Fix peer-related testsuite noise

Crashes like the one below after commit 0dad730b were just noise, but
caused by the process running peer:start_link/1 exiting with a
meaningful reason for the purpose of monitoring. Start from a middleman
process to work around it.

=ERROR REPORT==== 18-Mar-2022::10:50:22.260471 ===
** Generic server <0.139.0> terminating
** Last message in was {'EXIT',<0.135.0>,
                               {#Ref<0.2812022913.2652372994.101846>,[]}}
** When Server state == {peer_state,#{args =>
                                          ["-pa",
                                           "/home/anders/dev/otp/lib/diameter/test",
                                           "/home/anders/dev/otp/lib/diameter/test/../ebin",
                                           "-setcookie",
                                           "zzz"],
                                      name => "diametertcpserver"},
                                    diametertcpserver@elxa6vcfsf2,
                                    "/home/anders/dev/erlang/94c81bfd/erts-12.2.1/bin/erl",
                                    ["-sname","diametertcpserver","-detached",
                                     "-noinput","-user","peer","-origin",
                                     "g1hkACNkaWFtZXRlcl90ZXN0X19leGFtcGxlc0BlbHhhNnZjZnNmMgAAAIsAAAAAYjQeQw==",
                                     "-pa",
                                     "/home/anders/dev/otp/lib/diameter/test",
                                     "/home/anders/dev/otp/lib/diameter/test/../ebin",
                                     "-setcookie","zzz"],
                                    undefined,undefined,<<>>,running,
                                    {<0.135.0>,
                                     #Ref<0.2812022913.2652372994.101857>},
                                    0,#{}}
** Reason for termination ==
** {#Ref<0.2812022913.2652372994.101846>,[]}

=CRASH REPORT==== 18-Mar-2022::10:50:22.267130 ===
  crasher:
    initial call: peer:init/1
    pid: <0.139.0>
    registered_name: []
    exception exit: {#Ref<0.2812022913.2652372994.101846>,[]}
      in function  gen_server:decode_msg/9 (gen_server.erl, line 488)
    ancestors: [<0.135.0>]
    message_queue_len: 1
    messages: [{nodedown,diametertcpserver@elxa6vcfsf2}]
    links: []
    dictionary: []
    trap_exit: true
    status: running
    heap_size: 6772
    stack_size: 28
    reductions: 11827
  neighbours:

4 files changed, 47 insertions, 19 deletions

diff --git a/lib/diameter/test/diameter_dist_SUITE.erl b/lib/diameter/test/diameter_dist_SUITE.erl
index 7c66a711c8..81d2dfbf36 100644
--- a/lib/diameter/test/diameter_dist_SUITE.erl
+++ b/lib/diameter/test/diameter_dist_SUITE.erl
@@ -134,15 +134,11 @@ traffic() ->
 enslave() ->
     Here = filename:dirname(code:which(?MODULE)),
     Ebin = filename:join([Here, "..", "ebin"]),
-    Dirs = [Here, Ebin],
-    Args =  [["-pa" | [lists:flatten(io_lib:format("~s", [D])) || D <- Dirs]],
-             ["-setcookie", ?L(erlang:get_cookie())]],
-    [{N,S} || A <- [lists:append(Args)],
-              {M,S} <- ?NODES,
-              N <- [start(M, A)]].
+    Args = ["-pa", Here, Ebin],
+    [{N,S} || {M,S} <- ?NODES, N <- [start(M, Args)]].
 
 start(Name, Args) ->
-    {ok, _, Node} = peer:start_link(#{name => Name, args => Args}),
+    {ok, _, Node} = ?util:peer(#{name => Name, args => Args}),
     Node.
 
 %% ping/1
diff --git a/lib/diameter/test/diameter_distribution_SUITE.erl b/lib/diameter/test/diameter_distribution_SUITE.erl
index 364a90f3ba..013c42496e 100644
--- a/lib/diameter/test/diameter_distribution_SUITE.erl
+++ b/lib/diameter/test/diameter_distribution_SUITE.erl
@@ -132,15 +132,11 @@ traffic() ->
 enslave() ->
     Here = filename:dirname(code:which(?MODULE)),
     Ebin = filename:join([Here, "..", "ebin"]),
-    Dirs = [Here, Ebin],
-    Args = [["-pa" | [lists:flatten(io_lib:format("~s", [D])) || D <- Dirs]],
-            ["-setcookie", ?L(erlang:get_cookie())]],
-    [{N,S} || A <- [lists:append(Args)],
-              {M,S} <- ?NODES,
-              N <- [start(M, A)]].
+    Args = ["-pa", Here, Ebin],
+    [{N,S} || {M,S} <- ?NODES, N <- [start(M, Args)]].
 
 start(Name, Args) ->
-    {ok, _, Node} = peer:start_link(#{name => Name, args => Args}),
+    {ok, _, Node} = ?util:peer(#{name => Name, args => Args}),
     Node.
 
 %% ping/1
diff --git a/lib/diameter/test/diameter_examples_SUITE.erl b/lib/diameter/test/diameter_examples_SUITE.erl
index 2c68dc90a4..e5aab1a64a 100644
--- a/lib/diameter/test/diameter_examples_SUITE.erl
+++ b/lib/diameter/test/diameter_examples_SUITE.erl
@@ -285,11 +285,8 @@ enslave(Prefix) ->
               {ok, _, N} <- [slave(M,D)]].
 
 slave(Name, Dir) ->
-    Args = [["-pa" | [lists:flatten(io_lib:format("~s", [D]))
-                      || D <- [Dir, filename:join([Dir, "..", "ebin"])]]],
-            ["-setcookie", ?L(erlang:get_cookie())]],
-    {ok, _Pid, _Node} = peer:start_link(#{name => Name,
-                                          args => lists:append(Args)}).
+    Args = ["-pa", Dir, filename:join([Dir, "..", "ebin"])],
+    {ok, _Pid, _Node} = ?util:peer(#{name => Name, args => Args}).
 
 here() ->
     filename:dirname(code:which(?MODULE)).
diff --git a/lib/diameter/test/diameter_util.erl b/lib/diameter/test/diameter_util.erl
index f6c101b7f5..feb8a74e5f 100644
--- a/lib/diameter/test/diameter_util.erl
+++ b/lib/diameter/test/diameter_util.erl
@@ -34,6 +34,7 @@
          choose/1,
          tmpdir/0,
          mktemp/1,
+         peer/1,
          unique_string/0,
          have_sctp/0,
          eprof/1]).
@@ -212,6 +213,44 @@ mktemp(Prefix) ->
     Tmp.
 
 %% ---------------------------------------------------------------------------
+%% peer/1
+%%
+%% Start a peer that dies with the calling process, but start it from
+%% a spawned process to make it insensitive to the exit reason of the
+%% calling process.
+
+peer(#{name := _} = Opts) ->
+    Ref = make_ref(),
+    Pid = self(),
+    {_, MRef} = spawn_monitor(fun() -> peer(Opts, Pid, Ref) end),
+    receive
+        {Ref, T} ->
+            erlang:demonitor(MRef, [flush]),
+            T;
+        {'DOWN', MRef, process, _, T} ->
+            {error, T}
+    end;
+
+peer(#{} = Opts) ->
+    peer(Opts#{name => peer:random_name()}).
+
+%% peer/3
+
+peer(Opts, Pid, Ref) ->
+    Cookie = atom_to_list(erlang:get_cookie()),
+    Args = [str(B) || B <- maps:get(args, Opts, [])],
+    Pid ! {Ref, peer:start_link(Opts#{args => ["-setcookie", Cookie | Args]})},
+    MRef = monitor(process, Pid),
+    receive {'DOWN', MRef, process, _, _} -> ok end.
+
+%% str/1
+%%
+%% Turn possible iodata() into string().
+
+str(Bytes) ->
+    binary_to_list(iolist_to_binary(Bytes)).
+
+%% ---------------------------------------------------------------------------
 %% unique_string/0
 
 unique_string() ->