Fix Call.Unset() panic (issue stretchr#1236)

Unset changes len of a `ExpectedCalls` slice during iteration while using index from original slice, and under some conditions it tries to reslice element beyond of the slice boundaries. That causes panic.

The proposed solution uses independent write index to count elements kept in output slice.

Tests (the simplest and more complicated cases) and comments are included.

mock/mock.go

@@ -218,16 +218,22 @@ func (c *Call) Unset() *Call {
 
 	foundMatchingCall := false
 
-	for i, call := range c.Parent.ExpectedCalls {
+	// in-place filter slice for calls to be removed - iterate from 0'th to last skipping unnecessary ones
+	var index int // write index
+	for _, call := range c.Parent.ExpectedCalls {
 		if call.Method == c.Method {
 			_, diffCount := call.Arguments.Diff(c.Arguments)
 			if diffCount == 0 {
 				foundMatchingCall = true
-				// Remove from ExpectedCalls
-				c.Parent.ExpectedCalls = append(c.Parent.ExpectedCalls[:i], c.Parent.ExpectedCalls[i+1:]...)
+				// Remove from ExpectedCalls - just skip it
+				continue
 			}
 		}
+		c.Parent.ExpectedCalls[index] = call
+		index++
 	}
+	// trim slice up to last copied index
+	c.Parent.ExpectedCalls = c.Parent.ExpectedCalls[:index]
 
 	if !foundMatchingCall {
 		unlockOnce.Do(c.unlock)

mock/mock_test.go

@@ -569,6 +569,73 @@ func Test_Mock_UnsetIfAlreadyUnsetFails(t *testing.T) {
 	assert.Equal(t, 0, len(mockedService.ExpectedCalls))
 }
 
+func Test_Mock_UnsetByOnMethodSpec(t *testing.T) {
+	// make a test impl object
+	var mockedService = new(TestExampleImplementation)
+
+	mock1 := mockedService.
+		On("TheExampleMethod", 1, 2, 3).
+		Return(0, nil)
+
+	assert.Equal(t, 1, len(mockedService.ExpectedCalls))
+	mock1.On("TheExampleMethod", 1, 2, 3).
+		Return(0, nil).Unset()
+
+	assert.Equal(t, 0, len(mockedService.ExpectedCalls))
+
+	assert.Panics(t, func() {
+		mock1.Unset()
+	})
+
+	assert.Equal(t, 0, len(mockedService.ExpectedCalls))
+}
+
+func Test_Mock_UnsetByOnMethodSpecAmongOthers(t *testing.T) {
+	// make a test impl object
+	var mockedService = new(TestExampleImplementation)
+
+	_, filename, line, _ := runtime.Caller(0)
+	mock1 := mockedService.
+		On("TheExampleMethod", 1, 2, 3).
+		Return(0, nil).
+		On("TheExampleMethodVariadic", 1, 2, 3, 4, 5).Once().
+		Return(nil)
+	mock1.
+		On("TheExampleMethodFuncType", Anything).
+		Return(nil)
+
+	assert.Equal(t, 3, len(mockedService.ExpectedCalls))
+	mock1.On("TheExampleMethod", 1, 2, 3).
+		Return(0, nil).Unset()
+
+	assert.Equal(t, 2, len(mockedService.ExpectedCalls))
+
+	expectedCalls := []*Call{
+		{
+			Parent:          &mockedService.Mock,
+			Method:          "TheExampleMethodVariadic",
+			Repeatability:   1,
+			Arguments:       []interface{}{1, 2, 3, 4, 5},
+			ReturnArguments: []interface{}{nil},
+			callerInfo:      []string{fmt.Sprintf("%s:%d", filename, line+4)},
+		},
+		{
+			Parent:          &mockedService.Mock,
+			Method:          "TheExampleMethodFuncType",
+			Arguments:       []interface{}{Anything},
+			ReturnArguments: []interface{}{nil},
+			callerInfo:      []string{fmt.Sprintf("%s:%d", filename, line+7)},
+		},
+	}
+
+	// assert.Panics(t, func() {
+	// 	mock1.Unset()
+	// })
+
+	assert.Equal(t, 2, len(mockedService.ExpectedCalls))
+	assert.Equal(t, expectedCalls, mockedService.ExpectedCalls)
+}
+
 func Test_Mock_Return(t *testing.T) {
 
 	// make a test impl object