Create a copy of slices before comparing

We do so to avoid sorting the original slices unexpectedly.

Signed-off-by: Andrew Morgan (https://amorgan.xyz) <andrew@amorgan.xyz>

src/github.com/matrix-org/dendrite/clientapi/routing/register.go

@@ -418,25 +418,35 @@ func isValidMacLogin(
 	return hmac.Equal(givenMac, expectedMAC), nil
 }
 
-// checkFlows checks a single flow a against another, b. If a contains at least
-// all of the stages that b does, checkFlows returns true.
-func checkFlows(a []authtypes.LoginType, b []authtypes.LoginType) bool {
-	// Sort the slices for simple comparison
-	sort.Slice(a, func(i, j int) bool { return a[i] < a[j] })
-	sort.Slice(b, func(i, j int) bool { return b[i] < b[j] })
+// checkFlows checks a single completed flow against another required one. If
+// one contains at least all of the stages that the other does, checkFlows
+// returns true.
+func checkFlows(
+	completedStages []authtypes.LoginType,
+	requiredStages []authtypes.LoginType,
+) bool {
+	// Create temporary slices so they originals will not be modified on sorting
+	completed := make([]authtypes.LoginType, len(completedStages))
+	required := make([]authtypes.LoginType, len(requiredStages))
+	copy(completed, completedStages)
+	copy(required, requiredStages)
 
-	// Iterate through each slice, going to the next allowed slice only once
+	// Sort the slices for simple comparison
+	sort.Slice(completed, func(i, j int) bool { return completed[i] < completed[j] })
+	sort.Slice(required, func(i, j int) bool { return required[i] < required[j] })
+
+	// Iterate through each slice, going to the next required slice only once
 	// we've found a match.
 	i, j := 0, 0
-	for j < len(b) {
+	for j < len(required) {
 		// Exit if we've reached the end of our input without being able to
-		// match all of the allowed stages.
-		if i >= len(a) {
+		// match all of the required stages.
+		if i >= len(completed) {
 			return false
 		}
 
-		// If we've found a stage we want, move on to the next allowed stage.
-		if a[i] == b[j] {
+		// If we've found a stage we want, move on to the next required stage.
+		if completed[i] == required[j] {
 			j++
 		}
 		i++

src/github.com/matrix-org/dendrite/clientapi/routing/register_test.go

@@ -97,7 +97,7 @@ func TestFlowCheckingInvalidStage(t *testing.T) {
 
 // Should return true as we complete all stages of an allowed flow, though out
 // of order, as well as extraneous stages.
-func TestFlowCheckingUnorderedAndExtraneous(t *testing.T) {
+func TestFlowCheckingExtraneousUnordered(t *testing.T) {
 	testFlow := []authtypes.LoginType{
 		authtypes.LoginType("stage5"),
 		authtypes.LoginType("stage4"),
@@ -110,7 +110,7 @@ func TestFlowCheckingUnorderedAndExtraneous(t *testing.T) {
 	}
 }
 
-// Should return false as we're providing less stages than are required.
+// Should return false as we're providing fewer stages than are required.
 func TestFlowCheckingShortIncorrectInput(t *testing.T) {
 	testFlow := []authtypes.LoginType{
 		authtypes.LoginType("stage8"),
@@ -120,7 +120,7 @@ func TestFlowCheckingShortIncorrectInput(t *testing.T) {
 	}
 }
 
-// Should return false as we're providing less stages than are required.
+// Should return false as we're providing different stages than are required.
 func TestFlowCheckingExtraneousIncorrectInput(t *testing.T) {
 	testFlow := []authtypes.LoginType{
 		authtypes.LoginType("stage8"),