Hi, guys. In our project, we use KMP with JVM and ...

# stdlib

[Arthur Krukowski]

Hi, guys. In our project, we use KMP with JVM and JS targets. After testing a migration to Kotlin 2.0, we encountered crashes when running the JS target. Our research led us to LinkedHashMap (MutableMap has the same behavior). It appears that after calling the Iterator#remove method, the entry's key and value fields can be set to null. Here’s a simple test case to reproduce:

@Test
    fun test1() {
        val map = LinkedHashMap<String, Int>()
        val n = 3
        repeat(n) {
            map["$it"] = it
        }

        println(map)
        val iterator = map.iterator()
        var count = 0
        while (iterator.hasNext()) {
            val entry = iterator.next()
            iterator.remove()
            println("${entry.key}=${entry.value}")
//            assertEquals(count++, entry.value)
        }
    }

We got next results:

JVM (on all versions):

{0=0, 1=1, 2=2}
0=0
1=1
2=2

JS (Kotlin 1.9.24). Keys are null:

{0=0, 1=1, 2=2}
null=0
null=1
null=2

JS (Kotlin 2.0.21). Keys and values are null:

{0=0, 1=1, 2=2}
null=null
null=null
null=null

JS (Kotlin 2.1.0-RC). Throws exception when accessing entry after the remove method is called.

ConcurrentModificationException: The backing map has been modified after this entry was obtained.

In version 2.1.0-RC , at least we encounter a crash on JS that could serve as a hint for developers. Do we mention about this somewhere in documentation? And should the behavior be consistent among platforms? I mean, it looks like we crash only on JS, but not on JVM. Is it expected behavior for now?

[Abduqodiri Qurbonzoda [JB]]

Hi!
Here is the issue for changing the behavior: https://youtrack.jetbrains.com/issue/KTLC-23/Inconsistent-behaviour-of-MutableMap.Entry-between-implementations

[Abduqodiri Qurbonzoda [JB]]

The new behavior is indeed as expected. We acknowledge that it differs from the JVM, but you should not rely on any of the observed behaviors. The documentation states:

Map.Entry objects are valid only for the duration of the iteration; more formally, the behavior of a map entry is undefined if the backing map has been modified after the entry was returned by the iterator, except through the setValue operation on the map entry.

[Abduqodiri Qurbonzoda [JB]]

Do we mention about this somewhere in documentation?

As mentioned there are a few related public YouTrack issues where we have written about the change.
Also, we will write about this behavior change in the compatibility guide document for the Kotlin 2.1.0

[Arthur Krukowski]

Hi! Thank you for provided information.