Proper cause for Exceptions Deferred is resumed with

src/main/java/com/jakewharton/retrofit2/adapter/kotlin/coroutines/CoroutineCallAdapterFactory.kt

@@ -92,16 +92,18 @@ class CoroutineCallAdapterFactory private constructor() : CallAdapter.Factory()
         }
       }
 
+      val exceptionWithCapturedStack = CoroutineCallException()
       call.enqueue(object : Callback<T> {
         override fun onFailure(call: Call<T>, t: Throwable) {
+          (t.findRootCause() ?: t).initCause(exceptionWithCapturedStack)
           deferred.completeExceptionally(t)
         }
 
         override fun onResponse(call: Call<T>, response: Response<T>) {
           if (response.isSuccessful) {
             deferred.complete(response.body()!!)
           } else {
-            deferred.completeExceptionally(HttpException(response))
+            deferred.completeExceptionally(HttpException(response).apply { initCause(exceptionWithCapturedStack) })
           }
         }
       })
@@ -125,8 +127,10 @@ class CoroutineCallAdapterFactory private constructor() : CallAdapter.Factory()
         }
       }
 
+      val exceptionWithCapturedStack = CoroutineCallException()
       call.enqueue(object : Callback<T> {
         override fun onFailure(call: Call<T>, t: Throwable) {
+          (t.findRootCause() ?: t).initCause(exceptionWithCapturedStack)
           deferred.completeExceptionally(t)
         }
 
@@ -138,4 +142,19 @@ class CoroutineCallAdapterFactory private constructor() : CallAdapter.Factory()
       return deferred
     }
   }
+
 }
+
+internal fun Throwable.findRootCause(): Throwable? {
+  var cause = cause ?: return null
+
+  while (true) {
+    val nextCause = cause.cause
+    when (nextCause) {
+      null, cause -> return cause
+      else -> cause = nextCause
+    }
+  }
+}
+
+private class CoroutineCallException : RuntimeException("Originally called here:")

src/test/java/com/jakewharton/retrofit2/adapter/kotlin/coroutines/DeferredTest.kt

@@ -21,6 +21,7 @@ import kotlinx.coroutines.runBlocking
 import okhttp3.mockwebserver.MockResponse
 import okhttp3.mockwebserver.MockWebServer
 import okhttp3.mockwebserver.SocketPolicy.DISCONNECT_AFTER_REQUEST
+import org.junit.Assert.assertTrue
 import org.junit.Assert.fail
 import org.junit.Before
 import org.junit.Rule
@@ -60,23 +61,30 @@ class DeferredTest {
   @Test fun bodySuccess404() = runBlocking {
     server.enqueue(MockResponse().setResponseCode(404))
 
+    val currentFrame = Exception().stackTrace[0]
     val deferred = service.body()
     try {
       deferred.await()
       fail()
     } catch (e: HttpException) {
       assertThat(e).hasMessageThat().isEqualTo("HTTP 404 Client Error")
+
+      val rootCauseIsCurrentMethod = e.findRootCause()?.stackTrace?.any { it.equalsExceptLine(currentFrame) } ?: false
+      assertTrue("Root cause needs to be current method", rootCauseIsCurrentMethod)
     }
   }
 
   @Test fun bodyFailure() = runBlocking {
     server.enqueue(MockResponse().setSocketPolicy(DISCONNECT_AFTER_REQUEST))
 
+    val currentFrame = Exception().stackTrace[0]
     val deferred = service.body()
     try {
       deferred.await()
       fail()
     } catch (e: IOException) {
+      val rootCauseIsCurrentMethod = e.findRootCause()?.stackTrace?.any { it.equalsExceptLine(currentFrame) } ?: false
+      assertTrue("Root cause needs to be current method", rootCauseIsCurrentMethod)
     }
   }
 
@@ -101,11 +109,17 @@ class DeferredTest {
   @Test fun responseFailure() = runBlocking {
     server.enqueue(MockResponse().setSocketPolicy(DISCONNECT_AFTER_REQUEST))
 
+    val currentFrame = Exception().stackTrace[0]
     val deferred = service.response()
     try {
       deferred.await()
       fail()
     } catch (e: IOException) {
+      val rootCauseIsCurrentMethod = e.findRootCause()?.stackTrace?.any { it.equalsExceptLine(currentFrame) } ?: false
+      assertTrue("Root cause needs to be current method", rootCauseIsCurrentMethod)
     }
   }
+
+  private fun StackTraceElement.equalsExceptLine(other: StackTraceElement) =
+      other.className == className && other.fileName == fileName && other.methodName == methodName
 }