From b0900fc2f7daab1c77dd8878c39ab24591bb076b Mon Sep 17 00:00:00 2001
From: Xavier Noria <fxn@hashref.com>
Date: Thu, 5 Apr 2018 01:40:20 +0200
Subject: Explain why the AM becomes group leader

---
 lib/kernel/doc/src/application.xml    | 10 +++++++---
 lib/kernel/src/application_master.erl |  4 ++++
 2 files changed, 11 insertions(+), 3 deletions(-)

(limited to 'lib/kernel')

diff --git a/lib/kernel/doc/src/application.xml b/lib/kernel/doc/src/application.xml
index 886286b76d..be914aee87 100644
--- a/lib/kernel/doc/src/application.xml
+++ b/lib/kernel/doc/src/application.xml
@@ -318,8 +318,13 @@ Nodes = [cp1@cave, {cp2@cave, cp3@cave}]</code>
           <c>{error,{not_started,App}}</c> is returned, where <c>App</c>
           is the name of the missing application.</p>
         <p>The application controller then creates an <em>application master</em>
-	  for the application. The application master is
-          the group leader of all the processes in the application.
+          for the application. The application master becomes the
+          group leader of all the processes in the application. I/O is
+          forwarded to the previous group leader, though, this is just
+          a way to identify processes that belong to the application.
+          Used for example to find itself from any process, or,
+          reciprocally, to kill them all when it terminates.</p>
+        <p>
           The application master starts the application by calling
           the application callback function <c>Module:start/2</c> as
           defined by the application specification key <c>mod</c>.</p>
@@ -608,4 +613,3 @@ Nodes = [cp1@cave, {cp2@cave, cp3@cave}]</code>
       <seealso marker="app">app(4)</seealso></p>
   </section>
 </erlref>
-
diff --git a/lib/kernel/src/application_master.erl b/lib/kernel/src/application_master.erl
index 5da2b0b06c..06991b45e1 100644
--- a/lib/kernel/src/application_master.erl
+++ b/lib/kernel/src/application_master.erl
@@ -118,6 +118,10 @@ init(Parent, Starter, ApplData, Type) ->
     link(Parent),
     process_flag(trap_exit, true),
     OldGleader = group_leader(),
+    %% We become the group leader, but forward all I/O to OldGleader.
+    %% This is just a way to identify processes that belong to the
+    %% application. Used for example to find ourselves from any
+    %% process, or, reciprocally, to kill them all when we terminate.
     group_leader(self(), self()),
     %% Insert ourselves as master for the process.  This ensures that
     %% the processes in the application can use get_env/1 at startup.
-- 
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