[Bug][kotlin2cpg] flow suddenly disappears if the sink is in the sources set

[xavierpinho]

We've noticed that having a sink in the sources set changes the outcome of reachableBy in unexpected ways. Below are three test cases showing this behaviour.

Joern version: 4.0.131

Test case 1 ✅

Source is literals, and works as expected.

val cpg = code("""
        |const val SRC = "src"
        |fun doStuff() {
        | println(SRC)
        |}
        |""".stripMargin)
val literals = cpg.literal.l
val sink     = cpg.call("println").argument(1).l
val source   = literals
sink.reachableByFlows(source).map(flowToResultPairs).l shouldBe List(
  List(("const val SRC = \"src\"", Some(2)), ("println(SRC)", Some(4)))
)

Test case 2 ✅

Source is sink, and works as expected.

val cpg = code("""
        |const val SRC = "src"
        |fun doStuff() {
        | println(SRC)
        |}
        |""".stripMargin)
val literals = cpg.literal.l
val sink     = cpg.call("println").argument(1).l
val source   = sink
sink.reachableByFlows(source).map(flowToResultPairs).l shouldBe List(
  List(("println(SRC)", Some(4)))
)

Test case 3 ❌

Source is literals plus sinks, and misses the result from Test case 1. That is, this test case's outcome is the same as that of Test case 2.

val cpg = code("""
        |const val SRC = "src"
        |fun doStuff() {
        | println(SRC)
        |}
        |""".stripMargin)
val literals = cpg.literal.l
val sink     = cpg.call("println").argument(1).l
val source   = literals ++ sink
sink.reachableByFlows(source).map(flowToResultPairs).l shouldBe List(
  List(("const val SRC = \"src\"", Some(2)), ("println(SRC)", Some(4))),
  List(("println(SRC)", Some(4)))
)

The expected behaviour would be for Test case 3 to output 2 results, namely those from Test case 1 and 2.

[DavidBakerEffendi]

@xavierpinho, from what I understand, there is a mechanism in the data-flow engine where the shortest path is returned if found, but also a path length ≤ 1 is filtered out - these might be running in an order where flows disappear.