storage: Show notification when rootfs is read-only

Tests:

- Change the partition to test to '/mnt'. Mount a loopback filesystem on /mnt.
'dd if=/dev/zero of=/test-file bs=1M count=100; mkfs.ext4 /test-file; mount -o
loop /test-file /mnt'. Turn it to read-only with 'mount -o remount,ro /mnt'.
Wait about 3 minutes for the notification to show up.

- The notification shows icon, title and message as expected. The button power
app appears and works as expected.

- When the filesystem is mount rw again, the notification goes away in 3
minutes. 'mount -o remount,rw /mnt'.

[sunil: Let glib.schedule decide time when debugging]
[sunil: Perform exact matching in partition mount options]
[sunil: Simplify notification message. Minor grammar change]
[sunil: Minor refactoring for styling]
Reviewed-by: Sunil Mohan Adapa <sunil@medhas.org>

plinth/modules/storage/__init__.py

@@ -68,6 +68,9 @@ class StorageApp(app_module.App):
         # Schedule initialization of UDisks2 initialization
         glib.schedule(3, udisks2.init, repeat=False)
 
+        # Check periodically for a read-only root filesystem
+        glib.schedule(600, _warn_about_read_only_filesystem)
+
     def setup(self, old_version):
         """Install and configure the app."""
         super().setup(old_version)
@@ -271,7 +274,7 @@ def get_error_message(error):
     return message_map.get(short_error, error)
 
 
-def warn_about_low_disk_space(request):
+def warn_about_low_disk_space(_data):
     """Warn about insufficient space on root partition."""
     from plinth.notification import Notification
 
@@ -341,3 +344,55 @@ def report_failing_drive(id, is_failing):
                                              'type': 'dismiss'
                                          }], data=data, group='admin')
     note.dismiss(should_dismiss=not is_failing)
+
+
+def is_partition_read_only(mount_point='/'):
+    """Return True if the partition is mounted read-only."""
+    for partition in psutil.disk_partitions(all=True):
+        if partition.mountpoint == mount_point:
+            return 'ro' in partition.opts.split(',')
+
+    raise ValueError('No such mount point')
+
+
+def _warn_about_read_only_filesystem(_data):
+    """Create a notification if the root filesystem is mounted read-only.
+
+    Remove the notification (if any) if the root filesystem is writable.
+    """
+    from plinth.notification import Notification
+
+    show = is_partition_read_only()
+    notification_id = 'storage-read-only-root-filesystem'
+
+    if not show:
+        try:
+            Notification.get(notification_id).delete()
+        except KeyError:
+            pass
+
+        return
+
+    message = gettext_noop(
+        # xgettext:no-python-format
+        'You cannot save configuration changes. Try rebooting the system. If '
+        'the problem persists after a reboot, check the storage device for '
+        'errors.')
+    title = gettext_noop('Read-only root filesystem')
+    data = {
+        'app_icon': 'fa-hdd-o',
+        'app_name': 'translate:' + gettext_noop('Storage'),
+    }
+
+    # This is a serious issue so the notification can't be dismissed.
+    actions = [{
+        'type': 'link',
+        'class': 'primary',
+        'text': gettext_noop('Go to Power'),
+        'url': 'power:index'
+    }]
+
+    Notification.update_or_create(id=notification_id, app_id='storage',
+                                  severity='error', title=title,
+                                  message=message, actions=actions, data=data,
+                                  group='admin')

plinth/modules/storage/tests/test_storage.py

@@ -7,9 +7,12 @@ import contextlib
 import re
 import subprocess
 import tempfile
+from unittest.mock import patch
 
+import psutil
 import pytest
 
+from plinth.modules import storage
 from plinth.modules.storage import privileged
 
 pytestmark = pytest.mark.usefixtures('mock_privileged')
@@ -343,3 +346,17 @@ def test_validate_directory_writable(path, error):
 def test_validate_directory_creatable(path, error):
     """Test that directory creatable validation returns expected output."""
     _assert_validate_directory(path, error, check_creatable=True)
+
+
+@patch('psutil.disk_partitions')
+def test_is_partition_read_only(disk_partitions):
+    """Test whether checking for ro partition works."""
+    partition = psutil._common.sdiskpart  # pylint: disable=protected-access
+    disk_partitions.return_value = [
+        partition('/dev/root', '/', 'btrfs', 'rw,nosuid', 42, 42),
+        partition('/dev/root', '/foo', 'btrfs', 'rw', 321, 321),
+        partition('/dev/foo', '/bar', 'extfs', 'ro', 123, 123)
+    ]
+    assert not storage.is_partition_read_only('/')
+    assert not storage.is_partition_read_only('/foo')
+    assert storage.is_partition_read_only('/bar')