Fix reversed ordering of arguments (#1648)

The source manager initialization function was defined as `sourceID`
followed by `jobID`, while the source initialization function is the
reverse. This is confusing and easy to mix up since the parameters are
the same type.

This commit adds a test to make sure the source manager initializes in
the correct order, but it doesn't prevent the library user to make the
same mistake. We may want to consider using different types.

pkg/sources/source_manager.go

@@ -19,7 +19,7 @@ type handle int64
 
 // SourceInitFunc is a function that takes a source and job ID and returns an
 // initialized Source.
-type SourceInitFunc func(ctx context.Context, sourceID int64, jobID int64) (Source, error)
+type SourceInitFunc func(ctx context.Context, jobID, sourceID int64) (Source, error)
 
 // sourceInfo is an aggregate struct to store source information provided on
 // initialization.
@@ -228,7 +228,7 @@ func (s *SourceManager) run(ctx context.Context, handle handle, jobID int64, rep
 		report.ReportError(Fatal{err})
 		return Fatal{err}
 	}
-	source, err := sourceInfo.initFunc(ctx, int64(handle), jobID)
+	source, err := sourceInfo.initFunc(ctx, jobID, int64(handle))
 	if err != nil {
 		report.ReportError(Fatal{err})
 		return Fatal{err}

pkg/sources/source_manager_test.go

@@ -245,3 +245,42 @@ func TestSourceManagerContextCancelled(t *testing.T) {
 	report := ref.Snapshot()
 	assert.Error(t, report.FatalError())
 }
+
+type DummyAPI struct {
+	registerSource func(context.Context, string, sourcespb.SourceType) (int64, error)
+	getJobID       func(context.Context, int64) (int64, error)
+}
+
+func (api DummyAPI) RegisterSource(ctx context.Context, name string, kind sourcespb.SourceType) (int64, error) {
+	return api.registerSource(ctx, name, kind)
+}
+
+func (api DummyAPI) GetJobID(ctx context.Context, id int64) (int64, error) {
+	return api.getJobID(ctx, id)
+}
+
+func TestSourceManagerJobAndSourceIDs(t *testing.T) {
+	mgr := NewManager(WithAPI(DummyAPI{
+		registerSource: func(context.Context, string, sourcespb.SourceType) (int64, error) {
+			return 1337, nil
+		},
+		getJobID: func(context.Context, int64) (int64, error) {
+			return 9001, nil
+		},
+	}))
+	var (
+		initializedJobID    int64
+		initializedSourceID int64
+	)
+	handle, err := mgr.Enroll(context.Background(), "dummy", 1337,
+		func(ctx context.Context, jobID, sourceID int64) (Source, error) {
+			initializedJobID = jobID
+			initializedSourceID = sourceID
+			return nil, fmt.Errorf("ignore")
+		})
+	assert.NoError(t, err)
+
+	_, _ = mgr.Run(context.Background(), handle)
+	assert.Equal(t, int64(1337), initializedSourceID)
+	assert.Equal(t, int64(9001), initializedJobID)
+}