"So don't do that"

Interesting... :)

I just upgraded to Mac OS X Snow Leopard and coincidentally started trying to dig into why my MacBook has been running out of battery "early", shutting down ungracefully.

Of course it just starts right back up and the worst I'm left with, is trying to figure out where I was. Which I make unnecessarily worse by stopping to check to see if there is anything in the logs, and getting further distracted. There never is - except today. Excellent. :)

I've been following Apple's docs on this and running the battery right down to the end, to calibrate it. This morning I made it happen again and when I jacked back into AC power, it started up just fine - and entered an infinite loop.

Strange; it looked like it was just about to display the Login Window, but then went back to the old-school text screen that comes before that - and then started cycling back and forth. Drat.

Double drat: I'd turned off ssh ("Remote Access" in the Sharing Preferences Pane) and so, couldn't ssh in, to check what was going on.

So I powered off and tried a shift-boot ("Safe Mode"), but no go; same behavior.

OK; So I booted into Single User Mode and started spelunking...

Several tangets later :) I saw a log message; something that was consulting /etc/authentication* had failed with an "unexpected character"... Sure enough, that file was hosed. (More on that in a moment.)

So... I pulled that file off the Install DVD and was on my way. (Running the Installer from DVD - amazingly well-written! - may have worked, though I didn't try.)

*More on /etc/authentication:

• Interestingly, /etc/authentication is a dynamic file; it was modified on reboot. The changes were not huge, though interesting to track down manually via the XML, since the order changes (which is not important to XML).
  
• This file is critical in determining who (or what process) is allowed to do what - and since it contained invalid data, Mac OS X could not even get to the Login Window.
  
• My fault: I had inadvertently caused etc/authentication to be at risk by running system_profiler in a tight shell loop, to eat battery (since it not only spews data, but also consults hardware) and it (I have since learned :) consults /etc/authorization - which my command had kept open (and vulnerable) FAR more than normal. Live and learn. :)