ServeFromSubPath: Redirect to URL with subpath when subpath missing (#66724)

* ServeFromSubPath: Redirect to URL with subpath when subpath missing

* Review fixes

* Added tests

* Use constant

* change to useMiddleware

* Update pkg/middleware/subpath_redirect.go

---------

Co-authored-by: Carl Bergquist <carl.bergquist@gmail.com>

pkg/middleware/subpath_redirect.go

@@ -0,0 +1,26 @@
+package middleware
+
+import (
+	"fmt"
+	"net/http"
+	"strings"
+
+	"github.com/grafana/grafana/pkg/setting"
+	"github.com/grafana/grafana/pkg/web"
+)
+
+// SubPathRedirect Redirects URLs that are missing the configured subpath to an URL that contains the subpath.
+func SubPathRedirect(cfg *setting.Cfg) web.Middleware {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
+			// Direct to url with subpath if the request is missing the subpath and is not an API request.
+			if !strings.HasPrefix(req.RequestURI, cfg.AppSubURL) && !strings.HasPrefix(req.RequestURI, "/api") {
+				newURL := fmt.Sprintf("%s%s", cfg.AppURL, strings.TrimPrefix(req.RequestURI, "/"))
+				http.Redirect(rw, req, newURL, http.StatusMovedPermanently)
+				return
+			}
+
+			next.ServeHTTP(rw, req)
+		})
+	}
+}