'Re: Review Request 121710: Avoid risk of starting two kscreen_launchers at the same having race cond'

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List:       kde-panel-devel
Subject:    Re: Review Request 121710: Avoid risk of starting two kscreen_launchers at the same having race cond
From:       "David Edmundson" <david () davidedmundson ! co ! uk>
Date:       2014-12-31 10:50:15
Message-ID: 20141231105015.18352.4271 () probe ! kde ! org
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> On Dec. 30, 2014, 2:06 p.m., Daniel Vrátil wrote:
> > I guess the code is note perfect and there are still race conditions, but this is \
> > a good step in the right direction, so let's ship it. I'll think about the \
> > lockfile solution, or some other and try to implement it (once I'm back at work). \
> >  The one-service-multiple-objects solution would not work, since you can't have \
> > multiple processes owning the same DBus service name, and I don't want to run \
> > multiple backends in one process.

Why would you ever want to run multiple backends at the same time?
(outside of testing, which could be done on a private DBus session)

- David

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This is an automatically generated e-mail. To reply, visit:
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On Dec. 28, 2014, 10:33 a.m., David Edmundson wrote:
> 
> -----------------------------------------------------------
> This is an automatically generated e-mail. To reply, visit:
> https://git.reviewboard.kde.org/r/121710/
> -----------------------------------------------------------
> 
> (Updated Dec. 28, 2014, 10:33 a.m.)
> 
> 
> Review request for Plasma, Solid and Daniel Vrátil.
> 
> 
> Repository: libkscreen
> 
> 
> Description
> -------
> 
> Avoid risk of starting two kscreen_launchers at the same having a race condition.
> 
> There were three possible bugs:
> CheckIsAlreadyRunning tried to register a service and check if it worked.
> This could clash with another process checking at the same time. Causing them both \
> to fail saying another is running Similarly, a daemon doing actual registering \
> could clash with another daemon just checking if the name is free, and then it \
> would fail saying we can't init() 
> There was also a risk that two launchers pass the check that nothing is running, \
> then both try to activate a session. DBus server handles this fine and one will \
> gracefully fail. Without this patch the second launcher would just die without \
> returning the path of the service that was activated causing the relevant app to do \
> nothing. 
> 
> --
> 
> IMHO, you'd be better off having a fixed service name and using DBus activation for \
> exactly these reasons. You could put the different backends at different object \
> paths, and have a method on the root object that says which object path to use \
> rather than using the stdout of a launcher. That's a discussion for another day \
> though. 
> 
> Diffs
> -----
> 
> src/backendlauncher/backendloader.cpp e7da8cd 
> src/backendlauncher/main.cpp f8bf323 
> 
> Diff: https://git.reviewboard.kde.org/r/121710/diff/
> 
> 
> Testing
> -------
> 
> Send it to bshah. Plasmashell started for him. Previously it didn't.
> 
> 
> Thanks,
> 
> David Edmundson
> 
> 

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      This is an automatically generated e-mail. To reply, visit:
      <a href="https://git.reviewboard.kde.org/r/121710/">https://git.reviewboard.kde.org/r/121710/</a>
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<blockquote style="margin-left: 1em; border-left: 2px solid #d0d0d0; padding-left: \
10px;">  <p style="margin-top: 0;">On December 30th, 2014, 2:06 p.m. UTC, <b>Daniel \
Vrátil</b> wrote:</p>  <blockquote style="margin-left: 1em; border-left: 2px solid \
#d0d0d0; padding-left: 10px;">  <pre style="white-space: pre-wrap; white-space: \
-moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: \
break-word;"><p style="padding: 0;text-rendering: inherit;margin: 0;line-height: \
inherit;white-space: inherit;">I guess the code is note perfect and there are still \
race conditions, but this is a good step in the right direction, so let's ship it. \
I'll think about the lockfile solution, or some other and try to implement it (once \
I'm back at work).</p> <p style="padding: 0;text-rendering: inherit;margin: \
0;line-height: inherit;white-space: inherit;">The one-service-multiple-objects \
solution would not work, since you can't have multiple processes owning the same DBus \
service name, and I don't want to run multiple backends in one process.</p></pre>  \
</blockquote>

</blockquote>

<pre style="white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: \
-pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;"><p style="padding: \
0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: inherit;">Why \
would you ever want to run multiple backends at the same time? (outside of testing, \
which could be done on a private DBus session)</p></pre> <br />

<p>- David</p>

<br />
<p>On December 28th, 2014, 10:33 a.m. UTC, David Edmundson wrote:</p>

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<div>Review request for Plasma, Solid and Daniel Vrátil.</div>
<div>By David Edmundson.</div>

<p style="color: grey;"><i>Updated Dec. 28, 2014, 10:33 a.m.</i></p>

<div style="margin-top: 1.5em;">
 <b style="color: #575012; font-size: 10pt;">Repository: </b>
libkscreen
</div>

<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Description </h1>
 <table width="100%" bgcolor="#ffffff" cellspacing="0" cellpadding="10" \
style="border: 1px solid #b8b5a0">  <tr>
  <td>
   <pre style="margin: 0; padding: 0; white-space: pre-wrap; white-space: \
-moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: \
break-word;"><p style="padding: 0;text-rendering: inherit;margin: 0;line-height: \
inherit;white-space: inherit;">Avoid risk of starting two kscreen_launchers at the \
same having a race condition.</p> <p style="padding: 0;text-rendering: \
inherit;margin: 0;line-height: inherit;white-space: inherit;">There were three \
possible bugs: CheckIsAlreadyRunning tried to register a service and check if it \
worked. This could clash with another process checking at the same time. Causing them \
both to fail saying another is running Similarly, a daemon doing actual registering \
could clash with another daemon just checking if the name is free, and then it would \
fail saying we can't init()</p> <p style="padding: 0;text-rendering: inherit;margin: \
0;line-height: inherit;white-space: inherit;">There was also a risk that two \
launchers pass the check that nothing is running, then both try to activate a \
session. DBus server handles this fine and one will gracefully fail. Without this \
patch the second launcher would just die without returning the path of the service \
that was activated causing the relevant app to do nothing.</p> <p style="padding: \
0;text-rendering: inherit;margin: 0;line-height: inherit;white-space: \
inherit;">--</p> <p style="padding: 0;text-rendering: inherit;margin: 0;line-height: \
inherit;white-space: inherit;">IMHO, you'd be better off having a fixed service name \
and using DBus activation for exactly these reasons. You could put the different \
backends at different object paths, and have a method on the root object that says \
which object path to use rather than using the stdout of a launcher. That's a \
discussion for another day though.</p></pre>  </td>
 </tr>
</table>

<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Testing </h1>
<table width="100%" bgcolor="#ffffff" cellspacing="0" cellpadding="10" style="border: \
1px solid #b8b5a0">  <tr>
  <td>
   <pre style="margin: 0; padding: 0; white-space: pre-wrap; white-space: \
-moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: \
break-word;"><p style="padding: 0;text-rendering: inherit;margin: 0;line-height: \
inherit;white-space: inherit;">Send it to bshah. Plasmashell started for him. \
Previously it didn't.</p></pre>  </td>
 </tr>
</table>

<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Diffs</b> </h1>
<ul style="margin-left: 3em; padding-left: 0;">

 <li>src/backendlauncher/backendloader.cpp <span style="color: \
grey">(e7da8cd)</span></li>

 <li>src/backendlauncher/main.cpp <span style="color: grey">(f8bf323)</span></li>

</ul>

<p><a href="https://git.reviewboard.kde.org/r/121710/diff/" style="margin-left: \
3em;">View Diff</a></p>

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