I use XFCE on Linux Mint 18.2.Apparently, my /sbin/upstart --user process is filling my~/.cache/upstart/startxfce4.log with gobs and gobs of:QGLContext::makeCurrent(): Cannot make invalid context current.Why would it be doing that and how can I make it stop?
Upstart fills up with gigabytes of QGLContext::makeCurrent(): Cannot make invalid context current
linux mint;logs;upstart
So,It's not upstart. The process which writes to that log may be upstart, but it's getting the messages from elsewhere.It's not even startxfce4, nor xfce4-session (which is run by startxfce4) - it's a specific app that's spamming the log.Since you're  getting constant spam, if you kill the app the log will stop increasing in size. So, kill your apps one by one (start with what seems to be using Qt, since QGLContext is a Qt class), until the log stops growing.... after doing that on my system, it turned out the culprit was amarok. Not that I know why it does that, but that's a subject for another question.