Prevent concurrent availability checks

lib/private/Files/Storage/Wrapper/Availability.php

@@ -40,9 +40,13 @@ class Availability extends Wrapper {
 	}
 
 	/**
+	 * Only called if availability === false
+	 *
 	 * @return bool
 	 */
 	private function updateAvailability() {
+		// reset availability to false so that multiple requests don't recheck concurrently
+		$this->setAvailability(false);
 		try {
 			$result = $this->test();
 		} catch (\Exception $e) {

tests/lib/files/storage/wrapper/availability.php

@@ -74,9 +74,12 @@ class Availability extends \Test\TestCase {
 		$storage->expects($this->once())
 			->method('test')
 			->willReturn(true);
-		$storage->expects($this->once())
+		$storage->expects($this->exactly(2))
 			->method('setAvailability')
-			->with($this->equalTo(true));
+			->withConsecutive(
+				[$this->equalTo(false)], // prevents concurrent rechecks
+				[$this->equalTo(true)] // sets correct availability
+			);
 		$storage->expects($this->once())
 			->method('mkdir');