mock: Deadlock if mocking String() #643
Description
This is a fairly contrived example, I admit. But this was a real head-scratcher in the wild.
The following code will never complete.
package main
import (
"fmt"
"github.com/stretchr/testify/mock"
)
type K struct {
mock.Mock
}
func (f *K) String() string {
return f.Called().Get(0).(string)
}
func (f *K) Foo(b B) string {
return f.Called(b).Get(0).(string)
}
type B struct {
Kay *K
}
func main() {
k := &K{}
b := B{
Kay: k,
}
k.On("String").Return("string-return")
k.On("Foo", b).Return("foo-return")
fmt.Printf(k.Foo(b))
}
This fails because Mock.Called calls Mock.MethodCalled, which locks the Mutex, and eventually calls Arguments.Diff. When Arguments.Diff calls fmt.Sprintf, fmt.Sprintf sees the Kay field of B. Since K implements fmt.Stringer, fmt.Sprintf calls K.String. Since this is a mocked function, K.String jumps into Mock.Called, which calls Mock.MethodCalled which waits for the still-locked mutex to unlock, which never happens, because the lock is further up the call chain.
I don't know if this should be considered a bug or not, but it certainly was unexpected, and took a lot of digging to figure out.