internal/frontend: extend /latest-version for packages

Previously, the /latest-version endpoint returned the latest module
version. This wasn't always correct for packages: the latest version
of a package might not be the latest version of its module (since it
might have been removed from the module in later versions).

Example: github.com/docker/docker is both a package and a module.
- The package's latest version is v0.9.1.
- The module's latest version is v1.13.1.

Visiting /github.com/docker/docker takes you to v0.9.1. As of this CL,
the badge says "Latest".  Previously it said "Go to latest," but the
link took you back to the same page.

Fixes b/143833787.

Change-Id: I9600ac7e1db22ae0c4168e310a633f757dae9cd6
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/590622
Reviewed-by: Robert Findley <rfindley@google.com>

content/static/html/pages/details.tmpl

@@ -21,6 +21,7 @@
           data-href="mod/{{$header.Path}}">
         {{- else -}}
           data-mpath="{{$header.Module.Path}}"
+          data-ppath="{{$header.Path}}"
           data-href="{{$header.Path}}">
         {{- end }}
       </div>
@@ -108,7 +109,12 @@ if (copyButton) {
 
 async function latest() {
   const latestBadge = document.querySelector('.DetailsHeader-badge');
-  const url = '/latest-version/' + latestBadge.getAttribute('data-mpath');
+  let url = '/latest-version/' + latestBadge.getAttribute('data-mpath');
+  const ppath = latestBadge.getAttribute('data-ppath');
+  if (ppath) {
+    url += '?pkg=' + ppath;
+  }
+  console.log(url);
   const res = await fetch(url);
   if (!res.ok) {
     // server-side error; do nothing

go.sum

@@ -166,6 +166,7 @@ github.com/google/licensecheck v0.0.0-20190611011822-e07107a3842f/go.mod h1:ORkR
 github.com/google/martian v2.1.0+incompatible h1:/CP5g8u/VJHijgedC/Legn3BAbAaWPgecwXBIDzw5no=
 github.com/google/martian v2.1.0+incompatible/go.mod h1:9I4somxYTbIHy5NJKHRl3wXiIaQGbYVAs8BPL6v8lEs=
 github.com/google/pprof v0.0.0-20181206194817-3ea8567a2e57/go.mod h1:zfwlbNMJ+OItoe0UupaVj+oy1omPYYDuagoSzA8v9mc=
+github.com/google/pprof v0.0.0-20190515194954-54271f7e092f h1:Jnx61latede7zDD3DiiP4gmNz33uK0U5HDUaF0a/HVQ=
 github.com/google/pprof v0.0.0-20190515194954-54271f7e092f/go.mod h1:zfwlbNMJ+OItoe0UupaVj+oy1omPYYDuagoSzA8v9mc=
 github.com/google/renameio v0.1.0/go.mod h1:KWCgfxg9yswjAJkECMjeO8J8rahYeXnNhOm40UhjYkI=
 github.com/googleapis/gax-go/v2 v2.0.4/go.mod h1:0Wqv26UfaUD9n4G6kQubkQ+KchISgw+vpHVxEJEs9eg=

internal/frontend/details.go

@@ -352,9 +352,16 @@ func parseDetailsURLPath(urlPath string) (pkgPath, modulePath, version string, e
 	return pkgPath, modulePath, version, nil
 }
 
+// handleLatestVersion writes a JSON string with the latest version of the package or module.
+// It expects URLs of the form
+//   /latest-version/MODULE_PATH
+// for modules, or
+//   /latest-version/MODULE_PATH?pkg=PACKAGE_PATH
+// for packages.
 func (s *Server) handleLatestVersion(w http.ResponseWriter, r *http.Request) {
 	modulePath := strings.TrimPrefix(r.URL.Path, "/latest-version/")
-	v, err := s.latestVersion(r.Context(), modulePath)
+	packagePath := r.URL.Query().Get("pkg")
+	v, err := s.latestVersion(r.Context(), modulePath, packagePath)
 	if err != nil {
 		log.Errorf("handleLatestVersion(%q): %v", modulePath, err)
 		http.Error(w, `""`, http.StatusInternalServerError) // send valid json
@@ -364,12 +371,23 @@ func (s *Server) handleLatestVersion(w http.ResponseWriter, r *http.Request) {
 		log.Errorf("handleLatestVersion: fmt.Fprintf: %v", err)
 	}
 }
-func (s *Server) latestVersion(ctx context.Context, modulePath string) (string, error) {
-	latestMod, err := s.ds.GetVersionInfo(ctx, modulePath, internal.LatestVersion)
-	if err != nil {
-		return "", err
+func (s *Server) latestVersion(ctx context.Context, modulePath, packagePath string) (_ string, err error) {
+	defer derrors.Wrap(&err, "latestVersion(ctx, %q, %q)", modulePath, packagePath)
+
+	var vi *internal.VersionInfo
+	if packagePath == "" {
+		vi, err = s.ds.GetVersionInfo(ctx, modulePath, internal.LatestVersion)
+		if err != nil {
+			return "", err
+		}
+	} else {
+		pkg, err := s.ds.GetPackage(ctx, packagePath, modulePath, internal.LatestVersion)
+		if err != nil {
+			return "", err
+		}
+		vi = &pkg.VersionInfo
 	}
-	v := latestMod.Version
+	v := vi.Version
 	if modulePath == stdlib.ModulePath {
 		v, err = stdlib.TagForVersion(v)
 		if err != nil {

internal/frontend/server_test.go

@@ -464,6 +464,24 @@ func TestServer(t *testing.T) {
 			http.StatusOK,
 			stdHeader,
 		},
+		{
+			"latest version for the standard library",
+			"/latest-version/std",
+			http.StatusOK,
+			[]string{`"go1.13"`},
+		},
+		{
+			"latest version for module",
+			"/latest-version/" + sample.ModulePath,
+			http.StatusOK,
+			[]string{`"v1.0.0"`},
+		},
+		{
+			"latest version for package",
+			fmt.Sprintf("/latest-version/%s?pkg=%s", sample.ModulePath, pkg2.Path),
+			http.StatusOK,
+			[]string{`"v1.0.0"`},
+		},
 	} {
 		t.Run(tc.name, func(t *testing.T) { // remove initial '/' for name
 			w := httptest.NewRecorder()