[packagekit] [Q] How "GetDepends" connect with "get_depends"

[NZzi]

Richard Hughes wrote:
> On Tue, 2008-09-16 at 15:03 +0800, NZzi wrote:
>> i wonder how "pkcon get-updates package" implement,
>> so i browse packagekit code, but i know little
>> about dbus, so after a whole day, i still can not
>> understand how "GetDepends" connect with "get_depends".
>>
>> i browse from pk-console.c:
>>
>> } else if (strcmp (mode, "get-depends") == 0) {
>> 	...
>> 	ret = pk_console_get_depends (client, filters, value, &error);
>> }
>>
>> pk_console_get_depends()->pk_client_get_depends()->
>> ret = dbus_g_proxy_call (client->priv->proxy, "GetDepends", error,...)
> 
> Correct, the pkcon program is just a front-end to the libpackagekit
> library[4], which in turn is a front-end to the DBUS interface.
> 
>> i study some dbus docs, i konw dbus_g_proxy_call() call
>> "GetDepends" method through proxy object(client->priv->proxy),
>> but the "real" dependency-process function is get-depends() in
>> yumBackend.py. and i found it's backend_get_depends()(in
>> pk-backend-yum.c) invokd the get-depends().
>>
>> but i don't know how dbus_g_proxy_call (client->priv->proxy, 
>> "GetDepends"...)
>> invoke backend_get_depends()(in pk-backend-yum.c)
>>
>> can anyone give me some hints, thanks advance
> 
> No problem, thanks for taking the time to read the code. The actual
> process is actually pretty simple:
> 
> The dbus server pk-engine.c takes the request for a TID [1] (transaction
> ID) and creates a new PkTransaction object (and files it in a
> PkTransactionList object. When this object is called with GetDepends,
> (pk-transaction.c) takes the request[3], and saves all the parameters,
> checks permissions and that sort of thing.
> 
> When PkTransactionList detects that nothing else is running, the new
> PkTransaction is "run" which means that it takes control of the single
> PkBackend instance and the backend is run and waits for that transaction
> to finish.


thanks very much for your detail explanations.

sorry, i know nothing about glib and dbus, so can you
explain more about the process of "daemon schedules requests".

after frontend(client) send "GetDepends" method call
request through proxy object(transaction interface),
how packagekitd react in source code? i only know
packagekitd go into glib loop, after create engine,
and etc.

After getting the "GetDepends" method call request,
how packagekitd call backend->desc->get_depends?


> 
> Now, that explains how the daemon schedules requests (as the backend can
> only ever do one thing at a time) but doesn't explain how the yum code
> is actually run.
> 
> In the /etc/PackageKit/PackageKit.conf file you'll have a DefaultBackend
> option, which will point to yum. When the daemon starts up, it dlopens a
> shared object file for whichever the default backend is, in this case
> you can see the code in pk-backend-yum.c
> 
> This code maps a vtable (a table of functions) to the PkBackend
> instance, so that when the functions in PkBackend are called, these are
> mapped to the functions in pk-backend-yum.c


how "GetDepends" map to backend_get_depends() in PkBackendDesc.

i read following doc:

http://www.packagekit.org/gtk-doc/backends-spawn-methods.html

which said "Get Depends" is mapped from the helper to either
command line programs or a dispatcher in the spawn helper, but
i browse all codes, still cannot understand how the map
implemented.


> 
> These functions are responsible for launching the yum executable
> yumBackend.py (using pk-backend-spawn.c, which in turn uses pk-spawn.c)
> and watches the standard out for replies, and then when it's done it
> kills the program. [2]
> 
> In the new dispatcher [5], it gets a little bit more complicated as the
> spawned executable is not closed down after every transaction, and
> instead can stay running in the backend for a few seconds. This makes
> using the GUI a lot snappier, as yum isn't being created and destroyed
> all the time.
> 
> Actually, when reading that back, it doesn't seem so simple anymore!
> 
> Feel free to ask about any of that stuff I've explained badly. I
> probably need to add something like this in the code anyway.
> 
> Richard.
> 
> [1] http://www.packagekit.org/gtk-doc/introduction-ideas-transactions.html
> [2] http://www.packagekit.org/gtk-doc/backend-spawn.html
> [3] http://www.packagekit.org/gtk-doc/Transaction.html
> [4] http://www.packagekit.org/gtk-doc/PkClient.html
> [5] http://article.gmane.org/gmane.comp.freedesktop.packagekit/3237
> 
> 
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