fix(jvm): make job cancellation hashcode not fail after deserialization

kotlinx-coroutines-core/jvm/src/Exceptions.kt

@@ -61,6 +61,10 @@ internal actual class JobCancellationException public actual constructor(
     override fun equals(other: Any?): Boolean =
         other === this ||
             other is JobCancellationException && other.message == message && other.job == job && other.cause == cause
-    override fun hashCode(): Int =
-        (message!!.hashCode() * 31 + job.hashCode()) * 31 + (cause?.hashCode() ?: 0)
+
+    override fun hashCode(): Int {
+        // since job is transient it is indeed nullable after deserialization
+        @Suppress("UNNECESSARY_SAFE_CALL")
+        return (message!!.hashCode() * 31 + (job?.hashCode() ?: 0)) * 31 + (cause?.hashCode() ?: 0)
+    }
 }

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

@@ -36,4 +36,30 @@ class JobCancellationExceptionSerializerTest : TestBase() {
             finish(4)
         }
     }
+
+    @Test
+    fun testHashCodeAfterDeserialization() = runTest {
+        try {
+            coroutineScope {
+                expect(1)
+                throw JobCancellationException(
+                    message = "Job Cancelled",
+                    job = Job(),
+                    cause = null,
+                )
+            }
+        } catch (e: Throwable) {
+            finish(2)
+            val outputStream = ByteArrayOutputStream()
+            ObjectOutputStream(outputStream).use {
+                it.writeObject(e)
+            }
+            val deserializedException =
+                ObjectInputStream(outputStream.toByteArray().inputStream()).use {
+                it.readObject() as JobCancellationException
+            }
+            // verify hashCode does not fail even though Job is transient
+            assert(deserializedException.hashCode() != 0)
+        }
+    }
 }