How can I test validation functionality in a terraform provider - go

Question

I have written some more sophisticated validation logic for fields I need to validate in a custom terraform provider. I can, of course, test these are unit tests, but that's insufficient; why if I forgot to actually apply the validator?
So, I need to actually use terraform config and have the provider do it's normal, natural thing.
Basically, I expect it to error. The documentation seems to indicate that I should do a regex match on the output. But this can't be right; it seems super brittle. Can someone tell me how this is done?
func TestValidation(t *testing.T) {
const userBasic = `
resource "my_user" "dude" {
name = "the.dude%d"
password = "Password1" // needs a special char to pass
email = "the.dude%d#domain.com"
groups = [ "readers" ]
}
`
rgx, _ := regexp.Compile("abc")
resource.UnitTest(t, resource.TestCase{
Providers: testAccProviders,
Steps: []resource.TestStep{
{
Config: userBasic,
ExpectError: rgx,
},
},
})
}
This code obviously doesn't work. And, a lot of research isn't yielding answers.

Answer 1

Since sdk version 2.3.0 you can set ErrorCheck function on resource.TestCase to provide more complex validation for errors.
For example:
resource.UnitTest(t, resource.TestCase{
Providers: testAccProviders,
ErrorCheck: func(err error) error {
if err == nil {
return errors.New("expected error but got none")
}
// your validation code here
// some simple example with string matching
if strings.Contains(err.Error(), "your expected error message blah") {
return nil
}
// return original error if no match
return err
},
Steps: []resource.TestStep{
{
Config: userBasic,
},
},
})