Your exception handler can encounter an exception

Raymond Chen

Consider the following code, written in C# just for kicks; the problem is generic to any environment that supports exception handling.

void ObliterateDocument()
{
 try {
  try {
   document.DestroyAll();
  } finally {
   document.Close();
   document.DestroyExtensions();
   document.DestroyPlugins();
  }
 } finally {
  document.Destroy();
 }
}

Some time later, you find yourself facing an assertion failure from document.Destroy() claiming that you are destroying the document while there are still active plugins. But there is your call to document.DestroyPlugins(), and it’s in a finally block, and the whole point of a finally block is that there is no way you can escape without executing it.

So why didn’t document.DestroyPlugins() execute?

Because your exception handler itself encountered an exception.

The exception handler is not active during its own finally clause. As a result, if an exception is thrown during document.Close(), the exception handler search begins at the block outside the finally block.

(That the exception handler is not active during its own finally clause should be obvious. It would mean that if an exception were to occur during the finally clause, the program would go into an infinite loop. And it also wouldn’t be possible to rethrow a caught exception; your throw would end up caught by yourself!)

In this case, the exception was caught by some outer caller, causing the remainder of the first finally block to be abandoned. The other finally blocks do run since they contain the one that died.

(This bug also exists in the proposed alternative to error-checking code posted by an anonymous commenter.)

Raymond Chen

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