feature #6953 Documented the Lockable Trait (javiereguiluz)

This PR was squashed before being merged into the master branch (closes #6953).

Discussion
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Documented the Lockable Trait

This fixes #6678

Commits
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b0495e0 Documented the Lockable Trait

Author: xabbuh

console/lockable_trait.rst

@@ -0,0 +1,38 @@
+Prevent Multiple Executions of a Console Command
+================================================
+
+A simple but effective way to prevent multiple executions of the same command in
+a single server is to use **file locks**. The Filesystem component provides a
+:doc:`LockHandler </components/filesystem/lock_handler>` class that eases the
+creation and release of these locks.
+
+In addition, the Console component provides a PHP trait called ``LockableTrait``
+that adds two convenient methods to lock and release commands::
+
+    // ...
+    use Symfony\Component\Console\Command\LockableTrait;
+
+    class UpdateContentsCommand extends Command
+    {
+        use LockableTrait;
+
+        // ...
+
+        protected function execute(InputInterface $input, OutputInterface $output)
+        {
+            if (!$this->lock()) {
+                $output->writeln('The command is already running in another process.');
+
+                return 0;
+            }
+
+            // If you prefer to wait until the lock is released, use this:
+            // $this->lock(true);
+
+            // ...
+
+            // if not released explicitly, Symfony releases the lock
+            // automatically when the execution of the command ends
+            $this->release();
+        }
+    }