fs: improve value parsing from command output

value extraction did not have error checking and was handled in
a one-liner. The actual implementation expands the parsing logic
and moves it into a function.

src/utils/fs.py

@@ -237,11 +237,28 @@ def _reduce_ntfsresize(partdev):
     # The first split operation leaves the wanted data at the second element of
     # the split ([1]). Finally do a second split with ' ' to get the data but
     # nothing else following it.
-    #
-    # In addition, increase by 10%+1K the reported shrink location on which the
-    # filesystem can be resized according to ntfsresize.
-    size = int(out_info.split('device size: ')[1].split(' ')[0])
-    new_size = int(int(out_info.split('resize at ')[1].split(' ')[0])*1.1+1024)
+    def parse_ntfsresize(output_data, pattern):
+        data_split = output_data.split(pattern)
+        # If we fail to match pattern in the split then data_split will contain [output_data]
+        if len(data_split) == 1:
+            logging.error(f'nfsresize: failed to find: "{pattern}"')
+            raise ValueError(f'nfsresize: failed to find: "{pattern}"')
+        value_str = data_split[1].split(' ')[0]
+
+        if not value_str.isdigit() or value_str.startswith('-'):
+            logging.error(f'nfsresize: failed to parse numeric value at "{pattern}"')
+            raise ValueError(f'nfsresize: failed to parse numeric value at "{pattern}"')
+        return int(value_str)
+
+    try:
+        size = parse_ntfsresize(out_info, 'device size: ')
+        new_size = parse_ntfsresize(out_info, 'resize at ')
+        # Increase by 10%+1K the indicated reduction by which the file system
+        # can be resized according to ntfsresize.
+        new_size = int(new_size * 1.1 + 1024)
+    except ValueError:
+        return -1
+
 
     # Run ntfsresize with -n to to probe for the smallest size, this loop is
     # intentional. Acumulate size until ntfsresize in dry-run mode fails, then
@@ -262,8 +279,13 @@ def _reduce_ntfsresize(partdev):
         if out_resize_dryrun.find('Needed relocations : ') == -1:
             break
 
-        extra_size = int(out_resize_dryrun.split('Needed relocations : ')[1].split(' ')[0])*1.1+1024
-        new_size += int(extra_size)
+        try:
+            extra_size = parse_ntfsresize(out_resize_dryrun, 'Needed relocations : ')
+            # Add size padding
+            extra_size = int(extra_size * 1.1 + 1024)
+            new_size += extra_size
+        except ValueError:
+                return -1
 
     if new_size < size:
         cmd_resize = shlex.split(f'ntfsresize -fs {new_size:.0f} {partdev}')