util/util-app: Include the exception in the error message printed whe…

…n the application encounters an error on startup

Problem

When the application exits prematurely due to an exception encountered on startup, this
exception is not printed to stderr, only the message "Exception thrown in main on startup".
The exception stack trace is printed to stdout, but this is unintuitive for the user.

Solution

Include the exception and stack trace in the exception printed to stderr.

Differential Revision: https://phabricator.twitter.biz/D1116753

CHANGELOG.rst

@@ -7,6 +7,13 @@ Note that ``PHAB_ID=#`` and ``RB_ID=#`` correspond to associated messages in com
 Unreleased
 ----------
 
+Runtime Behavior Changes
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+* util-app: When the application exits due to an error on startup, the error and
+  and stack trace are printed to stderr in addition to the existing stdout. ``PHAB_ID=D1116753``
+
+
 23.11.0
 -------
 

PROJECT.yml

@@ -4,4 +4,3 @@ owners:
   - csl-team:ldap
   - jillianc
   - mbezoyan
-  - nvong

util-app/src/main/scala/com/twitter/app/App.scala

@@ -3,6 +3,8 @@ package com.twitter.app
 import com.twitter.app.lifecycle.Event._
 import com.twitter.conversions.DurationOps._
 import com.twitter.util._
+import java.io.PrintWriter
+import java.io.StringWriter
 import java.lang.reflect.InvocationTargetException
 import java.util.concurrent.ConcurrentLinkedQueue
 import java.util.concurrent.atomic.AtomicReference
@@ -82,7 +84,13 @@ trait App extends ClosableOnce with CloseOnceAwaitably with Lifecycle {
         System.err.println(throwable.getMessage)
         System.exit(1)
       case _ =>
-        exitOnError("Exception thrown in main on startup")
+        util.Using(new StringWriter) { stringWriter =>
+          util.Using(new PrintWriter(stringWriter)) { printWriter =>
+            throwable.printStackTrace(printWriter)
+            printWriter.flush()
+            exitOnError(s"Exception thrown in main on startup: ${stringWriter.toString}")
+          }
+        }
     }
   }
 

util-app/src/test/scala/com/twitter/app/AppTest.scala

@@ -67,7 +67,7 @@ class AppTest extends AnyFunSuite {
 
     test1.main(Array())
 
-    assert(test1.reason.contains("Exception thrown in main on startup"))
+    assert(test1.reason.exists(_.contains("Exception thrown in main on startup")))
   }
 
   test("App: propagate underlying exception from fields in app") {