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From Anis KADRI <anis.ka...@gmail.com>
Subject Re: Wiping plugins on navigation
Date Thu, 06 Sep 2012 18:35:29 GMT
I am also in favor of destroying plugins on page refreshes (not recreating
them). If anything, just to comply with the web app architecture which is
supposed to be somehow stateless.
This problem also applies to javascript-based platforms like Bada. Because
even though our implementation is only a javascript shim, the vendor's
implementation is also using some sort of a bridge [1].
The current mobile-spec doesn't work on Bada. The app just segfaults with a
big core dump, which a I didn't think was possible. I am guessing it is
because they're not doing any cleanup either, expecting people to develop
single page apps. Not much we can do other than implementing our own bridge
using [1] and not [2]

[1]
http://developer.bada.com/help_2.0/index.jsp?topic=/com.osp.cppappprogramming.help/html/tutorials/web_tutorial/using_javascriptbridge.htm
[2]
http://developer.bada.com/help_2.0/index.jsp?topic=/com.osp.webapireference.help/symbols/index/main.html

On Thu, Sep 6, 2012 at 9:57 AM, Andrew Grieve <agrieve@chromium.org> wrote:

> On Thu, Sep 6, 2012 at 11:41 AM, Braden Shepherdson <braden@chromium.org
> >wrote:
>
> > I disagree with the latency fears for regenerating plugins. I've just
> > sampled twenty plugins for Android and a handful for iOS (I don't know
> > Objective-C, so I can't reliably say much about these), and the large
> > majority of Android plugins don't define constructors at all. Those that
> do
> > are usually empty. I only saw one that had actual code, and it was simply
> > storing the result of a .getInstance() call on some other class into a
> > member variable. Similarly, I didn't see any member variables with
> > expensive initializers.
> >
> > Therefore I don't think we need to worry about the cost of destroying and
> > recreating a couple dozen classes with (nearly) empty constructors.
> >
> > As to Jimmy Jarvis' suggestion on the bug, I don't know if we can do IDs
> > that persist across refreshes. We could make callback IDs include the
> name
> > of the plugin, but each plugin over its lifetime may make many requests
> and
> > have multiple outstanding requests at a time. That requires a count, and
> we
> > can't persist that count across page reloads since it and the map of
> > callbacks reside in Javascript. We could use a hash function so that
> > callback IDs are only very unlikely to collide, but that seems less than
> > ideal.
> >
> Callback IDs do already include plugin names.
> I think his suggestion was to use a count, but just start the count at a
> random number instead of at zero.
>
>
> >
> > One advantage of the destroy-and-regenerate approach to plugins is that
> > they cannot get themselves into a bad state on navigation, because they
> > will always be starting fresh. As noted elsewhere, on purely
> > Javascript-powered platforms, is it even possible for plugins to persist
> > across navigation without playing games like using persistent storage?
> >
>
> Thinking about this more, I think it probably doesn't even help things much
> to recreate the plugin objects since that won't prevent old plugins from
> running. Old plugins can still send JS to be executed (unless we null out
> their webview reference maybe?), and if they have some background task
> ongoing, it will continue to run and be annoying / drain battery.
>
> I think really the only good option is to make sure reset() (or dispose()
> or whatever) is implemented on them when it needs to be. Beyond that, it
> would be helpful to detect when plugins misbehave, so maybe nulling out
> their webView reference and logging an error if they try to call
> sendJavascript is the answer to this part.
>
>
>
> >
> >
> > On Thu, Sep 6, 2012 at 11:27 AM, Braden Shepherdson <braden@chromium.org
> > >wrote:
> >
> > > Repeating here a comment on the bug:
> > >
> > > Jimmy Jarvis<
> > https://issues.apache.org/jira/secure/ViewProfile.jspa?name=jiminyjarvis
> >
> > added
> > > a comment - 05/Sep/12 23:02 - edited
> > >
> > > JSON-P uses a unique callback identifier, similar to the proposed fix
> > > above, and is far more reliable and easier to debug than reset with
> > > repeating IDs. It would be unfortunate overhead to have to reinitialize
> > our
> > > native logic every time a page reloads. The app would become far less
> > > responsive, at least for us. If a plugin is a bunch of stubs, it's not
> an
> > > big deal as you say – however, if it is a longer running setup process
> > ----
> > > I'd prefer ignoring unmatched callbacks. I agree sending the plugin a
> > Reset
> > > or Terminate message would enable the plugin authors an opportunity to
> do
> > > proper cleanup on pending requests, but even if they do not, unique
> ID's
> > > (like JSON-P) are a better solution than repeating IDs.
> > >
> > >
> > > On Wed, Sep 5, 2012 at 9:33 PM, Andrew Grieve <agrieve@chromium.org
> > >wrote:
> > >
> > >> Plugins could probably use static fields if they wanted to maintain
> some
> > >> state between page changes. I think it should be extremely rare to do
> so
> > >> though. e.g. For the platforms that implement their plugins in JS, is
> it
> > >> even possible to maintain state between page changes?
> > >>
> > >> One nice outcome of this is that it will make our mobile-spec pages
> less
> > >> likely to affect each other.
> > >>
> > >>
> > >> On Wed, Sep 5, 2012 at 6:59 PM, Brian LeRoux <b@brian.io> wrote:
> > >>
> > >> > Yes I do not refuting Cordova needs to work for both single page and
> > >> > multipage apps. Just saying there is a solution to this problem. ;P
> > >> >
> > >> >
> > >> > On Wed, Sep 5, 2012 at 3:49 PM, Jesse <purplecabbage@gmail.com>
> > wrote:
> > >> > > Whether it is an edge case, or a common case, multi-page apps
are
> a
> > >> > > reality, so we definitely need to notify plugins when the page
is
> > >> > > changing.
> > >> > > I don't necessarily agree that the plugin should be destroyed
and
> > >> > > recreated though, I can think of several cases where persistence
> > would
> > >> > > be nice to have.
> > >> > >
> > >> > > I also do not see this as a security issue.  Security is already
> > >> > > governed by the white-list, so non-trusted pages cannot access
> > device
> > >> > > functions.  If a plugin needs additional security, then it should
> be
> > >> > > built into the plugin, and not the responsibility of the
> framework.
> > >> > > ... Thinking of a SuperCookie plugin which uses the domain of
the
> > >> > > currently loaded page before deciding what to return, or something
> > >> > > similar.
> > >> > >
> > >> > > My 2 sense,
> > >> > >   Jesse
> > >> > >
> > >> > >
> > >> > > On Wed, Sep 5, 2012 at 3:30 PM, Bryce Curtis <
> > curtis.bryce@gmail.com>
> > >> > wrote:
> > >> > >> Sometimes multi-page apps are needed or you navigate from
your
> app
> > to
> > >> > >> another page.  One bug we ran into was that callback ids
are
> reused
> > >> > >> when loading a new page.  So, a plugin trying to send data
back
> to
> > >> the
> > >> > >> original page could be calling a recycled plugin with erroneous
> > data.
> > >> > >> In addition to the bugs, there is also a security issue with
a
> > >> > >> subsequent page being able to access a plugin that was used
in a
> > >> > >> previous page.
> > >> > >>
> > >> > >> The app/page lifecycle events are propagated to the plugins,
and
> > the
> > >> > >> plugins are destroyed when loading a new page.  However,
looking
> at
> > >> > >> the code, it appears this may be broken now.  (At least for
> > Android).
> > >> > >>
> > >> > >> On Wed, Sep 5, 2012 at 3:40 PM, Braden Shepherdson <
> > >> braden@chromium.org>
> > >> > wrote:
> > >> > >>> Sure, and I'm a fan of single-page apps (I do work for
Google,
> > after
> > >> > >>> all...), but this causes very chaotic, hard-to-track
bugs, so it
> > >> makes
> > >> > >>> sense to be robust over a refresh/navigation.
> > >> > >>>
> > >> > >>>
> > >> > >>> On Wed, Sep 5, 2012 at 5:25 PM, Brian LeRoux <b@brian.io>
> wrote:
> > >> > >>>
> > >> > >>>> One thing to note, we tend to advise ppl author single
page web
> > >> apps
> > >> > >>>> which makes state visibility change an app logic
concern (and
> > avoid
> > >> > >>>> this issue from manifesting). Generally, we can say
a page
> > refresh
> > >> is
> > >> > >>>> not a great user experience in apps.
> > >> > >>>>
> > >> > >>>> On Wed, Sep 5, 2012 at 1:15 PM, Braden Shepherdson
<
> > >> > braden@chromium.org>
> > >> > >>>> wrote:
> > >> > >>>> > This is intended as a continuation of the discussion
started
> in
> > >> > >>>> > https://issues.apache.org/jira/browse/CB-1318
.
> > >> > >>>> >
> > >> > >>>> > The bug in question is one where one page starts
a long
> native
> > >> side
> > >> > >>>> action
> > >> > >>>> > such as a network call. Then the user navigates
the app to
> > >> another
> > >> > page.
> > >> > >>>> > When the long action completes, the call returns
and the
> > >> appropriate
> > >> > >>>> > Javascript callback is looked up and called.
> > >> > >>>> >
> > >> > >>>> > However when the page is navigated, the counter
that provides
> > >> > supposedly
> > >> > >>>> > unique names for callbacks is reset, allowing
a callback on
> the
> > >> new
> > >> > page
> > >> > >>>> to
> > >> > >>>> > have the same name as the callback from the
old page. It then
> > >> gets
> > >> > called
> > >> > >>>> > incorrectly, potentially introducing weird and
transient
> bugs.
> > >> > >>>> >
> > >> > >>>> > The proposed solution is to do the following
on navigation:
> > >> > >>>> > - Call a destroy() call on all plugins, which
by default does
> > >> > nothing.
> > >> > >>>> This
> > >> > >>>> > allows the plugins a chance to cancel any outstanding
network
> > >> > requests or
> > >> > >>>> > do any other cleanup work.
> > >> > >>>> > - Delete the plugin instance and recreate it.
> > >> > >>>> >
> > >> > >>>> > In the bug I also said one step would be to
wipe the callback
> > >> table
> > >> > in
> > >> > >>>> the
> > >> > >>>> > Javascript, but that isn't necessary since it
would have been
> > >> wiped
> > >> > by
> > >> > >>>> the
> > >> > >>>> > navigation anyway.
> > >> > >>>> >
> > >> > >>>> > This issue is cross-platform-ish. It (probably)
doesn't apply
> > to
> > >> > >>>> web-based
> > >> > >>>> > platforms like WebOS or Bada, because the plugins
are
> > Javascript
> > >> > shims
> > >> > >>>> > rather than native code, and are wiped on navigation
like any
> > >> other
> > >> > >>>> > Javascript. However this issue does exist on
at least Android
> > and
> > >> > iOS,
> > >> > >>>> and
> > >> > >>>> > probably a few others as well.
> > >> > >>>> >
> > >> > >>>> > I'm proposing to implement the solution outlined
above on
> > Android
> > >> > and
> > >> > >>>> iOS.
> > >> > >>>> > I don't have the devices or environment to do
any other
> > >> platforms,
> > >> > nor
> > >> > >>>> am I
> > >> > >>>> > sure which are necessary. The maintainers of
other platforms
> > will
> > >> > have to
> > >> > >>>> > consider this problem for their platform. I
would also update
> > the
> > >> > core
> > >> > >>>> > plugins to define a destroy() method if they
have relevant
> > >> cleanups
> > >> > to
> > >> > >>>> make.
> > >> > >>>> >
> > >> > >>>> > Thoughts on the approach, things I'm missing?
> > >> > >>>> >
> > >> > >>>> > Braden
> > >> > >>>>
> > >> > >
> > >> > >
> > >> > >
> > >> > > --
> > >> > > @purplecabbage
> > >> > > risingj.com
> > >> >
> > >>
> > >
> > >
> >
>

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