How does updating running application binaries during an upgrade work?

I just upgraded my ubuntu lucid to natty , during the upgrade it replaced most of the running applications with a newer version.

How does this work? (Won't the applications crash?) What will happen to the application if a lib file got upgraded and a running application which is looking for a older lib tries to load it?

Solution 1:

Linux (and other UNIXes) draws a distinction between the name of a file (the link), the file itself (often identified with the inode), and open handles to the file. When you go to delete a file, you call the unlink() call - this erases the link to the file (you could also use rename() to overwrite it with a different inode). However, if open handles to the file (or other links - files can have multiple hardlinks) remain, the inode remains, and so does the file content, until all links and handles go away.

So running programs using the library or whatever keep a handle to the old version (often implicitly through a memory mapping), so it stays on disk. It just doesn't have a filename anymore, and will be cleaned up when all programs using it shut down (or on the next reboot, during the filesystem check or journal replay).

Further, note that programs expecting the 'old library' will do just fine with newer versions of the library. Linux libraries are assigned a filename ('soname') that reflects the version of the ABI (Application Binary Interface) offered by the library. For example, the C library on my system is libc.so.6. Any program compiled against an older version of libc, but still a version of libc implementing the version 6 ABI, will work fine with it. Really old programs will look for a libc.so.5 or libc.so.4 or something instead; in this case, you'd need to keep the old version around as well - but since the filename is different, this isn't a problem.

Solution 2:

Unlike Windows, you can delete or replace an open file; to give a simplified explanation, new requests for the files open the new file, existing handles use the file that existed when they were created. In other words, in Linux you can have files/file versions that still exist, although there's no longer a pointer to them in the directory structure; those cease existing there is no pointer to them at all (closed and all).

Usually a running app loads the needed libraries up front, so the problem you describe would only happen in very specific timing situations while the package is being installed: the running apps are still using the old version of the library, newly started apps use the new one.

This is used not only in distro upgrades, but happens on every package upgrade (the dist-upgrade just adds a couple more automagic steps to that process).