KTOR-2427 fix trailing slash toString (#2419)

ktor-server/ktor-server-core/jvm/src/io/ktor/routing/Route.kt

@@ -110,14 +110,16 @@ public open class Route(
         }
     }
 
-    override fun toString(): String = when {
-        parent == null -> "/$selector"
-        parent.parent == null -> parent.toString().let { parentText ->
-            when {
-                parentText.endsWith('/') -> "$parentText$selector"
-                else -> "$parentText/$selector"
+    override fun toString(): String {
+        return when (val parentRoute = parent?.toString()) {
+            null -> when (selector) {
+                is TrailingSlashRouteSelector -> "/"
+                else -> "/$selector"
+            }
+            else -> when (selector) {
+                is TrailingSlashRouteSelector -> if (parentRoute.endsWith('/')) parentRoute else "$parentRoute/"
+                else -> if (parentRoute.endsWith('/')) "$parentRoute$selector" else "$parentRoute/$selector"
             }
         }
-        else -> "$parent/$selector"
     }
 }

ktor-server/ktor-server-core/jvm/test/io/ktor/tests/routing/RouteTest.kt

@@ -0,0 +1,32 @@
+/*
+ * Copyright 2014-2021 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package io.ktor.tests.routing
+
+import io.ktor.routing.*
+import kotlin.test.*
+
+class RouteTest {
+
+    @Test
+    fun testToStringSimple() {
+        val root = Route(parent = null, selector = PathSegmentConstantRouteSelector("root"))
+        val simpleChild = Route(parent = root, selector = PathSegmentConstantRouteSelector("simpleChild"))
+        val simpleGrandChild =
+            Route(parent = simpleChild, selector = PathSegmentConstantRouteSelector("simpleGrandChild"))
+
+        val slashChild = Route(parent = root, selector = TrailingSlashRouteSelector)
+        val slashGrandChild = Route(parent = slashChild, selector = TrailingSlashRouteSelector)
+        val simpleChildInSlash = Route(parent = slashGrandChild, PathSegmentConstantRouteSelector("simpleChildInSlash"))
+        val slashChildInSimpleChild = Route(parent = simpleChildInSlash, TrailingSlashRouteSelector)
+
+        assertEquals("/root", root.toString())
+        assertEquals("/root/simpleChild", simpleChild.toString())
+        assertEquals("/root/simpleChild/simpleGrandChild", simpleGrandChild.toString())
+        assertEquals("/root/", slashChild.toString())
+        assertEquals("/root/", slashGrandChild.toString())
+        assertEquals("/root/simpleChildInSlash", simpleChildInSlash.toString())
+        assertEquals("/root/simpleChildInSlash/", slashChildInSimpleChild.toString())
+    }
+}