for it now:</p>
+    {{ end }}
     <form method="POST" action="/" class="flex flex-wrap">
       <div class="flex">
         <label for=short class="flex my-2 px-2 items-center bg-gray-100 border border-r-0 border-gray-300 rounded-l-md text-gray-700">http://go/</label>
         <input id=short name=short required type=text size=15 placeholder="shortname" value="{{.Short}}" pattern="\w[\w\-\.]*" title="Must start with letter or number; may contain letters, numbers, dashes, and periods."
           class="p-2 my-2 rounded-r-md border-gray-300 placeholder:text-gray-400">
         <span class="flex m-2 items-center">&rarr;</span>
       </div>
-      <input name=long required type=text size=40 placeholder="https://destination-url"{{if .Short}} autofocus{{end}} class="p-2 my-2 mr-2 max-w-full rounded-md border-gray-300 placeholder:text-gray-400">
+      <input name=long required type=text size=40 placeholder="https://destination-url"{{if .Short}} value="{{.Long}}" autofocus{{end}} class="p-2 my-2 mr-2 max-w-full rounded-md border-gray-300 placeholder:text-gray-400">
       <button type=submit class="py-2 px-4 my-2 rounded-md bg-blue-500 border-blue-500 text-white hover:bg-blue-600 hover:border-blue-600">Create</button>
     </form>
     <p class="text-sm text-gray-500"><a class="text-blue-600 hover:underline" href="/.help">Help and advanced options</a></p>