Mock Golang Tests (If we want to test methodA, while it will call methodB internally, how can we mock the tests?)

demo.go

package demo

import (
	"context"
)

type ProviderInterface interface {
	MethodA(ctx context.Context, id int) error
	MethodB(ctx context.Context, id int) error
	MethodC(ctx context.Context, id int) error
}

type providerImpl struct {

}

func (impl *providerImpl) MethodA(ctx context.Context, id int) error  {
	// do something

	// then call method B
	return impl.MethodB(ctx, id)
}

func (impl *providerImpl) MethodB(ctx context.Context, id int) error  {
	// do something

	return nil
}

func (impl *providerImpl) MethodC(ctx context.Context, id int) error  {
	// do something

	return nil
}

deomo_test.go

package demo

import (
	"context"
	"testing"
)

type mockProvider struct {
	methodAFunc func(ctx context.Context, id int) error
	methodBFunc func(ctx context.Context, id int) error
	methodCFunc func(ctx context.Context, id int) error
}

func (mp *mockProvider) MethodA(ctx context.Context, id int) error {
	if mp.methodAFunc != nil {
		return mp.methodAFunc(ctx, id)
	}
	return nil
}

func (mp *mockProvider) MethodB(ctx context.Context, id int) error {
	if mp.methodBFunc != nil {
		return mp.methodBFunc(ctx, id)
	}
	return nil
}

func (mp *mockProvider) MethodC(ctx context.Context, id int) error {
	if mp.methodCFunc != nil {
		return mp.methodCFunc(ctx, id)
	}
	return nil
}

func TestMethodA(t *testing.T) {
	// we're testing methodA, which will call methodB

	actualImpl := &providerImpl{}

	tests := []struct {
		name string
		mp   ProviderInterface
	}{
		{
			mp: &mockProvider{
				methodAFunc: actualImpl.MethodA, // we can use actual providerImpl
				methodBFunc: func(ctx context.Context, id int) error {
					// mock some tests
					return nil
				},
				methodCFunc: nil,
			},
		},
		{
			mp: &mockProvider{
				methodAFunc: func(ctx context.Context, id int) error {
					// mock some tests
					return nil
				},
				methodBFunc: func(ctx context.Context, id int) error {
					// mock some tests
					return nil
				},
				methodCFunc: nil,
			},
		},
	}
	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			if err := tt.mp.MethodA(context.TODO(), 123); err != nil {
				t.Errorf("MethodA: %v", err)
			}
		})
	}
}