Correctly handle randomizing of Seed.Recursive modifications

utbot-framework/src/main/kotlin/org/utbot/engine/UtBotSymbolicEngine.kt

@@ -357,6 +357,12 @@ class UtBotSymbolicEngine(
             names,
             listOf(transform(ValueProvider.of(defaultValueProviders(defaultIdGenerator))))
         ) { thisInstance, descr, values ->
+            if (thisInstance?.model is UtNullModel) {
+                // We should not try to run concretely any models with null-this.
+                // But fuzzer does generate such values, because it can fail to generate any "good" values.
+                return@runJavaFuzzing BaseFeedback(Trie.emptyNode(), Control.PASS)
+            }
+
             val diff = until - System.currentTimeMillis()
             val thresholdMillisForFuzzingOperation = 0 // may be better use 10-20 millis as it might not be possible
             // to concretely execute that values because request to instrumentation process involves

utbot-fuzzing/src/main/kotlin/org/utbot/fuzzing/Api.kt

@@ -248,7 +248,7 @@ private object EmptyFeedback : Feedback<Nothing, Nothing> {
     }
 }
 
-private class NoSeedValueException(
+class NoSeedValueException internal constructor(
     // this type cannot be generalized because Java forbids types for [Throwable].
     val type: Any?
 ) : Exception("No seed candidates generated for type: $type")
@@ -463,7 +463,11 @@ private fun <TYPE, RESULT, DESCRIPTION : Description<TYPE>, FEEDBACK : Feedback<
                 State(state.recursionTreeDepth + 1, state.cache, state.missedTypes)
             ),
             modify = task.modify
-                .shuffled(random)
+                .toMutableList()
+                .transformIfNotEmpty {
+                    shuffle(random)
+                    take(random.nextInt(size + 1))
+                }
                 .mapTo(arrayListOf()) { routine ->
                     fuzz(
                         routine.types,
@@ -474,15 +478,13 @@ private fun <TYPE, RESULT, DESCRIPTION : Description<TYPE>, FEEDBACK : Feedback<
                         routine,
                         State(state.recursionTreeDepth + 1, state.cache, state.missedTypes)
                     )
-                }.transformIfNotEmpty {
-                    take(random.nextInt(size + 1).coerceAtLeast(1))
                 }
         )
     } catch (nsv: NoSeedValueException) {
         @Suppress("UNCHECKED_CAST")
         state.missedTypes[nsv.type as TYPE] = task
         if (configuration.generateEmptyRecursiveForMissedTypes) {
-            Result.Empty { task.empty() }
+            Result.Empty { task.empty.builder() }
         } else {
             throw nsv
         }

utbot-fuzzing/src/main/kotlin/org/utbot/fuzzing/utils/Functions.kt

@@ -81,7 +81,7 @@ internal enum class BinaryFormat : (Int) -> Boolean {
     DOUBLE { override fun invoke(index: Int) = index % 16 == 0 && index != 0 },
 }
 
-internal fun <T> List<T>.transformIfNotEmpty(transform: List<T>.() -> List<T>): List<T> {
+internal fun <T> MutableList<T>.transformIfNotEmpty(transform: MutableList<T>.() -> List<T>): List<T> {
     return if (isNotEmpty()) transform() else this
 }
 

utbot-fuzzing/src/test/kotlin/org/utbot/fuzzing/FuzzerSmokeTest.kt

@@ -27,12 +27,15 @@ class FuzzerSmokeTest {
 
     @Test
     fun `fuzzing throws an exception if no values generated for some type`() {
-        assertThrows<IllegalStateException> {
+        assertThrows<NoSeedValueException> {
             runBlocking {
                 var count = 0
                 runFuzzing<Unit, Unit, Description<Unit>, BaseFeedback<Unit, Unit, Unit>>(
-                    { _, _ -> sequenceOf() },
-                    Description(listOf(Unit))
+                    provider = { _, _ -> sequenceOf() },
+                    description = Description(listOf(Unit)),
+                    configuration = Configuration(
+                        generateEmptyCollectionsForMissedTypes = false
+                    )
                 ) { _, _ ->
                     count += 1
                     BaseFeedback(Unit, Control.STOP)