fix(django): guard against cache truthiness [backport #5477 to 1.12] (#5646)

Fixes #5460 

Add guard against when a Django cache result has ambiguous truth because
the class doesn't implement the __bool__() function.

## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/contributing.html#Release-Note-Guidelines)
are followed.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] PR description includes explicit acknowledgement/acceptance of the
performance implications of this PR as reported in the benchmarks PR
comment.

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [x] Testing strategy adequately addresses listed risk(s).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] Release note makes sense to a user of the library.
- [x] Reviewer has explicitly acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment.

Co-authored-by: William Conti <[email protected]>

Author: Yun-Kim

ddtrace/contrib/django/patch.py

@@ -179,8 +179,9 @@ def traced_cache(django, pin, func, instance, args, kwargs):
         elif command_name == "get":
             # if valid result and check for special case for Django~3.0 that returns an empty Sentinel object as
             # missing key
-            if result and result != getattr(instance, "_missing_key", None):
+            if result is not None and result != getattr(instance, "_missing_key", None):
                 span.set_metric(db.ROWCOUNT, 1)
+            # else result is invalid or None, set row count to 0
             else:
                 span.set_metric(db.ROWCOUNT, 0)
         return result

releasenotes/notes/fix-django-rowcount-bug-52d4f30236049c1b.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    django: Adds fix for bug where Django cache return object throws an error if it does not implement ``__bool__()``.

tests/contrib/django/test_django.py

@@ -771,6 +771,46 @@ def test_cache_get_rowcount_missing_key(test_spans):
     assert_dict_issuperset(span.get_metrics(), {"db.row_count": 0})
 
 
+class NoBool:
+    def __bool__(self):
+        raise NotImplementedError
+
+
+def test_cache_get_rowcount_empty_key(test_spans):
+    # get the default cache
+    cache = django.core.cache.caches["default"]
+    cache.set(1, NoBool())
+
+    result = cache.get(1)
+
+    assert isinstance(result, NoBool) is True
+
+    spans = test_spans.get_spans()
+    assert len(spans) == 2
+
+    get_span = spans[1]
+    assert get_span.service == "django"
+    assert get_span.resource == "django.core.cache.backends.locmem.get"
+
+    assert_dict_issuperset(get_span.get_metrics(), {"db.row_count": 1})
+
+
+def test_cache_get_rowcount_missing_key_with_default(test_spans):
+    # get the default cache
+    cache = django.core.cache.caches["default"]
+
+    # This is the diff with `test_cache_get_rowcount_missing_key`,
+    # we are setting a default value to be returned in case of a cache miss
+    cache.get("missing_key", default="default_value")
+
+    spans = test_spans.get_spans()
+    assert len(spans) == 1
+    span = spans[0]
+    assert span.service == "django"
+    assert span.resource == "django.core.cache.backends.locmem.get"
+    assert_dict_issuperset(span.get_metrics(), {"db.row_count": 1})
+
+
 def test_cache_get_unicode(test_spans):
     # get the default cache
     cache = django.core.cache.caches["default"]