Use a separate list for exceptions registered to print after at_exit …

…handlers

spec/std/kernel_spec.cr

@@ -107,7 +107,7 @@ describe "at_exit" do
                        OUTPUT
   end
 
-  it "allow handlers to change the exit code with explicit `exit` call" do
+  it "allows handlers to change the exit code with explicit `exit` call" do
     status, output = build_and_run <<-'CODE'
       at_exit do |exit_code|
         puts "first handler code, exit code: #{exit_code}"
@@ -135,7 +135,7 @@ describe "at_exit" do
                        OUTPUT
   end
 
-  it "allow handlers to change the exit code with explicit `exit` call (2)" do
+  it "allows handlers to change the exit code with explicit `exit` call (2)" do
     status, output = build_and_run <<-'CODE'
       at_exit do |exit_code|
         puts "first handler code, exit code: #{exit_code}"
@@ -165,7 +165,7 @@ describe "at_exit" do
                        OUTPUT
   end
 
-  it "change final exit code when an handler raises an error" do
+  it "changes final exit code when an handler raises an error" do
     status, output, error = build_and_run <<-'CODE'
       at_exit do |exit_code|
         puts "first handler code, exit code: #{exit_code}"
@@ -204,4 +204,25 @@ describe "at_exit" do
     status.success?.should be_false
     error.should eq "Error running at_exit handler: Cannot use at_exit from an at_exit handler\n"
   end
+
+  it "shows unhandled exceptions after at_exit handlers" do
+    status, _, error = build_and_run <<-CODE
+      at_exit do
+        STDERR.puts "first handler code"
+      end
+
+      at_exit do
+        STDERR.puts "second handler code"
+      end
+
+      raise "Kaboom!"
+    CODE
+
+    status.success?.should be_false
+    error.should contain <<-OUTPUT
+                           second handler code
+                           first handler code
+                           Unhandled exception: Kaboom!
+                           OUTPUT
+  end
 end

src/kernel.cr

@@ -126,6 +126,7 @@ module AtExitHandlers
   @@running = false
 
   private class_getter(handlers) { [] of Int32 -> }
+  private class_getter(exceptions) { [] of Exception }
 
   def self.add(handler)
     raise "Cannot use at_exit from an at_exit handler" if @@running
@@ -134,15 +135,17 @@ module AtExitHandlers
   end
 
   def self.add_exception(ex : Exception)
-    # Prepend a handler for this exception. It will run after all at_exit
-    # handlers, to make sure the user sees the exception.
-    handlers.unshift ->(status : Int32) { ex.inspect_with_backtrace(STDERR) }
+    # at_exit handlers can't be cleared now, but might in the future.
+    # The exceptions are stored in a separate list to make sure it won't be cleared.
+
+    exceptions << ex
   end
 
   def self.run(status)
     return status unless handlers = @@handlers
     @@running = true
 
+    # Run the registered handlers in reverse order
     while handler = handlers.pop?
       begin
         handler.call status
@@ -152,6 +155,13 @@ module AtExitHandlers
       end
     end
 
+    # Print the registered exception(s) after all at_exit handlers, to make sure
+    # the user sees them.
+    while ex = exceptions.pop?
+      STDERR.print "Unhandled exception: "
+      ex.inspect_with_backtrace(STDERR)
+    end
+
     status
   end
 end