Fix a bug that unmounting devices on the policy update can be a busy loop.

chrome/browser/chromeos/file_manager/fake_disk_mount_manager.cc

Index: chrome/browser/chromeos/file_manager/fake_disk_mount_manager.cc
diff --git a/chrome/browser/chromeos/file_manager/fake_disk_mount_manager.cc b/chrome/browser/chromeos/file_manager/fake_disk_mount_manager.cc
index a740093551022ee1fc9fc72d4d0f6c29d7d0f54c..5fb85e4c3f130b2705f0b11e040f7e587997060d 100644
--- a/chrome/browser/chromeos/file_manager/fake_disk_mount_manager.cc
+++ b/chrome/browser/chromeos/file_manager/fake_disk_mount_manager.cc
@@ -107,7 +107,19 @@ void FakeDiskMountManager::UnmountPath(const std::string& mount_path,
                     OnMountEvent(DiskMountManager::UNMOUNTING,
                                  chromeos::MOUNT_ERROR_NONE,
                                  mount_point));
-  // Currently |callback| is just ignored.
+  // Enqueue callback so that |FakeDiskMountManager::FinishAllUnmountRequest()|
+  // can call them.
+  pending_unmount_callbacks_.push(callback);

[fukino, 2016/09/16 07:31:34]

Why do we need FinishAllUnmountPathRequests() called?
Is it possible to call each pending callbacks asynchronously without
FinishAllUnmountPathRequest()?

[yamaguchi, 2016/09/16 08:49:51]

The main purpose of this change in the test & the fake class is to detect the
bug with the original code.
If we combine the test with the existing code, it will form an infinite loop
(and will crash by overflow) because just calling the UnmountPath function does
not finish unmount right away.
("synchronous" here means to finish call in sync with the invocation of
UnmountPath,)

+}
+
+bool FakeDiskMountManager::FinishAllUnmountPathRequests() {
+  if (pending_unmount_callbacks_.empty())
+    return false;
+  while (!pending_unmount_callbacks_.empty()) {
+    pending_unmount_callbacks_.front().Run(chromeos::MOUNT_ERROR_NONE);
+    pending_unmount_callbacks_.pop();
+  }
+  return true;
 }
 
 void FakeDiskMountManager::FormatMountedDevice(const std::string& mount_path) {