2011-8-3 16:55:08

 

2011-8-3 16:55:08

System decomposition view
-------------------------

* Top-level view of Application Installer subsystem

The Application installer decomposes into the following functional
parts:

 - The "hildon-application-manager" program, "ham" in the sequel.

 - The "apt-worker" program.

 - The "libapt-pkg" library.

 - The "gpgv" program.

 - The "gnupg" program.

Ham is started when "Application Installer" is selected from the
"Others" menu or in response to a "mime-open" D-BUS request. 

从菜单或dbus请求激活

In turn,
it starts apt-worker via sudo so that apt-worker runs as root.  Ham
and apt-worker communicate over pipes using a private protocol.

ham和apt-work使用管道进行通信

Apt-worker is linked to libapt-pkg and uses it to perform all
operations related to the actual package management, like querying the
package data base, accessing repositories, managing the list of
repositories and running dpkg.  The stdout and stderr output of
apt-worker is caught by ham and made available to the user on request.

调用库来管理包数据库和仓库

Libapt-pkg can optionally use gpgv to verify digital signatures.  Gpgv
then runs as a subprocess of apt-worker.

Gnupg can optionally be used to manage the key-ring used by
libapt-pkg.  Gnupg would then be called by root from the command line.

Runtime view
------------

The normal course of actions can be illustrated by following the
process of installing a new package.

 - The user opens the "Application Installer", which starts a new
   process runnining ham.

启动了ham进程

 - Ham runs apt-worker as a subprocess via sudo.

创建子进程

 - Ham sets up the UI and goes to the usual event loop, doing
   everything in callbacks.

  创建UI 进入事件回调

 - Apt-worker initializes libapt-pkg and reads the package database
   cache.

 

 - Apt-worker then goes into a loop, reading requests from ham and
   sending back responses.  It only handles one request at any one
   time.

 - The user clicks on "Install Applications".

 - Ham sends a request to apt-worker asking for the list of installed
   user packages.  Ham resumes its normal event loop after sending the
   request and executes a callback when the response comes.

 - When the response comes, ham fills its own data structures and
   presents the appropriate list to the user.

 - Right after showing the list ham sends a request to apt-worker for
   more information about the first package in the list.  This
   additional information is more expensive to compute and includes
   things like whether all dependencies of a package are available and
   if so, how much data needs to be downloaded when actually
   installing the package.

 - While the response is outstanding, ham continues to handle events
   in the normal fashion.

 - When the response comes, ham augments its data structures and the
   displayed list with that additional information.  It then sends a
   similar request for the next package in the list until all packages
   are covered or the user goes to another view.

 - The user asks for the details of a particular packages.

 - Again, ham sends a request for this information to apt-worker and
   when the response comes, presents this information to the user.

 - The user decides to install the package.

 - Again, this request is forwarded to apt-worker.

 - Apt-worker uses libapt-pkg to perform the requested operation,
   which will in turn run dpkg and maybe gpgv as subprocesses.
   Progress reports are send to ham over the request/response channel.

 - Output of dpkg, gpgv, the maintainer scripts of packages etc and
   additional status information from libapt-pkg appears on stdout and
   stderr of apt-worker and is collected by ham.

 - Maintainer scripts of packages might interact with the user at this
   point by accessing the X-Server in the usual way.

 - When the operation has finished and ham has received the
   corresponding response from apt-worker, ham will re-request the
   list of packages and reinitialize its internal data structures.

Cancellation of requests sent to apt-worker has not been considered
extensively yet.

Security view
-------------

Apt-worker runs as root and can be used easily to perform arbitrary
actions as root; it provides instant privilege escalation.  Thus,
being "user" is equivalent to being "root" (security-wise) when
apt-worker is present in the system.