Change the contract of CoroutineContext.isActive to return 'true' for…

… contexts with no job in it. (#3639)

Otherwise, the API is becoming an error prone for being called from jobless entrypoints (i.e. 'suspend fun main' or Ktor handlers), as 'if (ctx.isActive)' is a well-established pattern for busy-wait or synchronous job.
It is now aligned with CoroutineScope.isActive behaviour.

Fixes #3300

kotlinx-coroutines-core/common/src/Job.kt

@@ -539,7 +539,7 @@ public fun Job.cancelChildren(cause: Throwable? = null) {
 
 /**
  * Returns `true` when the [Job] of the coroutine in this context is still active
- * (has not completed and was not cancelled yet).
+ * (has not completed and was not cancelled yet) or the context does not have a [Job] in it.
  *
  * Check this property in long-running computation loops to support cancellation
  * when [CoroutineScope.isActive] is not available:
@@ -550,11 +550,11 @@ public fun Job.cancelChildren(cause: Throwable? = null) {
  * }
  * ```
  *
- * The `coroutineContext.isActive` expression is a shortcut for `coroutineContext[Job]?.isActive == true`.
+ * The `coroutineContext.isActive` expression is a shortcut for `get(Job)?.isActive ?: true`.
  * See [Job.isActive].
  */
 public val CoroutineContext.isActive: Boolean
-    get() = this[Job]?.isActive == true
+    get() = get(Job)?.isActive ?: true
 
 /**
  * Cancels [Job] of this context with an optional cancellation cause.

kotlinx-coroutines-core/common/test/CoroutineScopeTest.kt

@@ -277,4 +277,15 @@ class CoroutineScopeTest : TestBase() {
 
     private fun scopePlusContext(c1: CoroutineContext, c2: CoroutineContext) =
         (ContextScope(c1) + c2).coroutineContext
+
+    @Test
+    fun testIsActiveWithoutJob() {
+        var invoked = false
+        suspend fun testIsActive() {
+            assertTrue(coroutineContext.isActive)
+            invoked = true
+        }
+        ::testIsActive.startCoroutine(Continuation(EmptyCoroutineContext){})
+        assertTrue(invoked)
+    }
 }