SERVER-67405: Fix is_patch lookup for build_variant_gen

(cherry picked from commit 1260ae9d64)

buildscripts/evergreen_gen_build_variant.py

@@ -59,7 +59,7 @@ class EvgExpansions(BaseModel):
     build_id: str
     build_variant: str
     exec_timeout_secs: Optional[int] = None
-    is_patch: Optional[bool]
+    is_patch: Optional[str]
     project: str
     max_tests_per_suite: Optional[int] = 100
     max_sub_suites: Optional[int] = 5
@@ -83,10 +83,21 @@ class EvgExpansions(BaseModel):
 
     def get_max_sub_suites(self) -> int:
         """Get the max_sub_suites to use."""
-        if not self.is_patch:
+        if not self.determine_is_patch():
             return self.mainline_max_sub_suites
         return self.max_sub_suites
 
+    def determine_is_patch(self) -> bool:
+        """
+        Determine if expansions indicate whether the script is being run in a patch build.
+
+        In a patch build, the `is_patch` expansion will be the string value of "true". In a
+        non-patch setting, it will not exist, or be an empty string.
+
+        :return: True if task is being run in a patch build.
+        """
+        return self.is_patch is not None and self.is_patch.lower() == "true"
+
     def build_suite_split_config(self, start_date: datetime,
                                  end_date: datetime) -> SuiteSplitConfig:
         """
@@ -113,7 +124,7 @@ class EvgExpansions(BaseModel):
         """
         return GenTaskOptions(
             create_misc_suite=True,
-            is_patch=self.is_patch,
+            is_patch=self.determine_is_patch(),
             generated_config_dir=GENERATED_CONFIG_DIR,
             use_default_timeouts=False,
             timeout_secs=self.timeout_secs,

buildscripts/tests/test_evergreen_gen_build_variant.py

@@ -87,7 +87,7 @@ def build_mock_orchestrator(build_expansions=None, task_def_list=None, build_tas
 class TestEvgExpansions(unittest.TestCase):
     def test_get_max_sub_suites_should_use_patch_value_in_patches(self):
         evg_expansions = under_test.EvgExpansions(
-            is_patch=True,
+            is_patch="true",
             max_sub_suites=5,
             mainline_max_sub_suites=1,
             build_id="build_id",
@@ -102,7 +102,7 @@ class TestEvgExpansions(unittest.TestCase):
 
     def test_get_max_sub_suites_should_use_mainline_value_in_non_patches(self):
         evg_expansions = under_test.EvgExpansions(
-            is_patch=False,
+            is_patch="false",
             max_sub_suites=5,
             mainline_max_sub_suites=1,
             build_id="build_id",
@@ -133,6 +133,60 @@ class TestEvgExpansions(unittest.TestCase):
                          evg_expansions.mainline_max_sub_suites)
 
 
+class TestDetermineIsPatch(unittest.TestCase):
+    def test_is_patch_is_none_should_return_false(self):
+        evg_expansions = under_test.EvgExpansions(
+            is_patch=None,
+            build_id="build_id",
+            build_variant="build variant",
+            project="project",
+            revision="abc123",
+            task_name="task name",
+            task_id="task_314",
+        )
+
+        self.assertFalse(evg_expansions.determine_is_patch())
+
+    def test_is_patch_is_false_should_return_false(self):
+        evg_expansions = under_test.EvgExpansions(
+            is_patch="false",
+            build_id="build_id",
+            build_variant="build variant",
+            project="project",
+            revision="abc123",
+            task_name="task name",
+            task_id="task_314",
+        )
+
+        self.assertFalse(evg_expansions.determine_is_patch())
+
+    def test_is_patch_is_empty_string_should_return_false(self):
+        evg_expansions = under_test.EvgExpansions(
+            is_patch="",
+            build_id="build_id",
+            build_variant="build variant",
+            project="project",
+            revision="abc123",
+            task_name="task name",
+            task_id="task_314",
+        )
+
+        self.assertFalse(evg_expansions.determine_is_patch())
+
+    def test_is_patch_is_true_should_return_true(self):
+        evg_expansions = under_test.EvgExpansions(
+            is_patch="true",
+            build_id="build_id",
+            build_variant="build variant",
+            project="project",
+            revision="abc123",
+            task_name="task name",
+            task_id="task_314",
+        )
+
+        self.assertTrue(evg_expansions.determine_is_patch())
+
+
 class TestTranslateRunVar(unittest.TestCase):
     def test_normal_value_should_be_returned(self):
         run_var = "some value"