Deprecate SendChannel.offer and replace its usages along the code base

kotlinx-coroutines-core/common/src/channels/AbstractChannel.kt

@@ -137,6 +137,7 @@ internal abstract class AbstractSendChannel<E>(
     }
 
     override fun offer(element: E): Boolean {
+        // Temporary migration for offer users who rely on onUndeliveredElement
         try {
             return super.offer(element)
         } catch (e: Throwable) {

kotlinx-coroutines-core/common/src/channels/Channel.kt

@@ -24,7 +24,7 @@ import kotlin.jvm.*
 public interface SendChannel<in E> {
     /**
      * Returns `true` if this channel was closed by an invocation of [close]. This means that
-     * calling [send] or [offer] will result in an exception.
+     * calling [send] will result in an exception.
      *
      * **Note: This is an experimental api.** This property may change its semantics and/or name in the future.
      */
@@ -51,7 +51,7 @@ public interface SendChannel<in E> {
      * Use [yield] or [CoroutineScope.isActive] to periodically check for cancellation in tight loops if needed.
      *
      * This function can be used in [select] invocations with the [onSend] clause.
-     * Use [offer] to try sending to this channel without waiting.
+     * Use [trySend] to try sending to this channel without waiting.
      */
     public suspend fun send(element: E)
 
@@ -64,23 +64,6 @@ public interface SendChannel<in E> {
      */
     public val onSend: SelectClause2<E, SendChannel<E>>
 
-    /**
-     * Immediately adds the specified [element] to this channel, if this doesn't violate its capacity restrictions,
-     * and returns `true`. Otherwise, just returns `false`. This is a synchronous variant of [send] which backs off
-     * in situations when `send` suspends.
-     *
-     * Throws an exception if the channel [is closed for `send`][isClosedForSend] (see [close] for details).
-     *
-     * When `offer` call returns `false` it guarantees that the element was not delivered to the consumer and it
-     * it does not call `onUndeliveredElement` that was installed for this channel. If the channel was closed,
-     * then it calls `onUndeliveredElement` before throwing an exception.
-     * See "Undelivered elements" section in [Channel] documentation for details on handling undelivered elements.
-     */
-    public fun offer(element: E): Boolean {
-        val result = trySend(element)
-        if (result.isSuccess) return true
-        throw recoverStackTrace(result.exceptionOrNull() ?: return false)
-    }
 
     /**
      * Immediately adds the specified [element] to this channel, if this doesn't violate its capacity restrictions,
@@ -103,7 +86,7 @@ public interface SendChannel<in E> {
      * on the side of [ReceiveChannel] starts returning `true` only after all previously sent elements
      * are received.
      *
-     * A channel that was closed without a [cause] throws a [ClosedSendChannelException] on attempts to [send] or [offer]
+     * A channel that was closed without a [cause] throws a [ClosedSendChannelException] on attempts to [send]
      * and [ClosedReceiveChannelException] on attempts to [receive][ReceiveChannel.receive].
      * A channel that was closed with non-null [cause] is called a _failed_ channel. Attempts to send or
      * receive on a failed channel throw the specified [cause] exception.
@@ -125,7 +108,7 @@ public interface SendChannel<in E> {
      * ```
      * val events = Channel(UNLIMITED)
      * callbackBasedApi.registerCallback { event ->
-     *   events.offer(event)
+     *   events.trySend(event)
      * }
      *
      * val uiUpdater = launch(Dispatchers.Main, parent = UILifecycle) {
@@ -146,6 +129,33 @@ public interface SendChannel<in E> {
      */
     @ExperimentalCoroutinesApi
     public fun invokeOnClose(handler: (cause: Throwable?) -> Unit)
+
+    /**
+     * **Deprecated** offer method.
+     *
+     * This method was deprecated in the favour of [trySend].
+     * It has proven itself as the most error-prone method in Channel API:
+     *
+     * * `Boolean` return type creates the false sense of security, implying that `false`
+     *    is returned instead of throwing an exception.
+     * * It was used mostly from non-suspending APIs where CancellationException triggered
+     *   internal failures in the application (the most common source of bugs).
+     * * Due to signature and explicit `if (ch.offer(...))` checks it was easy to
+     *   oversee such error during code review.
+     * * Its name was not aligned with the rest of the API and tried to mimic Java's queue instead.
+     *
+     * See https://github.com/Kotlin/kotlinx.coroutines/issues/974 for more context.
+     */
+    @Deprecated(
+        level = DeprecationLevel.WARNING,
+        message = "Deprecated in the favour of 'trySend' method",
+        replaceWith = ReplaceWith("trySend(element).isSuccess")
+    )
+    public fun offer(element: E): Boolean {
+        val result = trySend(element)
+        if (result.isSuccess) return true
+        throw recoverStackTrace(result.exceptionOrNull() ?: return false)
+    }
 }
 
 /**
@@ -544,14 +554,14 @@ public interface ChannelIterator<out E> {
  *
  * * When `capacity` is [Channel.UNLIMITED] &mdash; it creates a channel with effectively unlimited buffer.
  *   This channel has a linked-list buffer of unlimited capacity (limited only by available memory).
- *   [Sending][send] to this channel never suspends, and [offer] always returns `true`.
+ *   [Sending][send] to this channel never suspends, and [trySend] always succeeds.
  *
  * * When `capacity` is [Channel.CONFLATED] &mdash; it creates a _conflated_ channel
- *   This channel buffers at most one element and conflates all subsequent `send` and `offer` invocations,
+ *   This channel buffers at most one element and conflates all subsequent `send` and `trySend` invocations,
  *   so that the receiver always gets the last element sent.
  *   Back-to-back sent elements are conflated &mdash; only the last sent element is received,
  *   while previously sent elements **are lost**.
- *   [Sending][send] to this channel never suspends, and [offer] always returns `true`.
+ *   [Sending][send] to this channel never suspends, and [trySend] always succeeds.
  *
  * * When `capacity` is positive but less than [UNLIMITED] &mdash; it creates an array-based channel with the specified capacity.
  *   This channel has an array buffer of a fixed `capacity`.
@@ -598,8 +608,6 @@ public interface ChannelIterator<out E> {
  *
  * * When [send][SendChannel.send] operation throws an exception because it was cancelled before it had a chance to actually
  *   send the element or because the channel was [closed][SendChannel.close] or [cancelled][ReceiveChannel.cancel].
- * * When [offer][SendChannel.offer] operation throws an exception when
- *   the channel was [closed][SendChannel.close] or [cancelled][ReceiveChannel.cancel].
  * * When [receive][ReceiveChannel.receive], [receiveOrNull][ReceiveChannel.receiveOrNull], or [hasNext][ChannelIterator.hasNext]
  *   operation throws an exception when it had retrieved the element from the
  *   channel but was cancelled before the code following the receive call resumed.

kotlinx-coroutines-core/common/src/channels/ConflatedBroadcastChannel.kt

@@ -17,7 +17,7 @@ import kotlin.jvm.*
  * Back-to-send sent elements are _conflated_ -- only the the most recently sent value is received,
  * while previously sent elements **are lost**.
  * Every subscriber immediately receives the most recently sent element.
- * Sender to this broadcast channel never suspends and [offer] always returns `true`.
+ * Sender to this broadcast channel never suspends and [trySend] always succeeds.
  *
  * A secondary constructor can be used to create an instance of this class that already holds a value.
  * This channel is also created by `BroadcastChannel(Channel.CONFLATED)` factory function invocation.

kotlinx-coroutines-core/common/src/channels/ConflatedChannel.kt

@@ -9,11 +9,11 @@ import kotlinx.coroutines.internal.*
 import kotlinx.coroutines.selects.*
 
 /**
- * Channel that buffers at most one element and conflates all subsequent `send` and `offer` invocations,
+ * Channel that buffers at most one element and conflates all subsequent `send` and `trySend` invocations,
  * so that the receiver always gets the most recently sent element.
  * Back-to-send sent elements are _conflated_ -- only the most recently sent element is received,
  * while previously sent elements **are lost**.
- * Sender to this channel never suspends and [offer] always returns `true`.
+ * Sender to this channel never suspends and [trySend] always succeeds.
  *
  * This channel is created by `Channel(Channel.CONFLATED)` factory function invocation.
  */

kotlinx-coroutines-core/common/src/channels/LinkedListChannel.kt

@@ -9,7 +9,7 @@ import kotlinx.coroutines.selects.*
 
 /**
  * Channel with linked-list buffer of a unlimited capacity (limited only by available memory).
- * Sender to this channel never suspends and [offer] always returns `true`.
+ * Sender to this channel never suspends and [trySend] always succeeds.
  *
  * This channel is created by `Channel(Channel.UNLIMITED)` factory function invocation.
  *

kotlinx-coroutines-core/common/src/flow/SharedFlow.kt

@@ -92,7 +92,7 @@ import kotlin.native.concurrent.*
  *
  * To migrate [BroadcastChannel] usage to [SharedFlow], start by replacing usages of the `BroadcastChannel(capacity)`
  * constructor with `MutableSharedFlow(0, extraBufferCapacity=capacity)` (broadcast channel does not replay
- * values to new subscribers). Replace [send][BroadcastChannel.send] and [offer][BroadcastChannel.offer] calls
+ * values to new subscribers). Replace [send][BroadcastChannel.send] and [trySend][BroadcastChannel.trySend] calls
  * with [emit][MutableStateFlow.emit] and [tryEmit][MutableStateFlow.tryEmit], and convert subscribers' code to flow operators.
  *
  * ### Concurrency

kotlinx-coroutines-core/common/src/flow/StateFlow.kt

@@ -107,7 +107,7 @@ import kotlin.native.concurrent.*
  *
  * To migrate [ConflatedBroadcastChannel] usage to [StateFlow], start by replacing usages of the `ConflatedBroadcastChannel()`
  * constructor with `MutableStateFlow(initialValue)`, using `null` as an initial value if you don't have one.
- * Replace [send][ConflatedBroadcastChannel.send] and [offer][ConflatedBroadcastChannel.offer] calls
+ * Replace [send][ConflatedBroadcastChannel.send] and [trySend][ConflatedBroadcastChannel.trySend] calls
  * with updates to the state flow's [MutableStateFlow.value], and convert subscribers' code to flow operators.
  * You can use the [filterNotNull] operator to mimic behavior of a `ConflatedBroadcastChannel` without initial value.
  *

kotlinx-coroutines-core/common/test/channels/ArrayChannelTest.kt

@@ -86,9 +86,9 @@ class ArrayChannelTest : TestBase() {
     }
 
     @Test
-    fun testOfferAndPoll() = runTest {
+    fun testTryOp() = runTest {
         val q = Channel<Int>(1)
-        assertTrue(q.offer(1))
+        assertTrue(q.trySend(1).isSuccess)
         expect(1)
         launch {
             expect(3)
@@ -106,11 +106,11 @@ class ArrayChannelTest : TestBase() {
         expect(2)
         yield()
         expect(6)
-        assertTrue(q.offer(2))
+        assertTrue(q.trySend(2).isSuccess)
         expect(7)
-        assertTrue(q.offer(3))
+        assertTrue(q.trySend(3).isSuccess)
         expect(8)
-        assertFalse(q.offer(4))
+        assertFalse(q.trySend(4).isSuccess)
         yield()
         finish(12)
     }
@@ -157,7 +157,7 @@ class ArrayChannelTest : TestBase() {
         val capacity = 42
         val channel = Channel<Int>(capacity)
         repeat(4) {
-            channel.offer(-1)
+            channel.trySend(-1)
         }
         repeat(4) {
             channel.receiveCatching().getOrNull()

kotlinx-coroutines-core/common/test/channels/BasicOperationsTest.kt

@@ -15,8 +15,8 @@ class BasicOperationsTest : TestBase() {
     }
 
     @Test
-    fun testOfferAfterClose() = runTest {
-        TestChannelKind.values().forEach { kind -> testOffer(kind) }
+    fun testTrySendAfterClose() = runTest {
+        TestChannelKind.values().forEach { kind -> testTrySend(kind) }
     }
 
     @Test
@@ -39,7 +39,7 @@ class BasicOperationsTest : TestBase() {
             }
         }
         expect(1)
-        channel.offer(42)
+        channel.trySend(42)
         expect(2)
         channel.close(AssertionError())
         finish(4)
@@ -131,22 +131,21 @@ class BasicOperationsTest : TestBase() {
         finish(6)
     }
 
-    private suspend fun testOffer(kind: TestChannelKind) = coroutineScope {
+    private suspend fun testTrySend(kind: TestChannelKind) = coroutineScope {
         val channel = kind.create<Int>()
         val d = async { channel.send(42) }
         yield()
         channel.close()
 
         assertTrue(channel.isClosedForSend)
-        try {
-            channel.offer(2)
-            fail()
-        } catch (e: ClosedSendChannelException) {
-            if (!kind.isConflated) {
-                assertEquals(42, channel.receive())
+        channel.trySend(2)
+            .onSuccess { expectUnreached() }
+            .onFailure {
+                assertTrue { it is  ClosedSendChannelException}
+                if (!kind.isConflated) {
+                    assertEquals(42, channel.receive())
+                }
             }
-        }
-
         d.await()
     }
 

kotlinx-coroutines-core/common/test/channels/ChannelBufferOverflowTest.kt

@@ -11,13 +11,13 @@ class ChannelBufferOverflowTest : TestBase() {
     @Test
     fun testDropLatest() = runTest {
         val c = Channel<Int>(2, BufferOverflow.DROP_LATEST)
-        assertTrue(c.offer(1))
-        assertTrue(c.offer(2))
-        assertTrue(c.offer(3)) // overflows, dropped
+        assertTrue(c.trySend(1).isSuccess)
+        assertTrue(c.trySend(2).isSuccess)
+        assertTrue(c.trySend(3).isSuccess) // overflows, dropped
         c.send(4) // overflows dropped
         assertEquals(1, c.receive())
-        assertTrue(c.offer(5))
-        assertTrue(c.offer(6)) // overflows, dropped
+        assertTrue(c.trySend(5).isSuccess)
+        assertTrue(c.trySend(6).isSuccess) // overflows, dropped
         assertEquals(2, c.receive())
         assertEquals(5, c.receive())
         assertEquals(null, c.poll())
@@ -26,15 +26,15 @@ class ChannelBufferOverflowTest : TestBase() {
     @Test
     fun testDropOldest() = runTest {
         val c = Channel<Int>(2, BufferOverflow.DROP_OLDEST)
-        assertTrue(c.offer(1))
-        assertTrue(c.offer(2))
-        assertTrue(c.offer(3)) // overflows, keeps 2, 3
+        assertTrue(c.trySend(1).isSuccess)
+        assertTrue(c.trySend(2).isSuccess)
+        assertTrue(c.trySend(3).isSuccess) // overflows, keeps 2, 3
         c.send(4) // overflows, keeps 3, 4
         assertEquals(3, c.receive())
-        assertTrue(c.offer(5))
-        assertTrue(c.offer(6)) // overflows, keeps 5, 6
+        assertTrue(c.trySend(5).isSuccess)
+        assertTrue(c.trySend(6).isSuccess) // overflows, keeps 5, 6
         assertEquals(5, c.receive())
         assertEquals(6, c.receive())
         assertEquals(null, c.poll())
     }
-}
+}

kotlinx-coroutines-core/common/test/channels/ConflatedChannelTest.kt

@@ -15,9 +15,9 @@ open class ConflatedChannelTest : TestBase() {
     fun testBasicConflationOfferPoll() {
         val q = createConflatedChannel<Int>()
         assertNull(q.poll())
-        assertTrue(q.offer(1))
-        assertTrue(q.offer(2))
-        assertTrue(q.offer(3))
+        assertTrue(q.trySend(1).isSuccess)
+        assertTrue(q.trySend(2).isSuccess)
+        assertTrue(q.trySend(3).isSuccess)
         assertEquals(3, q.poll())
         assertNull(q.poll())
     }

kotlinx-coroutines-core/common/test/channels/LinkedListChannelTest.kt

@@ -12,7 +12,7 @@ class LinkedListChannelTest : TestBase() {
     fun testBasic() = runTest {
         val c = Channel<Int>(Channel.UNLIMITED)
         c.send(1)
-        check(c.offer(2))
+        assertTrue(c.trySend(2).isSuccess)
         c.send(3)
         check(c.close())
         check(!c.close())

kotlinx-coroutines-core/common/test/channels/RendezvousChannelTest.kt

@@ -82,7 +82,7 @@ class RendezvousChannelTest : TestBase() {
     @Test
     fun testOfferAndPool() = runTest {
         val q = Channel<Int>(Channel.RENDEZVOUS)
-        assertFalse(q.offer(1))
+        assertFalse(q.trySend(1).isSuccess)
         expect(1)
         launch {
             expect(3)
@@ -99,7 +99,7 @@ class RendezvousChannelTest : TestBase() {
         expect(2)
         yield()
         expect(5)
-        assertTrue(q.offer(2))
+        assertTrue(q.trySend(2).isSuccess)
         expect(6)
         yield()
         expect(8)

kotlinx-coroutines-core/common/test/flow/channels/ChannelBuildersFlowTest.kt

@@ -166,15 +166,15 @@ class ChannelBuildersFlowTest : TestBase() {
 
         var expected = 0
         launch {
-            assertTrue(channel.offer(1)) // Handed to the coroutine
-            assertTrue(channel.offer(2)) // Buffered
-            assertFalse(channel.offer(3)) // Failed to offer
+            assertTrue(channel.trySend(1).isSuccess) // Handed to the coroutine
+            assertTrue(channel.trySend(2).isSuccess) // Buffered
+            assertFalse(channel.trySend(3).isSuccess) // Failed to offer
             channel.send(3)
             yield()
             assertEquals(1, expected)
-            assertTrue(channel.offer(4)) // Handed to the coroutine
-            assertTrue(channel.offer(5)) // Buffered
-            assertFalse(channel.offer(6))  // Failed to offer
+            assertTrue(channel.trySend(4).isSuccess) // Handed to the coroutine
+            assertTrue(channel.trySend(5).isSuccess) // Buffered
+            assertFalse(channel.trySend(6).isSuccess)  // Failed to offer
             channel.send(6)
             assertEquals(2, expected)
         }