I’ve been meaning to follow up on my blog entry about the Restart Manager and reboots on the Windows platform in general. I saw a lot of incorrect conclusions being drawn from my blog entry. So, I want to quickly revisit the issue and hopefully clear up the confusion. Hopefully, this time I’ll communicate more clearly.
Jonathan Bastien-Filiatrault’s blog post goes right to the heart of the incorrect conclusion drawn by many. He states the following:
One thing that makes Unix in general not need this is that if a file is unlinked, as long as there are open file descriptors to this file, that file is not removed from disk (even if it cannot be referenced from the directory hierarchy) and the memory maps from an executable or library can be maintained.
I didn’t clearly note in my last blog entry that my experiments with Reid’s PowerBook proved to me that this is true. You can delete files even if they are in use on UNIX. That’s nice but it still doesn’t solve either of the two issues that are the real reason a reboot is required:
1. The old file is still running (i.e. you are still vulnerable to any security exploits)
2. New executions of the file get the updated version while the old executions of the file keep using the old file (i.e. you may have inter-/intra-process communication problems).
On Windows, replacing a file that is in use takes more steps (rename, replace, delete) than UNIX (delete, replace) but on either operating systems you need to stop the updated process for the update to truly take effect.
Jonathan also states:
When that file is unlinked, one can sneak a new library into place atomically.
I’m not sure what he means by "atomically" but unless the file system supports true ACID-like transactions then an "rm oldfile" "cp newfile oldfile" is not an atomic operation. The next version of the NT file system (shipping for the first time with Windows Vista, I believe) supports transactions such that "ren oldfile tempfile" "copy newfile oldfile" can actually be done atomically. But that is a conversation for a different day.
Anyway, I just wanted to be a bit more clear on the topic and note that fundamentally, file-in-use handling on the two operating systems aren’t really different and that the Restart Manager functionality would be useful on any operating system.