1.4.0-M1

Breaking changes

• The concept of atomic cancellation in channels is removed. All operations in channels
  and corresponding Flow operators are cancellable in non-atomic way (#1813).
• If CoroutineDispatcher throws RejectedExecutionException, cancel current Job and schedule its execution to Dispatchers.IO (#2003).
• CancellableContinuation.invokeOnCancellation is invoked if the continuation was cancelled while its resume has been dispatched (#1915).
• Flow.singleOrNull operator is aligned with standard library and does not longer throw IllegalStateException on multiple values (#2289).

New experimental features

• SharedFlow primitive for managing hot sources of events with support of various subscription mechanisms, replay logs and buffering (#2034).
• Flow.shareIn and Flow.stateIn operators to transform cold instances of flow to hot SharedFlow and StateFlow respectively (#2047).

Other

• Support leak-free closeable resources transfer via onUndeliveredElement in channels (#1936).
• Changed ABI in reactive integrations for Java interoperability (#2182).
• Fixed ProGuard rules for kotlinx-coroutines-core (#2046, #2266).
• Lint settings were added to Flow to avoid accidental capturing of outer CoroutineScope for cancellation check (#2038).

External contributions

• Allow nullable types in Flow.firstOrNull and Flow.singleOrNull by @ansman (#2229).
• Add Publisher.awaitSingleOrDefault|Null|Else extensions by @sdeleuze (#1993).
• awaitCancellation top-level function by @LouisCAD (#2213).
• Significant part of our Gradle build scripts were migrated to .kts by @turansky.

Thank you for your contributions and participation in the Kotlin community!