Make runtime directory optional

The runtime directory used to be used internally for a bunch of things, but at this point it's mainly used as a directory to write `edit` command buffers to.

Since those aren't crucial to the operation of PsySH, and since non-writable runtime directory configuration apparently happens sometimes, let's try making a writable runtime directory entirely optional.

See #443

src/Configuration.php

@@ -627,12 +627,14 @@ public function setRuntimeDir(string $dir)
      * overridden.
      *
      * @throws RuntimeException if no temporary directory is set and it is not possible to create one
+     *
+     * @param bool $create False to suppress directory creation if it does not exist
      */
-    public function getRuntimeDir(): string
+    public function getRuntimeDir($create = true): string
     {
         $runtimeDir = $this->configPaths->runtimeDir();
 
-        if (!\is_dir($runtimeDir)) {
+        if ($create && !\is_dir($runtimeDir)) {
             if (!@\mkdir($runtimeDir, 0700, true)) {
                 throw new RuntimeException(\sprintf('Unable to create PsySH runtime directory. Make sure PHP is able to write to %s in order to continue.', \dirname($runtimeDir)));
             }

src/Shell.php

@@ -214,7 +214,7 @@ protected function getDefaultCommands(): array
             new Command\TraceCommand(),
             new Command\BufferCommand(),
             new Command\ClearCommand(),
-            new Command\EditCommand($this->config->getRuntimeDir()),
+            new Command\EditCommand($this->config->getRuntimeDir(false)),
             // new Command\PsyVersionCommand(),
             $sudo,
             $hist,

test/ConfigurationTest.php

@@ -57,6 +57,21 @@ public function testGettersAndSetters()
         $this->assertSame('wheee', $config->getConfigDir());
     }
 
+    public function testGetRuntimeDir()
+    {
+        $dirName = \tempnam(\sys_get_temp_dir(), 'psysh-config-test-');
+        \unlink($dirName);
+
+        $config = $this->getConfig();
+        $config->setRuntimeDir($dirName);
+
+        $this->assertSame($config->getRuntimeDir(false), $dirName);
+        $this->assertDirectoryDoesNotExist($dirName);
+
+        $this->assertSame($config->getRuntimeDir(true), $dirName);
+        $this->assertDirectoryExists($dirName);
+    }
+
     /**
      * @group isolation-fail
      */