fix edge case in logic and update tests

python/src/filesystem.rs

@@ -635,7 +635,7 @@ impl ObjectOutputStream {
                 // this will never overflow
                 let remaining = self.max_buffer_size - self.buffer.content_length();
                 // if we have enough space to store this chunk, just append it
-                if chunk.len() <= remaining {
+                if chunk.len() < remaining {
                     self.buffer.extend_from_slice(chunk);
                     break;
                 }
@@ -646,6 +646,8 @@ impl ObjectOutputStream {
                 self.upload_buffer()?;
                 // len(second) will always be < max_buffer_size, and we just
                 // emptied the buffer by flushing, so we won't overflow
+                // if len(chunk) just happened to be == remaining,
+                // the second slice is empty. this is a no-op
                 self.buffer.extend_from_slice(second);
             }
             Ok(len as i64)

python/tests/test_fs.py

@@ -232,12 +232,18 @@ def test_roundtrip_azure_decoded_sas(azurite_sas_creds, sample_data: pa.Table):
     assert dt.version() == 0
 
 
-def test_warning_for_small_max_buffer_size(tmp_path):
-    for storage_size in [1, 4 * 1024 * 1024, 5 * 1024 * 1024 - 1]:
-        storage_opts = {"max_buffer_size": str(storage_size)}
-        store = DeltaStorageHandler(str(tmp_path.absolute()), options=storage_opts)
-        with pytest.warns(UserWarning):
-            store.open_output_stream("test")
+@pytest.mark.parametrize("storage_size", [1, 4 * 1024 * 1024, 5 * 1024 * 1024 - 1])
+def test_warning_for_small_max_buffer_size(tmp_path, storage_size):
+    storage_opts = {"max_buffer_size": str(storage_size)}
+    store = DeltaStorageHandler(str(tmp_path.absolute()), options=storage_opts)
+    with pytest.warns(UserWarning) as warnings:
+        store.open_output_stream("test")
+
+    assert len(warnings) == 1
+    assert (
+        f"You specified a `max_buffer_size` of {storage_size} bits less than {5*1024*1024} bits"
+        in str(warnings[0].message)
+    )
 
 
 def test_pickle_roundtrip(tmp_path):