[5.6] Fix cache for loggers (#23118)

* Cache loggers

* Test LogManager caches Logger instances

* Use assertSame

src/Illuminate/Log/LogManager.php

@@ -113,7 +113,7 @@ protected function get($name)
     {
         try {
             return $this->channels[$name] ?? with($this->resolve($name), function ($logger) use ($name) {
-                return $this->tap($name, new Logger($logger, $this->app['events']));
+                return $this->channels[$name] = $this->tap($name, new Logger($logger, $this->app['events']));
             });
         } catch (Throwable $e) {
             return tap($this->createEmergencyLogger(), function ($logger) use ($e) {

tests/Log/LogManagerTest.php

@@ -0,0 +1,19 @@
+<?php
+
+namespace Illuminate\Tests\Log;
+
+use Illuminate\Log\LogManager;
+use Orchestra\Testbench\TestCase;
+
+class LogManagerTest extends TestCase
+{
+    public function testLogManagerCachesLoggerInstances()
+    {
+        $manager = new LogManager($this->app);
+
+        $logger1 = $manager->channel('single')->getLogger();
+        $logger2 = $manager->channel('single')->getLogger();
+
+        $this->assertSame($logger1, $logger2);
+    }
+}