BUG: fatal interaction between application:ensure_all_started(A) and permit(B, false)
Michael Truog
mjtruog@REDACTED
Tue Mar 23 20:45:25 CET 2021
Most usage of application:ensure_all_started/1 wouldn't want it to hang
due to permissions because that wouldn't be fail-fast and the usage
would often be for testing that needs to fail (the system state is not
as expected).
Your desire to have the permissions cause the application start sequence
to hang would be part of manual interaction with the application start
sequence. So, it seems best to have that in a separate function that is
specifically for manual interaction with the applications, like
application:await_all_started/1 .
I would want to avoid functionality that can cause the execution to
hang, so I would avoid permissions and application:await_all_started/1
to keep the execution of higher-level source code predictable. One way
of resolving these separate use-cases is to have any hang-execution
functionality in a separate module, like application_shell to clearly
indicate the functionality is for manual interaction only.
Best Regards,
Michael
On 3/23/21 5:45 AM, Ulf Wiger wrote:
> Permissions are a bit tricky. :)
>
> The issue I encountered was the use of
> `application:ensure_all_started()` in `rebar3 shell`, which I think is
> perfectly appropriate use of the function.
> But if that use is supposed to mimic the starting of applications via
> a boot script, then it should either return `ok` (counter-intuitive),
> or hang until the permission flag(s) turn to true.
>
> The `application:close()` function should definitely remove the
> corresponding entries in `start_p_false`, and if there are such
> entries, not complain that the app hasn't been started.
>
> Basically, once an application has been started, the permission flag
> should toggle it on or off - if you call `permit(A, false)` on running
> application A, it should be stopped. A subsequent call to `permit(A,
> true)` should start it again.
>
> This is why I think it might be better for `ensure_all_started()` to
> hang rather than returning an error tuple. Alternatively, that a new
> function, `await_all_started()` does this, if that's what's desired.
> The question then becomes which one 'rebar3 shell` should use...
>
> BR,
> Ulf W
>
> On Tue, Mar 23, 2021 at 1:13 PM Fred Hebert <mononcqc@REDACTED
> <mailto:mononcqc@REDACTED>> wrote:
>
> I initially implemented `application:ensure_all_started/1,2'
> because it made for an easier get-started mechanism than getting a
> full blown release, and avoided having people do the annoying "try
> until it works" garbage routine by hand (which the function is now
> doing), or having to just line up all the start calls at the
> beginning of a test run.
>
> I would be in favour of just letting it fail and return back
> {error, {b, {permit, false}}} -- there are more variations we
> could use, but this term allows to:
> - specifically mention it's b that isn't allowed to start
> - uses the 'permit' keyword that matches the function name, such
> that googling something like "erlang permit false" will yield
> things such as the doc page for application:permit/2 (or this
> thread, given how google indexing works)
>
> That being said, it does not line up with the current API usage,
> which would probably need some fixing of some sort? Look at this
> session:
>
> 1> application:load(ssl).
> ok
> 2> application:permit(ssl, false).
> ok
> 3> application:ensure_all_started(crypto).
> {ok,[crypto]}
> 4> application:ensure_all_started(public_key).
> {ok,[asn1,public_key]}
> 5> application:start(ssl).
> ok
> 6> application:stop(ssl).
> {error,{not_started,ssl}}
>
> Starting the application "works" even if it does not with
> permissions; the call silently fails while returning it succeeded.
> Since the current documentation for ensure_all_started states the
> following:
>
> The function reports {error, {AppName,Reason}} for errors,
> where Reason is any possible reason returned by start/1,2
> <http://erlang.org/doc/apps/kernel/application.html#start-1>
> when starting a specific dependency.
>
> We are in a situation where ensure_all_started can't or won't know
> that permissions are at play (I didn't when I implemented it), and
> it appears that we would need to do an explicit permission check
> before each run to properly return the error explaining the issue
> without breaking the compatibility of start/1,2. The gotcha here
> is that there's apparently no way to access this status aside from
> application_controller internals that would need some extra
> visibility, which start/1,2 calls on its own as well.
>
> Regards,
> Fred.
>
> On Mon, Mar 22, 2021 at 1:06 PM Ulf Wiger <ulf@REDACTED
> <mailto:ulf@REDACTED>> wrote:
>
> Hmm, trying some more with OTP 24, it addresses the problem
> with the memory growth, but still isn't permission-aware.
>
> Consider test apps a and b, where a depends on b.
>
> 15> application:permit(b,false).
> ok
> 16> application:ensure_all_started(a).
> {error,{a,{not_started,b}}}
> 17> application:which_applications().
> [{stdlib,"ERTS CXC 138 10","3.15"},
> {kernel,"ERTS CXC 138 10","8.0"}]
> 18> application:permit(b,true).
> ok
> 19> application:which_applications().
> [{b,"test app","0.1"},
> {stdlib,"ERTS CXC 138 10","3.15"},
> {kernel,"ERTS CXC 138 10","8.0"}]
>
> The call to application:ensure_all_started(a) fails, and
> supposedly all child apps that were started will have been
> stopped again, and it does look that way.
>
> But if we later permit b to run, it turns out that the start
> request wasn't actually removed, and b pops up.
>
> This is for sure a much less serious problem than the previous
> one.
>
> However, I'm not sure if returning error is actually the right
> thing to do there. The call SHOULD probably hang.
>
> Comments?
>
> BR,
> Ulf W
>
> On Mon, Mar 22, 2021 at 1:23 PM Ulf Wiger <ulf@REDACTED
> <mailto:ulf@REDACTED>> wrote:
>
> When I started looking closer into this, it would appear
> as if there is a long-standing bug in the
> application_controller regarding permissions.
>
> And with "long-standing" I mean that it was there even
> when Kostis did some Tidier-based cleanup 11 years ago.
> Kostis didn't introduce it, though.
>
> When servicing a start request, the
> application_controller, if permission(App) == false, adds
> a new entry to the `start_p_false` list, i.e. a new entry
> for each request.
> https://github.com/erlang/otp/blob/master/lib/kernel/src/application_controller.erl#L689-L690
>
> ... but when servicing a subsequent {permit_application,
> App, true}, it uses lists:keydelete/3 to remove the App
> from the `start_p_false` list.
> https://github.com/erlang/otp/blob/master/lib/kernel/src/application_controller.erl#L759-L761
>
> lists:keydelete/3 obviously only removes the first
> matching entry.
>
> Earlier in that function, it also only locates the first
> pending request (or rather, chronologically the last), and
> uses the `From` in `spawn_starter()'.
>
> The rest of the pending requests should be handled
> somewhere - likely in `handle_application_started/3`, but
> aren't.
>
> BR,
> Ulf W
>
> On Sat, Mar 20, 2021 at 1:45 PM Mikael Pettersson
> <mikpelinux@REDACTED <mailto:mikpelinux@REDACTED>> wrote:
>
> On Sat, Mar 20, 2021 at 8:46 AM Ulf Wiger
> <ulf@REDACTED <mailto:ulf@REDACTED>> wrote:
> >
> > I had the brilliant idea of using application
> permissions for a particular use case. This seemed to
> work perfectly, until I ran `rebar3 shell`, and
> spotted some disturbing behavior.
> >
> > The bug, apparently, lies in that
> `application:ensure_all_started(A)` ends up
> busy-looping if A depends on B, and permission(B) ->
> false. What's worse, for each call to start(B), the
> application controller notices the permission flag,
> returns `ok` and inserts an entry in its internal
> `start_p_false` list. This amounts to a memory leak.
> >
> > I commented it in a tweet, then decided to try to
> find the source, esp. since I suspected
> `application:ensure_all_started/1`.
> >
> > https://twitter.com/uwiger/status/1372944356781531136
> >
> > In short, if permission(B) -> false, what happens is:
> > start(A) -> {error, {not_started, B}}
> > start(B) -> ok
> > start(A) -> {error, {not_started, B}}
> > ... [repeat endlessly]
> >
> > Now, it could be fixed by adding a permission check
> in the looping function, but this raises the question
> of what should happen in the above case. Three
> alternatives:
> >
> > 1. ensure_all_started(A) returns {error,
> {not_permitted, B}}, or something
> > 2. the call hangs until the flag(s) change, but
> start(B) is only called once.
> > 3. Warn against the use of permissions in the docs,
> and deprecate them.
> >
> > I'm assuming that most of you may not even know
> about permissions. They were introduced back in
> 1996-97 (I believe), when I and Martin Björklund were
> going back and forth on how to support distributed
> applications and cluster control. Eventually, this led
> to dist_ac and the protocol being defined, so that
> users could write a controller app taking control of
> an application and giving instructions on where it
> should run. In the AXD301, this was done by the RCM
> application. I believe I talked about it at the EUC
> 1997, but it's hard to find information about that on
> the web. :)
> >
> > Anyway, permissions were left in the API, and ARE
> documented.
> >
> > Thoughts?
>
> I know we've used the permissions mechanism
> occasionally during
> maintenance or live upgrades. Off-hand I don't know if
> we'd want
> alternative 1 or 2 (my colleague Daniel Szoboszlay
> might know more
> about this).
>
> /Mikael
>
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