[Bug 830492] Re: Downloads should not lock apt

[Julian Andres Klode]

> This could be solved be checking the cache just after locking for install. 
Not at all. Synaptic requires more information then found in the cache, and it needs the database on the system, that is, /var/lib/dpkg/status and other lists to not change in order to be able to display the information correctly. Otherwise, it tries reading at the wrong offset in the file and who knows what happens then.

> This would prevent 2 applications from downloading the same file.
Yes, but you still have the file in the download lists in both runs. The problem here is that you can't display correct progress information anymore and would have to wait until the other process finishes the download and you probably can't download something else in between, without major changes.

On the other things, it's just too complicated to lock for installation,
then regenerate the cache and transfer the modifications to the new
cache (you'd need to correctly map from the old cache to the new one).
Then you might even notice that you need to download some more packages,
because someone removed packages while you downloaded, etc. In Summary:
It's just too complicated to do right.

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https://bugs.launchpad.net/bugs/830492

Title:
  Downloads should not lock apt

Status in APT:
  New
Status in The synaptic package manager:
  New
Status in “apt” package in Ubuntu:
  New
Status in “synaptic” package in Ubuntu:
  New

Bug description:
  It should be possible to download while installing, or download 2
  things at once.

  Currently an apt-get command with '--download-only' will lock apt
  (/var/lib/dpkg/lock). Synaptic even locks apt just by being open!

  Examples:
  * I might be updating my system. The update is still in the download stage, and I want to download and install a small package for something. Currently I would have to stop the download, install the package, and resume the download.
  * I'm installing lots of application from the Software Centre. While 1 app is installing the next should be downloading.
  * Run apt-get from command line while browsing software in Synaptic.

  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). It may also be
  necessary to prevent updating package lists (sudo apt-get update) when
  any download lock exists as well as apt's main lock.

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