If corrupt user prefs are encountered, reset prefs instead of excepting

hab/user_prefs.py

@@ -93,7 +93,7 @@ def filename(self, value):
 
     def load(self, force=False):
         """Load user preferences from self.filename if they haven't already
-        been loaded.
+        been loaded. Does not use pyjson5 as this file is entirely managed by hab.
 
         Args:
             force (bool, optional): Force re-loading of preferences.
@@ -108,7 +108,16 @@ def load(self, force=False):
             return True
 
         with self.filename.open() as fle:
-            data = utils.json.load(fle)
+            try:
+                # NOTE: This file is always saved using the native json library
+                # there is no reason to support json5 encoded files here.
+                data = json.load(fle)
+            except ValueError as error:
+                # When encountering corrupt saved user pref data, clear and
+                # reset the user prefs. Warn about it this isn't entirely hidden
+                logger.warning("User pref file corrupt, resetting.")
+                logger.info("User pref exception suppressed:", exc_info=error)
+                data = {}
             self.update(data)
             self._is_loaded = True
         return True

tests/test_user_prefs.py

@@ -1,4 +1,5 @@
 import datetime
+import logging
 
 import pytest
 
@@ -157,3 +158,60 @@ def test_uri(resolver, tmpdir, monkeypatch):
     # Check UriObj.__str__ returns a string even if uri is None
     prefs_g = user_prefs.UriObj()
     assert isinstance(prefs_g.__str__(), str)
+
+
+@pytest.mark.parametrize(
+    "test_text",
+    (
+        # File was opened in write mode but no contents written
+        None,
+        # Process was killed half way through writing json output.
+        '{\n    "uri": "app',
+    ),
+)
+def test_corruption(resolver, tmpdir, monkeypatch, caplog, test_text):
+    """Check how UserPrefs handles trying to load an incomplete or empty existing
+    json document.
+    """
+    caplog.set_level(logging.INFO)
+
+    def assert_log_exists(level, msg):
+        for record in caplog.records:
+            if record.levelname != level:
+                continue
+            if msg in record.message:
+                print(record)
+                break
+        else:
+            raise AssertionError(
+                f"No logging message was made matching: {level} and {msg}"
+            )
+
+    # TODO: This duplicated code should probably be moved into a fixture
+    # Force the prefs to be saved into the test directory.
+    if utils.Platform.name() == "windows":
+        monkeypatch.setenv("LOCALAPPDATA", str(tmpdir))
+    else:
+        monkeypatch.setenv("HOME", str(tmpdir))
+    # Ensure we are using the modified filepath
+    prefs = user_prefs.UserPrefs(resolver)
+    assert prefs.filename.parent == tmpdir
+
+    prefs_file = tmpdir / ".hab_user_prefs.json"
+
+    # Create a empty file that won't resolve properly for json.
+    with prefs_file.open("w") as fle:
+        if test_text:
+            fle.write(test_text)
+
+    # Check that the expected log messages are emitted when invalid
+    # json file contents are encountered
+    caplog.clear()
+    prefs = user_prefs.UserPrefs(resolver)
+    # Even with invalid contents True will be returned
+    assert prefs.load()
+    # When corrupt prefs are encountered, default empty dict results
+    assert prefs == {}
+
+    assert_log_exists("WARNING", "User pref file corrupt, resetting.")
+    assert_log_exists("INFO", "User pref exception suppressed:")