Comment 2 for bug 830492

a: This could be solved be checking the cache just after locking for install. If any changes have occurred that conflict with the install then Synaptic should remove the install lock and ask the user what do to about the conflict. This would be failsafe because it is checking /after/ locking for install, but Synaptic could additionally check periodically to give users an early warning.

b: I already considered b in the bug description:
"To avoid multiple instances of apt downloading the same file, each partial file should have it's own lock (For example, /var/cache/apt/archives/partial/inkscape_0.48.1-2ubuntu2_i386.deb would have inkscape_0.48.1-2ubuntu2_i386.lock)."

This would prevent 2 applications from downloading the same file.

c: So while an update is still in the download phase, I install a package that conflicts with the update. This is very rare because if the package /is/ going to conflict, it will most likely conflict /before/ the update. But in this case, when the update finishes downloading, it should lock for install, check the cache and notice the conflict, remove the install lock, print a message about the conflict, and quit.