datastore: Put fails with "invalid Value type []datastore.Property" when a PropertyLoadSaver map field is nil

[vaibhavnaagar]

Client

Datastore

Environment

Any. Reproduced on macOS arm64 / go1.26 against both the Datastore emulator and unit-level proto conversion, on every release from datastore/v1.3.0 through current HEAD.

Code

// Custom map type implementing PropertyLoadSaver, since maps are not
// natively supported property values:
type CustomMap map[string]string

func (m *CustomMap) Save() ([]datastore.Property, error) {
	b, err := json.Marshal(m)
	if err != nil {
		return nil, err
	}
	return []datastore.Property{{Name: "json", Value: b, NoIndex: true}}, nil
}

func (m *CustomMap) Load(props []datastore.Property) error {
	for _, p := range props {
		if p.Name == "json" {
			return json.Unmarshal(p.Value.([]byte), m)
		}
	}
	return nil
}

type MyEntity struct {
	Name  string    `datastore:"name"`
	MyMap CustomMap `datastore:"mymap"`
	Count int       `datastore:"count"`
}

// Works: MyMap is non-nil.
_, err := client.Put(ctx, key, &MyEntity{Name: "a", MyMap: CustomMap{"k": "v"}})

// Fails: MyMap is nil (the zero value of any entity that never initialized it).
_, err = client.Put(ctx, key, &MyEntity{Name: "a", Count: 2})

Expected behavior

The nil map field is saved the same way the non-nil field is: as a nested entity built from the Save() output (or flattened, with the flatten tag).

Actual behavior

datastore: invalid Value type []datastore.Property for a Property with Name "mymap"

Root cause

Non-nil PropertyLoadSaver fields are handled by plsFieldSave, which wraps the Save() output correctly: p.Value = &Entity{Properties: subProps}. But plsForSave intentionally skips nil values of nil-able kinds, so a nil map falls through to the fallback PropertyLoadSaver block in reflectFieldSave (save.go), which stores the slice raw:

val, err := pSaver.Save()
...
p.Value = val // []Property — not a valid Property value type

[]Property is not a legal Property.Value type, so interfaceToProto rejects it and every Put/Mutate through this path fails. The same block also ignores the flatten option, unlike plsFieldSave.

This block was added by #2794 to fix #2611 ("PropertyLoadSaver ignored with nil map types") — it correctly made Save() get called via the field's address, but stored the raw []Property instead of wrapping it. Its unit test only asserted SaveStruct output (where the invalid value goes unnoticed), so #2611's scenario has kept failing since, just with a different error message.

Workaround

Initialize the map field before saving.

I have a fix ready (wrap the Save() output in &Entity{...} and honor flatten, matching plsFieldSave) and will send a PR shortly.