firestore, datastore: implement DetectProjectID for emulator

We can use any project ID when the emulator is being used.

Try to look detect a real-looking one, and fallback to a dummy ID.

Fixes #1751.

datastore/datastore.go

@@ -75,6 +75,12 @@ func NewClient(ctx context.Context, projectID string, opts ...option.ClientOptio
 			option.WithoutAuthentication(),
 			option.WithGRPCDialOption(grpc.WithInsecure()),
 		}
+		if projectID == DetectProjectID {
+			projectID, _ = detectProjectID(ctx, opts...)
+			if projectID == "" {
+				projectID = "dummy-emulator-datastore-project"
+			}
+		}
 	} else {
 		o = []option.ClientOption{
 			option.WithEndpoint(prodAddr),
@@ -96,16 +102,11 @@ func NewClient(ctx context.Context, projectID string, opts ...option.ClientOptio
 	o = append(o, opts...)
 
 	if projectID == DetectProjectID {
-		creds, err := transport.Creds(ctx, o...)
+		detected, err := detectProjectID(ctx, opts...)
 		if err != nil {
-			return nil, fmt.Errorf("fetching creds: %v", err)
+			return nil, err
 		}
-
-		if creds.ProjectID == "" {
-			return nil, errors.New("datastore: see the docs on DetectProjectID")
-		}
-
-		projectID = creds.ProjectID
+		projectID = detected
 	}
 
 	if projectID == "" {
@@ -122,6 +123,17 @@ func NewClient(ctx context.Context, projectID string, opts ...option.ClientOptio
 	}, nil
 }
 
+func detectProjectID(ctx context.Context, opts ...option.ClientOption) (string, error) {
+	creds, err := transport.Creds(ctx, opts...)
+	if err != nil {
+		return "", fmt.Errorf("fetching creds: %v", err)
+	}
+	if creds.ProjectID == "" {
+		return "", errors.New("datastore: see the docs on DetectProjectID")
+	}
+	return creds.ProjectID, nil
+}
+
 var (
 	// ErrInvalidEntityType is returned when functions like Get or Next are
 	// passed a dst or src argument of invalid type.

firestore/client.go

@@ -68,18 +68,21 @@ func NewClient(ctx context.Context, projectID string, opts ...option.ClientOptio
 			return nil, fmt.Errorf("firestore: dialing address from env var FIRESTORE_EMULATOR_HOST: %s", err)
 		}
 		o = []option.ClientOption{option.WithGRPCConn(conn)}
+		if projectID == DetectProjectID {
+			projectID, _ = detectProjectID(ctx, opts...)
+			if projectID == "" {
+				projectID = "dummy-emulator-firestore-project"
+			}
+		}
 	}
 	o = append(o, opts...)
 
 	if projectID == DetectProjectID {
-		creds, err := transport.Creds(ctx, o...)
+		detected, err := detectProjectID(ctx, o...)
 		if err != nil {
-			return nil, fmt.Errorf("fetching creds: %v", err)
-		}
-		if creds.ProjectID == "" {
-			return nil, errors.New("firestore: see the docs on DetectProjectID")
+			return nil, err
 		}
-		projectID = creds.ProjectID
+		projectID = detected
 	}
 
 	vc, err := vkit.NewClient(ctx, o...)
@@ -93,7 +96,17 @@ func NewClient(ctx context.Context, projectID string, opts ...option.ClientOptio
 		databaseID: "(default)", // always "(default)", for now
 	}
 	return c, nil
+}
 
+func detectProjectID(ctx context.Context, opts ...option.ClientOption) (string, error) {
+	creds, err := transport.Creds(ctx, opts...)
+	if err != nil {
+		return "", fmt.Errorf("fetching creds: %v", err)
+	}
+	if creds.ProjectID == "" {
+		return "", errors.New("firestore: see the docs on DetectProjectID")
+	}
+	return creds.ProjectID, nil
 }
 
 // Close closes any resources held by the client.