Don't store child that returns ignore in simple supervisor

If a child of a simple_one_for_one returns ignore from its start
function no longer store the child for any restart type. It is not
possible to restart or delete the child because the supervisor is a
simple_one_for_one.

Previously a simple_one_for_one would crash, potentially without
shutting down all of its children, when it tried to shutdown a child
with undefined pid.

Previous only one undefined pid child was stored in a simple_one_for_one
supervisor no matter how many times the child start function returned
ignore.

1 files changed, 9 insertions, 3 deletions

diff --git a/lib/stdlib/doc/src/supervisor.xml b/lib/stdlib/doc/src/supervisor.xml
index ffac1c0bd7..6ff477a42d 100644
--- a/lib/stdlib/doc/src/supervisor.xml
+++ b/lib/stdlib/doc/src/supervisor.xml
@@ -386,9 +386,15 @@
           added to the supervisor and the function returns the same
           value.</p>
         <p>If the child process start function returns <c>ignore</c>,
-          the child specification is added to the supervisor, the pid
-          is set to <c>undefined</c>, and the function returns
-          <c>{ok,undefined}</c>.</p>
+          the child specification is added to the supervisor (unless the
+          supervisor is a <c>simple_one_for_one</c> supervisor, see below),
+          the pid is set to <c>undefined</c> and the function returns
+          <c>{ok,undefined}</c>.
+        </p>
+        <p>In the case of a <c>simple_one_for_one</c> supervisor, when a child
+          process start function returns <c>ignore</c> the functions returns
+          <c>{ok,undefined}</c> and no child is added to the supervisor.
+        </p>
         <p>If the child process start function returns an error tuple or
           an erroneous value, or if it fails, the child specification is
           discarded, and the function returns <c>{error,Error}</c> where