util/slicesx: add package for generic slice functions, use

Now that we're using rand.Shuffle in a few locations, create a generic
shuffle function and use it instead. While we're at it, move the
interleaveSlices function to the same package for use.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I0b00920e5b3eea846b6cedc30bd34d978a049fd3

net/dnscache/dnscache.go

@@ -24,6 +24,7 @@ import (
 	"tailscale.com/types/logger"
 	"tailscale.com/util/cloudenv"
 	"tailscale.com/util/singleflight"
+	"tailscale.com/util/slicesx"
 )
 
 var zaddr netip.Addr
@@ -577,7 +578,7 @@ func (dc *dialCall) raceDial(ctx context.Context, ips []netip.Addr) (net.Conn, e
 			iv4 = append(iv4, ip)
 		}
 	}
-	ips = interleaveSlices(iv6, iv4)
+	ips = slicesx.Interleave(iv6, iv4)
 
 	go func() {
 		for i, ip := range ips {
@@ -636,21 +637,6 @@ func (dc *dialCall) raceDial(ctx context.Context, ips []netip.Addr) (net.Conn, e
 	}
 }
 
-// interleaveSlices combines two slices of the form [a, b, c] and [x, y, z]
-// into a slice with elements interleaved; i.e. [a, x, b, y, c, z].
-func interleaveSlices[T any](a, b []T) []T {
-	var (
-		i   int
-		ret = make([]T, 0, len(a)+len(b))
-	)
-	for i = 0; i < len(a) && i < len(b); i++ {
-		ret = append(ret, a[i], b[i])
-	}
-	ret = append(ret, a[i:]...)
-	ret = append(ret, b[i:]...)
-	return ret
-}
-
 func v4addrs(aa []netip.Addr) (ret []netip.Addr) {
 	for _, a := range aa {
 		a = a.Unmap()

net/dnscache/dnscache_test.go

@@ -141,30 +141,6 @@ func TestResolverAllHostStaticResult(t *testing.T) {
 	}
 }
 
-func TestInterleaveSlices(t *testing.T) {
-	testCases := []struct {
-		name string
-		a, b []int
-		want []int
-	}{
-		{name: "equal", a: []int{1, 3, 5}, b: []int{2, 4, 6}, want: []int{1, 2, 3, 4, 5, 6}},
-		{name: "short_b", a: []int{1, 3, 5}, b: []int{2, 4}, want: []int{1, 2, 3, 4, 5}},
-		{name: "short_a", a: []int{1, 3}, b: []int{2, 4, 6}, want: []int{1, 2, 3, 4, 6}},
-		{name: "len_1", a: []int{1}, b: []int{2, 4, 6}, want: []int{1, 2, 4, 6}},
-		{name: "nil_a", a: nil, b: []int{2, 4, 6}, want: []int{2, 4, 6}},
-		{name: "nil_all", a: nil, b: nil, want: []int{}},
-	}
-
-	for _, tc := range testCases {
-		t.Run(tc.name, func(t *testing.T) {
-			merged := interleaveSlices(tc.a, tc.b)
-			if !reflect.DeepEqual(merged, tc.want) {
-				t.Errorf("got %v; want %v", merged, tc.want)
-			}
-		})
-	}
-}
-
 func TestShouldTryBootstrap(t *testing.T) {
 	oldDebug := debug
 	t.Cleanup(func() { debug = oldDebug })