Add chroot context manager example to debootstrap

Closes #132

api-daemon/provisioner/examples/debootstrap_script.py

@@ -34,6 +34,29 @@
 # with that.
 
 import os
+from contextlib import contextmanager
+
+
+# Create a chroot context manager
+# This can be used later in the script to chroot to the destination directory
+# for instance to run commands within the target.
+@contextmanager
+def chroot_target(destination):
+    try:
+        real_root = os.open("/", os.O_RDONLY)
+        os.chroot(destination)
+        fake_root = os.open("/", os.O_RDONLY)
+        os.fchdir(fake_root)
+        yield
+    finally:
+        os.fchdir(real_root)
+        os.chroot(".")
+        os.fchdir(real_root)
+        os.close(fake_root)
+        os.close(real_root)
+        del fake_root
+        del real_root
+
 
 # Installation function - performs a debootstrap install of a Debian system
 # Note that the only arguments are keyword arguments.
@@ -193,13 +216,7 @@ GRUB_DISABLE_LINUX_UUID=false
         fh.write(data)
 
     # Chroot, do some in-root tasks, then exit the chroot
-    # EXITING THE CHROOT IS VERY IMPORTANT OR THE FOLLOWING STAGES OF THE PROVISIONER
-    # WILL FAIL IN UNEXPECTED WAYS! Keep this in mind when using chroot in your scripts.
-    real_root = os.open("/", os.O_RDONLY)
-    os.chroot(temporary_directory)
-    fake_root = os.open("/", os.O_RDONLY)
-    os.fchdir(fake_root)
-
+    with chroot_target(temporary_directory):
         # Install and update GRUB
         os.system(
             "grub-install --force /dev/rbd/{}/{}_{}".format(root_disk['pool'], vm_name, root_disk['disk_id'])
@@ -219,15 +236,6 @@ GRUB_DISABLE_LINUX_UUID=false
             "systemctl enable cloud-init.target"
         )
 
-    # Restore our original root/exit the chroot
-    # EXITING THE CHROOT IS VERY IMPORTANT OR THE FOLLOWING STAGES OF THE PROVISIONER
-    # WILL FAIL IN UNEXPECTED WAYS! Keep this in mind when using chroot in your scripts.
-    os.fchdir(real_root)
-    os.chroot(".")
-    os.fchdir(real_root)
-    os.close(fake_root)
-    os.close(real_root)
-
     # Unmount the bound devfs
     os.system(
         "umount {}/dev".format(
@@ -235,8 +243,4 @@ GRUB_DISABLE_LINUX_UUID=false
         )
     )
 
-    # Clean up file handles so paths can be unmounted
-    del fake_root
-    del real_root
-
     # Everything else is done via cloud-init user-data

api-daemon/provisioner/examples/dummy_script.py

@@ -29,7 +29,7 @@
 # This script will run under root privileges as the provisioner does. Be careful
 # with that.
 
-# Installation function - performs a debootstrap install of a Debian system
+# Installation function - performs no actions then returns
 # Note that the only arguments are keyword arguments.
 def install(**kwargs):
     # The provisioner has already mounted the disks on kwargs['temporary_directory'].