Confluence keeps thumbnail imageio temp files open after they have expired

Symptoms

Confluence is filling the temp directory with files named like imageio*.tmp. These are used in the generation of thumbnails, for example by the gallery macro. These files are being held open by Confluence, and eventually Confluence's process runs out of available file handles, and produces "Too many open files" errors in the log.

Cause

The imageio objects are not being finalised correctly, holding both the file handle and the object open. This results in a file handle leak, but also in a memory/object leak.

Solution

This issue does not occur in any version later than 3.5.x