fix: preserve empty text and bytes in memory

Merge #7072

PiperOrigin-RevId: 981274973

Author: baba9811

src/google/adk/artifacts/in_memory_artifact_service.py

@@ -47,6 +47,23 @@ class _ArtifactEntry:
   artifact_version: ArtifactVersion
 
 
+# Runner._compute_artifact_delta_for_rewind marks an artifact as
+# inaccessible by saving exactly this part. Match it exactly rather than
+# treating every empty payload as absent, so a caller that saves a
+# legitimately empty artifact can read it back.
+#
+# Notes:
+# 1. A caller that saves an empty artifact with mime type exactly
+#    application/octet-stream will still read back None. That collision is
+#    inherent to using content shape as a tombstone; narrowing the match
+#    shrinks the hole from every empty artifact to one specific mime type.
+# 2. This tombstone convention is in-memory only; other artifact services
+#    (such as GcsArtifactService) do not perform this empty-payload check.
+_REWIND_TOMBSTONE = types.Part(
+    inline_data=types.Blob(mime_type="application/octet-stream", data=b"")
+)
+
+
 class InMemoryArtifactService(BaseArtifactService, BaseModel):
   """An in-memory implementation of the artifact service.
 
@@ -208,11 +225,7 @@ async def load_artifact(
           version=parsed_uri.version,
       )
 
-    if (
-        artifact_data == types.Part()
-        or artifact_data == types.Part(text="")
-        or (artifact_data.inline_data and not artifact_data.inline_data.data)
-    ):
+    if artifact_data == types.Part() or artifact_data == _REWIND_TOMBSTONE:
       return None
     return artifact_data
 

tests/unittests/artifacts/test_artifact_service.py

@@ -2954,12 +2954,15 @@ async def test_save_load_text_artifact(
 @pytest.mark.parametrize(
     "service_type",
     [
+        ArtifactServiceType.IN_MEMORY,
         ArtifactServiceType.GCS,
         ArtifactServiceType.FILE,
     ],
 )
+@pytest.mark.parametrize("filename", ["empty.txt", "user:empty.txt"])
+@pytest.mark.parametrize("version", [None, 0])
 async def test_save_load_empty_text_artifact(
-    service_type, artifact_service_factory
+    service_type, artifact_service_factory, filename, version
 ):
   """Tests that empty text artifacts survive round-trip save/load."""
   artifact_service = artifact_service_factory(service_type)
@@ -2969,20 +2972,57 @@ async def test_save_load_empty_text_artifact(
       app_name="app0",
       user_id="user0",
       session_id="123",
-      filename="empty.txt",
+      filename=filename,
       artifact=artifact,
   )
   loaded = await artifact_service.load_artifact(
       app_name="app0",
       user_id="user0",
       session_id="123",
-      filename="empty.txt",
+      filename=filename,
+      version=version,
   )
   assert loaded is not None
   assert loaded.text == ""
   assert loaded.inline_data is None
 
 
+@pytest.mark.parametrize(
+    "service_type",
+    [
+        ArtifactServiceType.IN_MEMORY,
+        ArtifactServiceType.GCS,
+        ArtifactServiceType.FILE,
+    ],
+)
+@pytest.mark.parametrize("filename", ["empty.bin", "user:empty.bin"])
+@pytest.mark.parametrize("version", [None, 0])
+async def test_save_load_empty_bytes_artifact(
+    service_type, artifact_service_factory, filename, version
+):
+  """Tests that empty bytes artifacts survive round-trip save/load."""
+  artifact_service = artifact_service_factory(service_type)
+  artifact = types.Part.from_bytes(data=b"", mime_type="application/pdf")
+
+  await artifact_service.save_artifact(
+      app_name="app0",
+      user_id="user0",
+      session_id="123",
+      filename=filename,
+      artifact=artifact,
+  )
+  loaded = await artifact_service.load_artifact(
+      app_name="app0",
+      user_id="user0",
+      session_id="123",
+      filename=filename,
+      version=version,
+  )
+  assert loaded is not None
+  assert loaded.inline_data is not None
+  assert loaded.inline_data.data == b""
+
+
 def _write_tampered_metadata(
     root: Path,
     *,

tests/unittests/runners/test_runner_rewind.py

@@ -76,7 +76,8 @@ def setup_method(self):
     )
 
   @pytest.mark.asyncio
-  async def test_rewind_async_with_state_and_artifacts(self):
+  @pytest.mark.parametrize("initial_text", ["f1v0", ""])
+  async def test_rewind_async_with_state_and_artifacts(self, initial_text):
     """Tests rewind_async rewinds state and artifacts."""
     runner = self.runner
     user_id = "test_user"
@@ -93,7 +94,7 @@ async def test_rewind_async_with_state_and_artifacts(self):
         user_id=user_id,
         session_id=session_id,
         filename="f1",
-        artifact=types.Part.from_text(text="f1v0"),
+        artifact=types.Part.from_text(text=initial_text),
     )
     event1 = Event(
         invocation_id="invocation1",
@@ -177,7 +178,7 @@ async def test_rewind_async_with_state_and_artifacts(self):
         user_id=user_id,
         session_id=session_id,
         filename="f1",
-    ) == types.Part.from_text(text="f1v0")
+    ) == types.Part.from_text(text=initial_text)
     # f2 should not exist
     assert (
         await runner.artifact_service.load_artifact(