btrfs: support resizing if root is mounted ro.

Resize of btrfs fails if the mount point for the file system we are trying
to resize, i.e. the root of the filesystem is read only. With this change
we use a known (currently snapper specific) rw location to work around a
flaw that blocks resizing of the ro filesystem.

LP: #1734787

4 files changed, 95 insertions, 8 deletions

diff --git a/cloudinit/config/cc_resizefs.py b/cloudinit/config/cc_resizefs.py
index 0d282e63..cec22bb7 100644
--- a/cloudinit/config/cc_resizefs.py
+++ b/cloudinit/config/cc_resizefs.py
@@ -59,7 +59,17 @@ __doc__ = get_schema_doc(schema)  # Supplement python help()
 
 
 def _resize_btrfs(mount_point, devpth):
-    return ('btrfs', 'filesystem', 'resize', 'max', mount_point)
+    # If "/" is ro resize will fail. However it should be allowed since resize
+    # makes everything bigger and subvolumes that are not ro will benefit.
+    # Use a subvolume that is not ro to trick the resize operation to do the
+    # "right" thing. The use of ".snapshot" is specific to "snapper" a generic
+    # solution would be walk the subvolumes and find a rw mounted subvolume.
+    if (not util.mount_is_read_write(mount_point) and
+            os.path.isdir("%s/.snapshots" % mount_point)):
+        return ('btrfs', 'filesystem', 'resize', 'max',
+                '%s/.snapshots' % mount_point)
+    else:
+        return ('btrfs', 'filesystem', 'resize', 'max', mount_point)
 
 
 def _resize_ext(mount_point, devpth):
diff --git a/cloudinit/tests/test_util.py b/cloudinit/tests/test_util.py
new file mode 100644
index 00000000..ba6bf699
--- /dev/null
+++ b/cloudinit/tests/test_util.py
@@ -0,0 +1,46 @@
+# This file is part of cloud-init. See LICENSE file for license information.
+
+"""Tests for cloudinit.util"""
+
+import logging
+
+import cloudinit.util as util
+
+from cloudinit.tests.helpers import CiTestCase, mock
+
+LOG = logging.getLogger(__name__)
+
+MOUNT_INFO = [
+    '68 0 8:3 / / ro,relatime shared:1 - btrfs /dev/sda1 ro,attr2,inode64',
+    '153 68 254:0 / /home rw,relatime shared:101 - xfs /dev/sda2 rw,attr2'
+]
+
+
+class TestUtil(CiTestCase):
+
+    def test_parse_mount_info_no_opts_no_arg(self):
+        result = util.parse_mount_info('/home', MOUNT_INFO, LOG)
+        self.assertEqual(('/dev/sda2', 'xfs', '/home'), result)
+
+    def test_parse_mount_info_no_opts_arg(self):
+        result = util.parse_mount_info('/home', MOUNT_INFO, LOG, False)
+        self.assertEqual(('/dev/sda2', 'xfs', '/home'), result)
+
+    def test_parse_mount_info_with_opts(self):
+        result = util.parse_mount_info('/', MOUNT_INFO, LOG, True)
+        self.assertEqual(
+            ('/dev/sda1', 'btrfs', '/', 'ro,relatime'),
+            result
+        )
+
+    @mock.patch('cloudinit.util.get_mount_info')
+    def test_mount_is_rw(self, m_mount_info):
+        m_mount_info.return_value = ('/dev/sda1', 'btrfs', '/', 'rw,relatime')
+        is_rw = util.mount_is_read_write('/')
+        self.assertEqual(is_rw, True)
+
+    @mock.patch('cloudinit.util.get_mount_info')
+    def test_mount_is_ro(self, m_mount_info):
+        m_mount_info.return_value = ('/dev/sda1', 'btrfs', '/', 'ro,relatime')
+        is_rw = util.mount_is_read_write('/')
+        self.assertEqual(is_rw, False)
diff --git a/cloudinit/util.py b/cloudinit/util.py
index df0aa5db..9976400f 100644
--- a/cloudinit/util.py
+++ b/cloudinit/util.py
@@ -2059,7 +2059,7 @@ def expand_package_list(version_fmt, pkgs):
     return pkglist
 
 
-def parse_mount_info(path, mountinfo_lines, log=LOG):
+def parse_mount_info(path, mountinfo_lines, log=LOG, get_mnt_opts=False):
     """Return the mount information for PATH given the lines from
     /proc/$$/mountinfo."""
 
@@ -2121,11 +2121,16 @@ def parse_mount_info(path, mountinfo_lines, log=LOG):
 
         match_mount_point = mount_point
         match_mount_point_elements = mount_point_elements
+        mount_options = parts[5]
 
-    if devpth and fs_type and match_mount_point:
-        return (devpth, fs_type, match_mount_point)
+    if get_mnt_opts:
+        if devpth and fs_type and match_mount_point and mount_options:
+            return (devpth, fs_type, match_mount_point, mount_options)
     else:
-        return None
+        if devpth and fs_type and match_mount_point:
+            return (devpth, fs_type, match_mount_point)
+
+    return None
 
 
 def parse_mtab(path):
@@ -2195,7 +2200,7 @@ def parse_mount(path):
     return None
 
 
-def get_mount_info(path, log=LOG):
+def get_mount_info(path, log=LOG, get_mnt_opts=False):
     # Use /proc/$$/mountinfo to find the device where path is mounted.
     # This is done because with a btrfs filesystem using os.stat(path)
     # does not return the ID of the device.
@@ -2227,7 +2232,7 @@ def get_mount_info(path, log=LOG):
     mountinfo_path = '/proc/%s/mountinfo' % os.getpid()
     if os.path.exists(mountinfo_path):
         lines = load_file(mountinfo_path).splitlines()
-        return parse_mount_info(path, lines, log)
+        return parse_mount_info(path, lines, log, get_mnt_opts)
     elif os.path.exists("/etc/mtab"):
         return parse_mtab(path)
     else:
@@ -2613,4 +2618,10 @@ def wait_for_files(flist, maxwait, naplen=.5, log_pre=""):
     return need
 
 
+def mount_is_read_write(mount_point):
+    """Check whether the given mount point is mounted rw"""
+    result = get_mount_info(mount_point, get_mnt_opts=True)
+    mount_opts = result[-1].split(',')
+    return mount_opts[0] == 'rw'
+
 # vi: ts=4 expandtab
diff --git a/tests/unittests/test_handler/test_handler_resizefs.py b/tests/unittests/test_handler/test_handler_resizefs.py
index 29d5574d..5aa3c498 100644
--- a/tests/unittests/test_handler/test_handler_resizefs.py
+++ b/tests/unittests/test_handler/test_handler_resizefs.py
@@ -1,7 +1,7 @@
 # This file is part of cloud-init. See LICENSE file for license information.
 
 from cloudinit.config.cc_resizefs import (
-    can_skip_resize, handle, maybe_get_writable_device_path)
+    can_skip_resize, handle, maybe_get_writable_device_path, _resize_btrfs)
 
 from collections import namedtuple
 import logging
@@ -293,5 +293,25 @@ class TestMaybeGetDevicePathAsWritableBlock(CiTestCase):
             " per kernel cmdline",
             self.logs.getvalue())
 
+    @mock.patch('cloudinit.util.mount_is_read_write')
+    @mock.patch('cloudinit.config.cc_resizefs.os.path.isdir')
+    def test_resize_btrfs_mount_is_ro(self, m_is_dir, m_is_rw):
+        """Do not resize / directly if it is read-only. (LP: #1734787)."""
+        m_is_rw.return_value = False
+        m_is_dir.return_value = True
+        self.assertEqual(
+            ('btrfs', 'filesystem', 'resize', 'max', '//.snapshots'),
+            _resize_btrfs("/", "/dev/sda1"))
+
+    @mock.patch('cloudinit.util.mount_is_read_write')
+    @mock.patch('cloudinit.config.cc_resizefs.os.path.isdir')
+    def test_resize_btrfs_mount_is_rw(self, m_is_dir, m_is_rw):
+        """Do not resize / directly if it is read-only. (LP: #1734787)."""
+        m_is_rw.return_value = True
+        m_is_dir.return_value = True
+        self.assertEqual(
+            ('btrfs', 'filesystem', 'resize', 'max', '/'),
+            _resize_btrfs("/", "/dev/sda1"))
+
 
 # vi: ts=4 expandtab