add ErrnoUtils library

Summary:
# This diff

Adds a library function for determining whether a given errno could stem from a hanging mount point. We can expand this library later if we discover more hanging mount failure modes

# Context

Crashes during graceful restart are plaguing macOS. Although these happen in the background, these greatly affect users because they cause Eden daemons to exit uncleanly. Because of the unclean exit, mount points get left in the mount table and cause all sorts of issues. The biggest issues are:

1) Any attempts to perform IO into the mount point fail
2) Any subsequent attempts to remount the mount point fail. These attempts fail because of #1, and Eden's startup/mount codepaths haven't been taught how to deal with this.

The goal of this stack is to:

1) Introduce a mechanism to easily repro graceful restart crashes (both crashes that occur in the TakeoverClient and TakeoverServer)
2) Use the new mechanism to troubleshoot issue #2 above
3) Use the learnings from #2 to make EdenFS automatically recover from graceful restart crashes (unmount stale mounts and successfully remount any mounts that were mounted prior to the crash).

Reviewed By: jdelliot

Differential Revision: D72490699

fbshipit-source-id: 2d42f9961f39953abb6ad0bc12b305498e446a3e

Author: MichaelCuevas

eden/common/utils/ErrnoUtils.h

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <cerrno>
+
+namespace facebook::eden {
+
+inline bool isErrnoFromHangingMount(int err, bool isNFS) {
+  if (isNFS) {
+    // Hard NFS mounts tend to return EIO, whereas soft NFS mounts return
+    // ETIMEDOUT
+    return err == ENOTCONN || err == EIO || err == ETIMEDOUT;
+  } else {
+    // FUSE mounts (seem to) always return ENOTCONN when the mount is hanging
+    return err == ENOTCONN || err == EIO;
+  }
+}
+} // namespace facebook::eden