Root cause

ConcurrentMapNative.entries returns the raw LinkedHashMap delegate's entries under a lock, but iteration happens outside the lock. When an entry is modified/removed during iteration (e.g., CIOEngine.close() iterating endpoints while endpoint.close() triggers onDone callback that removes entries), a ConcurrentModificationException is thrown.

File: ktor-utils/posix/src/io/ktor/util/collections/ConcurrentMapNative.kt

actual override val entries: MutableSet<MutableMap.MutableEntry<Key, Value>>
    get() = synchronized(lock) { delegate.entries }  // returns raw view, not a copy

The same issue exists for keys and values properties.

Observed in CI

• io.ktor.client.tests.ContentTest.testEmptyContent[mingwX64] — ConcurrentModificationException at HashMap.EntriesItr#checkForComodification → CIOEngine#close
• Intermittent across macOS/Windows native targets

Suggested fix

Return a snapshot copy instead of the raw delegate view:

actual override val entries: MutableSet<MutableMap.MutableEntry<Key, Value>>
    get() = synchronized(lock) { LinkedHashSet(delegate.entries) }

Or better: make CIOEngine.close() collect endpoint values before iterating.

Context

Discovered while investigating CI failures on PR #5379 (CaseInsensitiveMap). The failures are unrelated to that PR's changes.