Fix hot reload on latest JDKs to work without any special options.

ktor-server/ktor-server-host-common/jvm/src/io/ktor/server/engine/ClassLoaders.kt

@@ -23,7 +23,8 @@ internal fun ClassLoader.allURLs(): Set<URL> {
 /**
  * This only works in JDK9+ with VM option `--add-opens java.base/jdk.internal.loader=ALL-UNNAMED`
  * This is required since [allURLs] function is unable to lookup url list due to modules and class loaders
- * reorganisation in JDK9+.
+ * reorganisation in JDK9+. However, if failed, it fallbacks to [urlClassPathByPackagesList] implementation
+ * that should always work.
  */
 private fun ClassLoader.urlClassPath(): List<URL>? {
     try {
@@ -39,12 +40,41 @@ private fun ClassLoader.urlClassPath(): List<URL>? {
         val urls = getURLsMethod.invoke(ucpInstance) as Array<URL>?
 
         return urls?.toList()
-    } catch (cause: Throwable) {
-        return null
+    } catch (_: Throwable) {
+        return try {
+            urlClassPathByPackagesList()
+        } catch (_: Throwable) {
+            null
+        }
     }
 }
 
+/**
+ * Extract classloader's packages list and guess URLs by package segments.
+ * Unlike the old way, this doesn't require any black magic so works well on all JDKs
+ * from JDK6 to the latest.
+ */
+private fun ClassLoader.urlClassPathByPackagesList(): List<URL>? {
+    val allPackagePaths = ClassLoaderDelegate(this).packagesList().map { it.replace('.', '/') }
+        .flatMapTo(HashSet<String>()) { packageName ->
+            val segments = packageName.split('/')
+            (1 .. segments.size).
+                map { segments.subList(0, it).joinToString("/") } + packageName
+        }.sortedBy { it.count { character -> character == '/' } } + ""
+
+    return allPackagePaths.flatMap { path -> getResources(path)?.toList() ?: emptyList() }
+        .distinctBy { it.path.substringBefore('!') }
+}
+
 private fun Class<*>.findURLClassPathField(): Field? {
     declaredFields.firstOrNull { it.name == "ucp" && it.type.simpleName == "URLClassPath" }?.let { return it }
     return superclass?.findURLClassPathField() ?: return null
 }
+
+/**
+ * This is auxillary classloader that is not used for loading classes. The purpose is just
+ * to get access to [getPackages] function that is unfortunately protected.
+ */
+private class ClassLoaderDelegate(delegate: ClassLoader) : ClassLoader(delegate) {
+    internal fun packagesList(): List<String> = getPackages().map { it.name }
+}