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

[Dániel Szoboszlay]

Hi,

The use case Mikael mentioned is that we have an application (not a
distributed application by the way) that may or may not be started at the
time of the upgrade; but if it is started, it needs to be stopped just
before the upgrade and restarted right after it. This can be easily
achieved by simply disabling the application for the time of the upgrade.
The application controller will remember whether it was started when we
disabled it and will restart if iaccordingly when we enable it again. It is
just more convenient this way than storing the original started/stopped
state of the application somewhere else for the time of the upgrade.

For our use case 1. and 2. are equally acceptable, but 3. is obviously not.

I'm not sure which solution should be preferred. I have a feeling that
there are some significant semantic inconsistencies on how application
dependencies and permissions work. Like application:start(B) would return
ok when B is not permitted, but then you cannot start A. But if later on
you permit B it gets started, but A does not. And even though you can only
start A if B is both permitted and started, once A is running, stopping or
disabling B would leave A running. Maybe all applications shall have a new
state that says "start blocked by disabled dependency". Then
ensure_all_started(A) could simply return ok, and leave A in this state
until you permit B.

/Daniel

On Sat, 20 Mar 2021 at 13:45, 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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