Copy CopyableThreadContextElement when switching context with flowOn (#3778)

`flowOn` uses its own undispatched coroutine start when it detects a fast path. Previously, it concatenated the context missing the copy of CopyableThreadContextElement.
Fixed by replacing concatenation with `newCoroutineContext`.

Fixes #3787 

Co-authored-by: Vsevolod Tolstopyatov <qwwdfsad@gmail.com>

Author: wanyingd1996

kotlinx-coroutines-core/common/src/flow/internal/ChannelFlow.kt

@@ -161,7 +161,7 @@ internal abstract class ChannelFlowOperator<S, T>(
         // Fast-path: When channel creation is optional (flowOn/flowWith operators without buffer)
         if (capacity == Channel.OPTIONAL_CHANNEL) {
             val collectContext = coroutineContext
-            val newContext = collectContext + context // compute resulting collect context
+            val newContext = collectContext.newCoroutineContext(context) // compute resulting collect context
             // #1: If the resulting context happens to be the same as it was -- fallback to plain collect
             if (newContext == collectContext)
                 return flowCollect(collector)

kotlinx-coroutines-core/jvm/test/ThreadContextElementTest.kt

@@ -7,6 +7,7 @@ package kotlinx.coroutines
 import org.junit.Test
 import kotlin.coroutines.*
 import kotlin.test.*
+import kotlinx.coroutines.flow.*
 
 class ThreadContextElementTest : TestBase() {
 
@@ -37,7 +38,7 @@ class ThreadContextElementTest : TestBase() {
     }
 
     @Test
-    fun testUndispatched()= runTest {
+    fun testUndispatched() = runTest {
         val exceptionHandler = coroutineContext[CoroutineExceptionHandler]!!
         val data = MyData()
         val element = MyElement(data)
@@ -191,6 +192,21 @@ class ThreadContextElementTest : TestBase() {
 
         assertEquals(manuallyCaptured, captor.capturees)
     }
+
+    @Test
+    fun testThreadLocalFlowOn() = runTest {
+        val myData = MyData()
+        myThreadLocal.set(myData)
+        expect(1)
+        flow {
+            assertEquals(myData, myThreadLocal.get())
+            emit(1)
+        }
+            .flowOn(myThreadLocal.asContextElement() + Dispatchers.Default)
+            .single()
+        myThreadLocal.set(null)
+        finish(2)
+    }
 }
 
 class MyData
@@ -259,6 +275,7 @@ class CopyForChildCoroutineElement(val data: MyData?) : CopyableThreadContextEle
     }
 }
 
+
 /**
  * Calls [block], setting the value of [this] [ThreadLocal] for the duration of [block].
  *

kotlinx-coroutines-core/jvm/test/ThreadContextMutableCopiesTest.kt

@@ -4,6 +4,7 @@
 
 package kotlinx.coroutines
 
+import kotlinx.coroutines.flow.*
 import kotlin.coroutines.*
 import kotlin.test.*
 
@@ -131,4 +132,32 @@ class ThreadContextMutableCopiesTest : TestBase() {
             finish(2)
         }
     }
+
+    @Test
+    fun testDataIsCopiedThroughFlowOnUndispatched() = runTest {
+        expect(1)
+        val root = MyMutableElement(ArrayList())
+        val originalData = root.mutableData
+        flow {
+            assertNotSame(originalData, threadLocalData.get())
+            emit(1)
+        }
+            .flowOn(root)
+            .single()
+        finish(2)
+    }
+
+    @Test
+    fun testDataIsCopiedThroughFlowOnDispatched() = runTest {
+        expect(1)
+        val root = MyMutableElement(ArrayList())
+        val originalData = root.mutableData
+        flow {
+            assertNotSame(originalData, threadLocalData.get())
+            emit(1)
+        }
+            .flowOn(root + Dispatchers.Default)
+            .single()
+        finish(2)
+    }
 }