From a96fdcfabb15f2a46f7fda1dc4386c2f964595a3 Mon Sep 17 00:00:00 2001
From: Simon Coffey <simon.coffey@futurelearn.com>
Date: Mon, 16 Oct 2017 16:52:32 +0100
Subject: [PATCH] Clarify output when printing nested exception traces

Since v12.1.0, if an exception has a nested cause exception, the cause is also
displayed in the trace output.[1]

For heavily-nested exceptions, this output can be quite lengthy - for example,
Rails migrations nest DB errors twice over, resulting in an error message and
backtrace repeated three times.

To break up this output and make it clearer what each individual backtrace
relates to, this adds whitespace and a "Caused by:" label to each nested
exception being displayed.

To prevent "Caused by:" labels occurring on their own, I've moved the
exception loop shortcut return into the `#display_cause_details` method. This
doesn't alter the behaviour of the shortcut, as only the first exception will
be unconditionally printed (which was already the case, as the first exception
can't be already seen).

[1] https://github.com/ruby/rake/commit/fbb22e7f570fc573ad1bff9d5905df1ab1cbd475
[2] https://github.com/ruby/rake/commit/57c932cea12ef3201fcaeaf80ba6f4f545390269
---
 lib/rake/application.rb       | 17 +++++++++++++----
 test/test_rake_application.rb |  1 +
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/lib/rake/application.rb b/lib/rake/application.rb
index 89565a670..8420a942b 100644
--- a/lib/rake/application.rb
+++ b/lib/rake/application.rb
@@ -207,13 +207,22 @@ def display_error_message(ex) # :nodoc:
     end
 
     def display_exception_details(ex) # :nodoc:
-      seen = Thread.current[:rake_display_exception_details_seen] ||= []
-      return if seen.include? ex
-      seen << ex
+      display_exception_details_seen << ex
 
       display_exception_message_details(ex)
       display_exception_backtrace(ex)
-      display_exception_details(ex.cause) if has_cause?(ex)
+      display_cause_details(ex.cause) if has_cause?(ex)
+    end
+
+    def display_cause_details(ex) # :nodoc:
+      return if display_exception_details_seen.include? ex
+
+      trace "\nCaused by:"
+      display_exception_details(ex)
+    end
+
+    def display_exception_details_seen
+      Thread.current[:rake_display_exception_details_seen] ||= []
     end
 
     def has_cause?(ex) # :nodoc:
diff --git a/test/test_rake_application.rb b/test/test_rake_application.rb
index cc063c0d2..d17445c7e 100644
--- a/test/test_rake_application.rb
+++ b/test/test_rake_application.rb
@@ -97,6 +97,7 @@ def test_display_exception_details_cause
 
     assert_empty out
 
+    assert_match "Caused by:", err
     assert_match "cause a", err
     assert_match "cause b", err
   end