Prevent change in struct pointer values while testing - pointers

Question

How do I prevent any change done to struct values to remain in a subtest only i.e. to keep them unaffected outside the subtest. I can't make any changes in the structs as they are autogenerated using swagger codegen. Here is an example:
package main
import (
"testing"
)
func TestTyre(t *testing.T) {
type Tyre struct {
Color *string
}
type Vehicle struct {
Tyre Tyre
}
color := "black"
tyreForTest := Tyre{Color: &color}
expectedTyreColor := color
t.Run("negativeTest", func(t *testing.T) {
tyre := tyreForTest // would have worked if there weren't any pointer variables
*tyre.Color = "blue" // here I expect value to change only for this subtest
vehicle := Vehicle{Tyre: tyre}
actualTyreColor := vehicle.Tyre.Color
ok := (expectedTyreColor == *actualTyreColor)
if ok {
t.Error("Color should be blue")
}
})
t.Run("positiveTest", func(t *testing.T) {
tyre := tyreForTest
vehicle := Vehicle{Tyre: tyre}
actualTyreColor := vehicle.Tyre.Color
ok := (expectedTyreColor == *actualTyreColor)
if !ok {
t.Error("Color should be black, instead of", *actualTyreColor)
}
})
}
Output:
=== RUN TestTyre
=== RUN TestTyre/negativeTest
=== RUN TestTyre/positiveTest
TestTyre/positiveTest: prog.go:39: Color should be black, instead of blue
--- FAIL: TestTyre (0.00s)
--- PASS: TestTyre/negativeTest (0.00s)
--- FAIL: TestTyre/positiveTest (0.00s)
FAIL

Answer 1

You prevent that by not sharing color variable between test cases - each of your test cases gets a new Tyre but they both point to same memory storing color value. So, when you change tyre.Color in negative test case you update value of variable color and other test case tyre.Color points to that variable too.
Simple solution would be having a function makeTyre() that gets completely new Tyre with its own memory to store color and obtain Tyre for each test case:
package main
import (
"testing"
)
type Tyre struct {
Color *string
}
type Vehicle struct {
Tyre Tyre
}
func makeTyre() Tyre {
color := "black"
return Tyre{Color: &color}
}
func TestTyre(t *testing.T) {
expectedTyreColor := "black"
t.Run("negativeTest", func(t *testing.T) {
tyre := makeTyre()
*tyre.Color = "blue"
vehicle := Vehicle{Tyre: tyre}
actualTyreColor := vehicle.Tyre.Color
ok := (expectedTyreColor == *actualTyreColor)
if ok {
t.Error("Color should be blue")
}
})
t.Run("positiveTest", func(t *testing.T) {
tyre := makeTyre()
vehicle := Vehicle{Tyre: tyre}
actualTyreColor := vehicle.Tyre.Color
ok := (expectedTyreColor == *actualTyreColor)
if !ok {
t.Error("Color should be black, instead of", *actualTyreColor)
}
})
}