Check for empty pillar cache disk file and not call msgpack to load it, with test

changelog/63729.fixed.md

@@ -0,0 +1 @@
+Check file is not empty before attempting to read pillar disk cache file

salt/utils/cache.py

@@ -143,6 +143,10 @@ def _read(self):
         if not salt.utils.msgpack.HAS_MSGPACK or not os.path.exists(self._path):
             return
 
+        if 0 == os.path.getsize(self._path):
+            # File exists but empty, treat as empty cache
+            return
+
         try:
             with salt.utils.files.fopen(self._path, "rb") as fp_:
                 cache = salt.utils.msgpack.load(

tests/pytests/unit/pillar/test_pillar.py

@@ -1,7 +1,11 @@
+import logging
+from pathlib import Path
+
 import pytest
 
 import salt.loader
 import salt.pillar
+import salt.utils.cache
 from salt.utils.odict import OrderedDict
 
 
@@ -123,3 +127,33 @@ def test_pillar_envs_path_substitution(env, temp_salt_minion, tmp_path):
 
     # The __env__ string in the path has been substituted for the actual env
     assert pillar.opts["pillar_roots"] == expected
+
+
+def test_pillar_get_cache_disk(temp_salt_minion, caplog):
+    # create faked path for cache
+    with pytest.helpers.temp_directory() as temp_path:
+        tmp_cachedir = Path(str(temp_path) + "/pillar_cache/")
+        tmp_cachedir.mkdir(parents=True)
+        assert tmp_cachedir.exists()
+        tmp_cachefile = Path(str(temp_path) + "/pillar_cache/" + temp_salt_minion.id)
+        tmp_cachefile.touch()
+        assert tmp_cachefile.exists()
+
+        opts = temp_salt_minion.config.copy()
+        opts["pillarenv"] = None
+        opts["pillar_cache"] = True
+        opts["cachedir"] = str(temp_path)
+
+        caplog.at_level(logging.DEBUG)
+        pillar = salt.pillar.PillarCache(
+            opts=opts,
+            grains=salt.loader.grains(opts),
+            minion_id=temp_salt_minion.id,
+            saltenv="base",
+        )
+        fresh_pillar = pillar.fetch_pillar()
+        assert not (
+            f"Error reading cache file at '{tmp_cachefile}': Unpack failed: incomplete input"
+            in caplog.messages
+        )
+        assert fresh_pillar == {}

tests/pytests/unit/utils/test_cache.py

@@ -5,6 +5,7 @@
     Test the salt cache objects
 """
 
+import logging
 import pathlib
 import time
 
@@ -221,7 +222,7 @@ def test_everything(cache_dir):
         b"\xc3\x83\xc2\xa6\xc3\x83\xc2\xb8\xc3\x83\xc2\xa5",
     ],
 )
-def test_unicode_error(cache_dir, data):
+def test_unicode_error(cache_dir, data, caplog):
     """
     Test when the data in the cache raises a UnicodeDecodeError
     we do not raise an error.
@@ -240,9 +241,17 @@ def test_unicode_error(cache_dir, data):
             }
         }
     }
-    with patch.object(salt.utils.msgpack, "load", return_value=cache_data):
+    with patch.object(
+        salt.utils.msgpack, "load", return_value=cache_data
+    ), caplog.at_level(logging.DEBUG):
         cd = cache.CacheDisk(0.3, str(path))
-        assert cd._dict == cache_data
+        # this test used to rely on msgpack throwing errors if attempt to read an empty file
+        # code now checks if file empty and returns, so we should never attempt msgpack load
+        assert cd._dict == {}
+        assert not (
+            f"Error reading cache file at '{path}': Unpack failed: incomplete input"
+            in caplog.messages
+        )
 
 
 def test_cache_corruption(cache_dir):