From 51e3c384854e2bc27fd82bbd016ea52d4556f03d Mon Sep 17 00:00:00 2001
From: Jakub Witczak <kuba@erlang.org>
Date: Wed, 11 Sep 2024 16:55:27 +0200
Subject: [PATCH 1/2] stdlib: customize unlink_flush for noproc scenario

- handle termination race in supervisor
- race happens when supervisor shutdown procedure
- collides with child terminating from other reason
-
- in ssh tests this happens when client closes connection
- which propagates to server over network
- and results with termination of connection processes on server side
- in parallel, server being is shutdown during test cleanup
---
 lib/stdlib/src/supervisor.erl | 25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/lib/stdlib/src/supervisor.erl b/lib/stdlib/src/supervisor.erl
index 58b943d874f8..bb1b2456349a 100644
--- a/lib/stdlib/src/supervisor.erl
+++ b/lib/stdlib/src/supervisor.erl
@@ -968,16 +968,25 @@ shutdown(#child{pid=Pid, shutdown=Time} = Child) ->
         end
     end.
 
-unlink_flush(Pid, DefaultReason) ->
-    %% We call unlink in order to guarantee that the 'EXIT' has arrived
-    %% from the dead process. See the unlink docs for details.
+unlink_flush(Pid, noproc) ->
+    {links, Ls} = process_info(self(),links),
+    Timeout = case lists:member(Pid, Ls) of
+                  true -> infinity;
+                  false -> 0
+              end,
+    receive
+        {'EXIT', Pid, ExitReason} ->
+            ExitReason
+    after Timeout ->
+            naughty_child
+    end;
+unlink_flush(Pid, ExitReason) ->
     unlink(Pid),
     receive
-        {'EXIT', Pid, Reason} ->
-            Reason
-    after 0 ->
-        DefaultReason
-    end.
+        {'EXIT', Pid, _} -> ok
+    after 0 -> ok
+    end,
+    ExitReason.
 
 %%-----------------------------------------------------------------
 %% Func: terminate_dynamic_children/1

From 18cba2366be1abd901de0261b219497771f8e817 Mon Sep 17 00:00:00 2001
From: Jakub Witczak <u3s@users.noreply.github.com>
Date: Thu, 26 Sep 2024 09:03:38 +0200
Subject: [PATCH 2/2] Apply suggestions from code review

Co-authored-by: Rickard Green <rickard@erlang.org>
---
 lib/stdlib/src/supervisor.erl | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/lib/stdlib/src/supervisor.erl b/lib/stdlib/src/supervisor.erl
index bb1b2456349a..2cfa8a542559 100644
--- a/lib/stdlib/src/supervisor.erl
+++ b/lib/stdlib/src/supervisor.erl
@@ -970,6 +970,13 @@ shutdown(#child{pid=Pid, shutdown=Time} = Child) ->
 
 unlink_flush(Pid, noproc) ->
     {links, Ls} = process_info(self(),links),
+    %% We know that the process has terminated. If we still have a link, we are
+    %% guaranteed to eventually receive the 'EXIT' message containing the
+    %% actual exit reason (or a 'noconnection' exit reason if the connection is
+    %% lost). If we do not have a link, the 'EXIT' message is already present
+    %% in the message queue unless the child process behaved badly (unlinked
+    %% itself from us). If it behaved badly, we may or may not receive an 'EXIT'
+    %% message.
     Timeout = case lists:member(Pid, Ls) of
                   true -> infinity;
                   false -> 0
@@ -978,9 +985,10 @@ unlink_flush(Pid, noproc) ->
         {'EXIT', Pid, ExitReason} ->
             ExitReason
     after Timeout ->
-            naughty_child
+            child_process_unlinked
     end;
 unlink_flush(Pid, ExitReason) ->
+    %% Leave no 'EXIT' message from this process in the message queue.
     unlink(Pid),
     receive
         {'EXIT', Pid, _} -> ok