Add application:set_env/1 and application:set_env/2

It is equivalent to calling application:set_env/4 on
each application individually, except it is more efficient.

When given duplicate apps or duplicate keys, set_env/1
and set_env/2 will warn. The warning will also be emitted
during boot:

    $ erl -config dupkeys.config -s erlang halt
    2019-02-27 11:16:02.653100 application: kernel; duplicate parameter: key1
    =WARNING REPORT==== 27-Feb-2019::11:16:02.653100 ===
    application: kernel; duplicate parameter: key1

    $ erl -config dupapps.config -s erlang halt
    2019-02-27 11:16:02.653100 duplicate application config: kernel
    =WARNING REPORT==== 27-Feb-2019::11:16:02.653100 ===
    duplicate application config: kernel

Prior to this patch, the behaviour was unspecified,
and duplicate keys and duplicate apps would behave
different depending on the amount of config, the name
of the config files, and how those configs would be
listed. The goal is to raise an error in the future.

1 files changed, 35 insertions, 0 deletions

diff --git a/lib/kernel/doc/src/application.xml b/lib/kernel/doc/src/application.xml
index 4e32c1a3a5..5170502581 100644
--- a/lib/kernel/doc/src/application.xml
+++ b/lib/kernel/doc/src/application.xml
@@ -238,6 +238,41 @@ Nodes = [cp1@cave, {cp2@cave, cp3@cave}]</code>
       </desc>
     </func>
     <func>
+      <name name="set_env" arity="1" since="OTP @OTP-15642@"/>
+      <name name="set_env" arity="2" since="OTP @OTP-15642@"/>
+      <fsummary>Sets the configuration parameters of multiple applications.</fsummary>
+      <desc>
+        <p>Sets the configuration <c><anno>Config</anno></c> for multiple
+          applications. It is equivalent to calling <c>set_env/4</c> on
+          each application individially, except it is more efficient.
+          The given <c><anno>Config</anno></c> is validated before the
+          configuration is set.</p>
+        <p><c>set_env/2</c> uses the standard <c>gen_server</c> time-out
+          value (5000 ms). Option <c>timeout</c> can be specified
+          if another time-out value is useful, for example, in situations
+          where the application controller is heavily loaded.</p>
+        <p>Option <c>persistent</c> can be set to <c>true</c>
+          to guarantee that parameters set with <c>set_env/2</c>
+          are not overridden by those defined in the application resource
+          file on load. This means that persistent values will stick after the application
+          is loaded and also on application reload.</p>
+        <p>If an application is given more than once or if an application
+          has the same key given more than once, the behaviour is undefined
+          and a warning message will be logged. In future releases, an error
+          will be raised.</p>
+        <p><c>set_env/1</c> is equivalent to <c>set_env(Config, [])</c>.</p>
+        <warning>
+          <p>Use this function only if you know what you are doing,
+            that is, on your own applications. It is very
+            application-dependent and
+            configuration parameter-dependent when and how often
+            the value is read by the application. Careless use
+            of this function can put the application in a
+            weird, inconsistent, and malfunctioning state.</p>
+        </warning>
+      </desc>
+    </func>
+    <func>
       <name name="permit" arity="2" since=""/>
       <fsummary>Change the permission for an application to run at a node.</fsummary>
       <desc>