BUG: fatal interaction between application:ensure_all_started(A) and permit(B, false)

[Ulf Wiger]

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> 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>
> wrote:
>
>> On Sat, Mar 20, 2021 at 8:46 AM Ulf Wiger <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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