• Fixed StackOverflowError in Job.toString when Job is observed in its intermediate state (#2371).
• Improved liveness and latency of Dispatchers.Default and Dispatchers.IO in low-loaded mode (#2381).
• Improved performance of consecutive Channel.cancel invocations (#2384).
• SharingStarted is now fun interface (#2397).
• Additional lint settings for SharedFlow to catch programmatic errors early (#2376).
• Fixed bug when mutex and semaphore were not released during cancellation (#2390, thanks to @Tilps for reproducing).
• Some corner cases in cancellation propagation between coroutines and listenable futures are repaired (#1442, thanks to @vadimsemenov).
• Fixed unconditional cast to CoroutineStackFrame in exception recovery that triggered failures of instrumented code (#2386).
• Platform-specific dependencies are removed from kotlinx-coroutines-javafx (#2360).

This is a patch release with an important fix to the SharedFlow implementation.

• SharedFlow: Fix scenario with concurrent emitters and cancellation of a subscriber (#2359, thanks to @vehovsky for the bug report).

Improvements

• StateFlow, SharedFlow and corresponding operators are promoted to stable API (#2316).
• Flow.debounce operator with timeout selector based on each individual element is added (#1216, thanks to @mkano9!).
• CoroutineContext.job extension property is introduced (#2159).
• Flow.combine operator is reworked:
  • Complete fairness is maintained for single-threaded dispatchers.
  • Its performance is improved, depending on the use-case, by at least 50% (#2296).
  • Quadratic complexity depending on the number of upstream flows is eliminated (#2296).
  • crossinline and inline-heavy internals are removed, fixing sporadic SIGSEGV on Mediatek Android devices (#1683, #1743).
• Flow.zip operator performance is improved by 40%.
• Various API has been promoted to stable or its deprecation level has been raised (#2316).

Bug fixes

• Suspendable stateIn operator propagates exception to the caller when upstream fails to produce initial value (#2329).
• Fix SharedFlow with replay for subscribers working at different speed (#2325).
• Do not fail debug agent installation when security manager does not provide access to system properties (#2311).
• Cancelled lazy coroutines are properly cleaned up from debug agent output (#2294).
• BlockHound false-positives are correctly filtered out (#2302, #2190, #2303).
• Potential crash during a race between cancellation and upstream in Observable.asFlow is fixed (#2104, #2299, thanks to @LouisCAD and @drinkthestars).

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!

• Support of CoroutineContext in Flow.asPublisher and similar reactive builders (#2155).
• Kotlin updated to 1.4.0.
• Transition to new HMPP publication scheme for multiplatform usages:
  • Artifacts kotlinx-coroutines-core-common and kotlinx-coroutines-core-native are removed.
  • For multiplatform usages, it's enough to depend directly on kotlinx-coroutines-core in commonMain source-set.
  • The same artifact coordinates can be used to depend on a platform-specific artifact in platform-specific source-set.

New experimental features

• Added Flow.transformWhile operator (#2065).
• Replaced scanReduce with runningReduce to be consistent with the Kotlin standard library (#2139).

Bug fixes and improvements

• Improve user experience for the upcoming coroutines debugger (#2093, #2118, #2131).
• Debugger no longer retains strong references to the running coroutines (#2129).
• Fixed race in Flow.asPublisher (#2109).
• Fixed ensureActive to work in the empty context case to fix IllegalStateException when using flow from suspend fun main (#2044).
• Fixed a problem with AbortFlowException in the Flow.first operator to avoid erroneous NoSuchElementException (#2051).
• Fixed JVM dependency on Android annotations (#2075).
• Removed keep rules mentioning kotlinx.coroutines.android from core module (#2061 by @mkj-gram).
• Corrected some docs and examples (#2062, #2071, #2076, #2107, #2098, #2127, #2078, #2135).
• Improved the docs and guide on flow cancellation (#2043).
• Updated Gradle version to 6.3 (it only affects multiplatform artifacts in this release).

• Fixed problem that triggered Android Lint failure (#2004).
• New Flow.cancellable() operator for cooperative cancellation (#2026).
• Emissions from flow builder now check cancellation status and are properly cancellable (#2026).
• New currentCoroutineContext function to use unambiguously in the contexts with CoroutineScope in receiver position (#2026).
• EXACTLY_ONCE contract support in coroutine builders.
• Various documentation improvements.

Flow

• StateFlow, new primitive for state handling (#1973, #1816, #395). The StateFlow is designed to eventually replace ConflatedBroadcastChannel for state publication scenarios. Please, try it and share your feedback. Note, that Flow-based primitives to publish events will be added later. For events you should continue to either use BroadcastChannel(1), if you put events into the StateFlow, protect them from double-processing with flags.
• Flow.onEmpty operator is introduced (#1890).
• Behavioural change in Flow.onCompletion, it is aligned with invokeOnCompletion now and passes CancellationException to its cause parameter (#1693).
• A lot of Flow operators have left its experimental status and are promoted to stable API.

Other

• runInterruptible primitive to tie cancellation with thread interruption for blocking calls. Contributed by @jxdabc (#1947).
• Integration module with RxJava3 is introduced. Contributed by @ZacSweers (#1883)
• Integration with BlockHound in kotlinx-coroutines-debug module (#1821, #1060).
• Memory leak in ArrayBroadcastChannel is fixed (#1885).
• Behavioural change in suspendCancellableCoroutine, cancellation is established before invoking passed block argument (#1671).
• Debug agent internals are moved into kotlinx-coroutines-core for better integration with IDEA. It should not affect library users and all the redundant code should be properly eliminated with R8.
• ClassCastException with reusable continuations bug is fixed (#1966).
• More precise scheduler detection for Executor.asCoroutineDispatcher (#1992).
• Kotlin updated to 1.3.71.

Version 1.3.5

• firstOrNull operators. Contributed by @bradynpoulsen
• java.time adapters for Flow operators. Contributed by @fvasco
• kotlin.time.Duration support (#1402). Contributed by @fvasco
• Memory leak with a mix of reusable and non-reusable continuations is fixed (#1855)
• DebugProbes are ready for production installation: its performance is increased, the flag to disable creation stacktraces to reduce the footprint is introduced (#1379, #1372)
• Stacktrace recovery workaround for Android 6.0 and earlier bug (#1866).
• New integration module: kotlinx-coroutines-jdk9 with adapters for java.util.concurrent.Flow
• BroadcastChannel.close properly starts lazy coroutine (#1713).
• kotlinx-coroutines-bom is published without Gradle metadata.
• Make calls to service loader in reactor integrations optimizable by R8 (#1817)

Flow

• Detect missing awaitClose calls in callbackFlow to make it less error-prone when used with callbacks (#1762, #1770). This change makes callbackFlow different from channelFlow
• ReceiveChannel.asFlow extension is introduced (#1490)
• Enforce exception transparency invariant in flow builder (#1657)
• Proper Dispatcher support in Flow reactive integrations (#1765)
• Batch Subscription.request calls in Flow reactive integration (#766)
• ObservableValue.asFlow added to JavaFx integration module (#1695)
• ObservableSource.asFlow added to RxJava2 integration module (#1768)

Other changes

• kotlinx-coroutines-core is optimized for R8, making it much smaller for Android usages (75 KB for 1.3.4 release)
• Performance of Dispatchers.Default is improved (#1704, #1706)
• Kotlin is updated to 1.3.70
• CoroutineDispatcher and ExecutorCoroutineDispatcher experimental coroutine context keys are introduced (#1805)
• Performance of various Channel operations is improved (#1565)