KTOR-4009 Add flag to respond with 500 on exceptions in tests

ktor-server/ktor-server-test-host/jvmAndNix/src/io/ktor/server/testing/TestApplication.kt

@@ -133,7 +133,7 @@ public open class TestApplicationBuilder {
             developmentMode = true
             val oldConfig = config
             this@TestApplicationBuilder.environmentBuilder(this)
-            if (config == oldConfig) { // config was not set by user. load the default one
+            if (config == oldConfig) { // the user did not set config. load the default one
                 config = DefaultTestConfig()
             }
             parentCoroutineContext += this@TestApplicationBuilder.job

ktor-server/ktor-server-test-host/jvmAndNix/src/io/ktor/server/testing/TestApplicationEngine.kt

@@ -19,6 +19,10 @@ import kotlinx.atomicfu.*
 import kotlinx.coroutines.*
 import kotlin.coroutines.*
 
+@OptIn(InternalAPI::class)
+@PublicAPICandidate("2.2.0")
+internal const val CONFIG_KEY_THROW_ON_EXCEPTION = "ktor.test.throwOnException"
+
 /**
  * A test engine that provides a way to simulate application calls to the existing application module(s)
  * without actual HTTP connection.
@@ -96,7 +100,13 @@ class TestApplicationEngine(
     }
 
     private suspend fun PipelineContext<Unit, ApplicationCall>.handleTestFailure(cause: Throwable) {
-        tryRespondError(defaultExceptionStatusCode(cause) ?: throw cause)
+        val throwOnException = environment.config
+            .propertyOrNull(CONFIG_KEY_THROW_ON_EXCEPTION)
+            ?.getString()?.toBoolean() ?: true
+        tryRespondError(
+            defaultExceptionStatusCode(cause)
+                ?: if (throwOnException) throw cause else HttpStatusCode.InternalServerError
+        )
     }
 
     private suspend fun PipelineContext<Unit, ApplicationCall>.tryRespondError(statusCode: HttpStatusCode) {

ktor-server/ktor-server-test-host/jvmAndNix/test/TestApplicationTest.kt

@@ -8,8 +8,10 @@ import io.ktor.client.*
 import io.ktor.client.plugins.*
 import io.ktor.client.request.*
 import io.ktor.client.statement.*
+import io.ktor.http.*
 import io.ktor.http.content.*
 import io.ktor.server.application.*
+import io.ktor.server.config.*
 import io.ktor.server.plugins.*
 import io.ktor.server.request.*
 import io.ktor.server.response.*
@@ -248,4 +250,33 @@ class TestApplicationTest {
             request2.await()
         }
     }
+
+    @Test
+    fun testExceptionThrowsByDefault() = testApplication {
+        routing {
+            get("/boom") {
+                throw IllegalStateException("error")
+            }
+        }
+
+        val error = assertFailsWith<IllegalStateException> {
+            client.get("/boom")
+        }
+        assertEquals("error", error.message)
+    }
+
+    @Test
+    fun testExceptionRespondsWith500IfFlagSet() = testApplication {
+        environment {
+            config = MapApplicationConfig(CONFIG_KEY_THROW_ON_EXCEPTION to "false")
+        }
+        routing {
+            get("/boom") {
+                throw IllegalStateException("error")
+            }
+        }
+
+        val response = client.get("/boom")
+        assertEquals(HttpStatusCode.InternalServerError, response.status)
+    }
 }