arc: Abort booting ARC if the device is critically low on disk space.

chromeos/dbus/cryptohome_client.h

Index: chromeos/dbus/cryptohome_client.h
diff --git a/chromeos/dbus/cryptohome_client.h b/chromeos/dbus/cryptohome_client.h
index 8fc61f94b0e4f64241a565479d13f9aad0ed5e41..42dce8e25b6a536aaea798b6c867c6aadbf6274a 100644
--- a/chromeos/dbus/cryptohome_client.h
+++ b/chromeos/dbus/cryptohome_client.h
@@ -58,6 +58,10 @@ class CHROMEOS_EXPORT CryptohomeClient : public DBusClient {
       AsyncCallStatusWithDataHandler;
   // A callback to handle responses of AsyncXXX methods.
   typedef base::Callback<void(int async_id)> AsyncMethodCallback;
+  // A callback for GetFreeDiskSpace().
+  typedef base::Callback<void(DBusMethodCallStatus call_status,
+                              uint64_t disk_free_bytes)>
+      GetFreeDiskSpaceCallback;
   // A callback for GetSystemSalt().
   typedef base::Callback<void(DBusMethodCallStatus call_status,
                               const std::vector<uint8_t>& system_salt)>
@@ -153,10 +157,14 @@ class CHROMEOS_EXPORT CryptohomeClient : public DBusClient {
       const ProtobufMethodCallback& callback) = 0;
 
   // Calls GetAccountDiskUsage method. |callback| is called after the method
-  // call succeeds
+  // call succeeds.
   virtual void GetAccountDiskUsage(const cryptohome::Identification& account_id,
                                    const ProtobufMethodCallback& callback) = 0;
 
+  // Calls GetFreeDiskSpace method. |callback| is called after the method
+  // call succeeds.
+  virtual void GetFreeDiskSpace(const GetFreeDiskSpaceCallback& callback) = 0;

[Junichi Uekawa, 2016/07/15 00:29:40]

Why do we need a DBus message to cryptohome to call statfs/statvfs ?
Is that somehow disallowed? It should also be possible to call that from I/O
thread?

+
   // Calls GetSystemSalt method.  |callback| is called after the method call
   // succeeds.
   virtual void GetSystemSalt(const GetSystemSaltCallback& callback) = 0;