DynamicSupervisor max_children is not actually the max number of children

[sunboshan]

$ elixir -v
Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false] [dtrace]

Elixir 1.6.5 (compiled with OTP 20)

Mac OS X 10.12.6

DynamicSupervisor use internal state dynamic to track how many children has been started.

• The value increase by 1 when start_child/2 has been called. code
• The value decreased by 1 when delete_child/2 has been called. code

However, if a permanent child crashed, the value is decreased, which will make the dynamic value less than the actually number of children(since the child got restarted, however the dynamic value has decreased by 1).

Repro:

iex(1)> {:ok,dsup} = DynamicSupervisor.start_link(strategy: :one_for_one, max_children: 1)
{:ok, #PID<0.135.0>}
iex(2)> {:ok,child}=DynamicSupervisor.start_child(dsup,Foo)
{:ok, #PID<0.137.0>}
iex(3)> DynamicSupervisor.start_child(dsup,Foo)
{:error, :max_children}
iex(4)> Process.exit(child,1)
true
iex(5)> DynamicSupervisor.start_child(dsup,Foo)
{:ok, #PID<0.142.0>}
iex(6)> :sys.get_state(dsup)
%DynamicSupervisor{
  ...
  children: %{
    #PID<0.140.0> => {{Foo, :start_link, [[]]}, :permanent, 5000, :worker,
     [Foo]},
    #PID<0.142.0> => {{Foo, :start_link, [[]]}, :permanent, 5000, :worker,
     [Foo]}
  },
  dynamic: 1,
  ...
}

As shown, there are 2 children even given max_children as 1 for our dynamic supervisor.

[josevalim]

Perfect. I have a fix coming out soon.