.Net framework hotfix wreaks havoc

Last week all of us were baffled when suddenly one part of our application that uploads files to a FTP server stopped working. The strange thing was that the same build has been working without any issues for the past one week. We looked at everything that could have gone wrong, server, configuration, code but everything was setup fine and hadn't been changed. Also interestingly it stopped working for everyone except the developer who was responsible for the feature.

The first thing we did was to enable detailed logging to see what was happening, the logs showed two problems
  1. We were incorrectly formatting the path of file to upload
  2. The .Net framework code was changing folders after login to the root folder of the ftp where it didn't have permissions to upload the file
Further investigation showed that the second issue was not coming on the developer's machine. Most puzzling indeed.

Then I remembered that last week .Net had issued a critical hotfix for .Net 2.0, could this be the issue. We verified that the developer didn't have the hotfix and all machines which were failing did have, Strike 1! Next we uninstalled the hotfix from one of the machines and the FTP uploads started working, Strike 2!! Finally we fixed the incorrect formatting of the ftp url and the issue got resolved on all machines with or without the hotfix, Strike 3! Issue resolved!

The problem was that the hotfix changed the implementation of the FTP code inside the .Net framework so that it behaved differently when passed an incorrectly formatted url.

This was the first time I saw a working app fail because of the way an incorrect argument was handled by a newer version of the framework. It was a good learning experience though :) Also this strengthens my belief in asserting all assumptions in code because if we had asserted that the url was infact of the format that we were expecting, this issue would never have happened in the first place.

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