Check for presence of btrfs before trying to use it

If btrfs is not present in the kernel we end up with strange output like
this:

    Error creating disk image2014-06-10 16:00:40
    [devel-system-x86_64-generic][my-raw-disk-image][rawdisk.write]Failure
    to create disk image at /src/tmp/testdev.img
    ERROR: Command failed: mount -o loop /src/tmp/testdev.img
    /src/tmp/deployments/tmpQ7wXO1/tmp4lVDcu/tmpvHSzDE
    mount: mounting /dev/loop0 on
    /src/tmp/deployments/tmpQ7wXO1/tmp4lVDcu/tmpvHSzDE failed: Device or
    resource busy

To avoid this confusing error, Morph should explicitly check first.

1 files changed, 12 insertions, 1 deletions

diff --git a/writeexts.py b/writeexts.py
index d6f23e0d..74587bd1 100644
--- a/writeexts.py
+++ b/writeexts.py
@@ -104,7 +104,18 @@ class WriteExtension(cliapp.Application):
         
         self.output.write('%s\n' % (kwargs['msg'] % kwargs))
         self.output.flush()
-    
+
+    def check_for_btrfs_in_deployment_host_kernel(self):
+        with open('/proc/filesystems') as f:
+            text = f.read()
+        return '\tbtrfs\n' in text
+
+    def require_btrfs_in_deployment_host_kernel(self):
+        if not self.check_for_btrfs_in_deployment_host_kernel():
+            raise cliapp.AppException(
+                'Error: Btrfs is required for this deployment, but was not '
+                'detected in the kernel of the machine that is running Morph.')
+
     def create_local_system(self, temp_root, raw_disk):
         '''Create a raw system image locally.'''
         size = self.get_disk_size()