Add new Concat utility and use it to fix slice.append gotcha

src/internal/util/util.go

@@ -147,3 +147,24 @@ func RemoveDuplicates[T comparable](slice []T) []T {
 	}
 	return result
 }
+
+// Concat returns a new slice concatenating the passed in slices.
+//
+// Avoids a gotcha in Go where since append modifies the underlying memory of the input slice, doing
+// newSlice := append(slice1, slice2) can modify slice1. See https://go.dev/doc/effective_go#append
+// A std. library concat was added in go 1.22, but this is for backwards compatibility. https://pkg.go.dev/slices#Concat
+// This is mostly similiar to the std. library concat, but with a few differences so it compiles on go 1.20.
+func Concat[S ~[]E, E any](slices ...S) S {
+	size := 0
+	for _, s := range slices {
+		size += len(s)
+		if size < 0 {
+			panic("len out of range")
+		}
+	}
+	newSlice := make([]E, 0, size)
+	for _, s := range slices {
+		newSlice = append(newSlice, s...)
+	}
+	return newSlice
+}

src/internal/util/util_test.go

@@ -143,3 +143,28 @@ func TestRemoveDuplicates(t *testing.T) {
 		})
 	}
 }
+
+func TestConcat(t *testing.T) {
+	inputSlice1 := make([]int, 0, 10)
+	for i := 0; i < 3; i++ {
+		inputSlice1 = append(inputSlice1, i)
+	}
+	// Test naive append
+	newSlice1 := append(inputSlice1, 4)
+	newSlice2 := append(inputSlice1, 5)
+	newSlice3 := append(inputSlice1, 6)
+	// Shows test is working, you'd think that this would be Equal but it isn't. append() is modifying the inputSlice1
+	require.NotEqual(t, []int{0, 1, 2, 4}, newSlice1)
+	require.NotEqual(t, []int{0, 1, 2, 5}, newSlice2)
+	require.Equal(t, []int{0, 1, 2, 6}, newSlice3)
+	// Now try with new Concat
+	newSlice1 = Concat(inputSlice1, []int{4})
+	newSlice2 = Concat(inputSlice1, []int{5})
+	newSlice3 = Concat(inputSlice1, []int{6})
+	require.Len(t, inputSlice1, 3)
+	require.Equal(t, 10, cap(inputSlice1))
+	require.Equal(t, []int{0, 1, 2}, inputSlice1)
+	require.Equal(t, []int{0, 1, 2, 4}, newSlice1)
+	require.Equal(t, []int{0, 1, 2, 5}, newSlice2)
+	require.Equal(t, []int{0, 1, 2, 6}, newSlice3)
+}

src/zdns/alookup.go

@@ -18,6 +18,8 @@ import (
 
 	"github.com/pkg/errors"
 	"github.com/zmap/dns"
+
+	"github.com/zmap/zdns/src/internal/util"
 )
 
 // DoTargetedLookup performs a lookup of the given domain name against the given nameserver, looking up both IPv4 and IPv6 addresses
@@ -57,7 +59,7 @@ func (r *Resolver) DoTargetedLookup(name, nameServer string, ipMode IPVersionMod
 		}
 	}
 
-	combinedTrace := append(ipv4Trace, ipv6Trace...)
+	combinedTrace := util.Concat(ipv4Trace, ipv6Trace)
 
 	// In case we get no IPs and a non-NOERROR status from either
 	// IPv4 or IPv6 lookup, we return that status.

src/zdns/lookup.go

@@ -292,7 +292,7 @@ func (r *Resolver) LookupAllNameservers(q *Question, nameServer string) (*Combin
 	for _, nserver := range nsResults.Servers {
 		// Use all the ipv4 and ipv6 addresses of each nameserver
 		nameserver := nserver.Name
-		ips := append(nserver.IPv4Addresses, nserver.IPv6Addresses...)
+		ips := util.Concat(nserver.IPv4Addresses, nserver.IPv6Addresses)
 		for _, ip := range ips {
 			curServer = net.JoinHostPort(ip, "53")
 			res, trace, status, _ := r.ExternalLookup(q, curServer)