util/cstruct: add package for decoding padded C structures (#5429)

I was working on my "dump iptables rules using only syscalls" branch and
had a bunch of C structure decoding to do. Rather than manually
calculating the padding or using unsafe trickery to actually cast
variable-length structures to Go types, I'd rather use a helper package
that deals with padding for me.

Padding rules were taken from the following article:
  http://www.catb.org/esr/structure-packing/

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

util/cstruct/cstruct_test.go

@@ -0,0 +1,151 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cstruct
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"testing"
+)
+
+func TestPadBytes(t *testing.T) {
+	testCases := []struct {
+		offset int
+		size   int
+		want   int
+	}{
+		// No padding at beginning of structure
+		{0, 1, 0},
+		{0, 2, 0},
+		{0, 4, 0},
+		{0, 8, 0},
+
+		// No padding for single bytes
+		{1, 1, 0},
+
+		// Single byte padding
+		{1, 2, 1},
+		{3, 4, 1},
+
+		// Multi-byte padding
+		{1, 4, 3},
+		{2, 8, 6},
+	}
+	for _, tc := range testCases {
+		t.Run(fmt.Sprintf("%d_%d_%d", tc.offset, tc.size, tc.want), func(t *testing.T) {
+			got := padBytes(tc.offset, tc.size)
+			if got != tc.want {
+				t.Errorf("got=%d; want=%d", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestDecoder(t *testing.T) {
+	t.Run("UnsignedTypes", func(t *testing.T) {
+		dec := func(n int) *Decoder {
+			buf := make([]byte, n)
+			buf[0] = 1
+
+			d := NewDecoder(buf)
+
+			// Use t.Cleanup to perform an assertion on this
+			// decoder after the test code is finished with it.
+			t.Cleanup(func() {
+				if err := d.Err(); err != nil {
+					t.Fatal(err)
+				}
+			})
+			return d
+		}
+		if got := dec(2).Uint16(); got != 1 {
+			t.Errorf("uint16: got=%d; want=1", got)
+		}
+		if got := dec(4).Uint32(); got != 1 {
+			t.Errorf("uint32: got=%d; want=1", got)
+		}
+		if got := dec(8).Uint64(); got != 1 {
+			t.Errorf("uint64: got=%d; want=1", got)
+		}
+		if got := dec(pointerSize / 8).Uintptr(); got != 1 {
+			t.Errorf("uintptr: got=%d; want=1", got)
+		}
+	})
+
+	t.Run("SignedTypes", func(t *testing.T) {
+		dec := func(n int) *Decoder {
+			// Make a buffer of the exact size that consists of 0xff bytes
+			buf := make([]byte, n)
+			for i := 0; i < n; i++ {
+				buf[i] = 0xff
+			}
+
+			d := NewDecoder(buf)
+
+			// Use t.Cleanup to perform an assertion on this
+			// decoder after the test code is finished with it.
+			t.Cleanup(func() {
+				if err := d.Err(); err != nil {
+					t.Fatal(err)
+				}
+			})
+			return d
+		}
+		if got := dec(2).Int16(); got != -1 {
+			t.Errorf("int16: got=%d; want=-1", got)
+		}
+		if got := dec(4).Int32(); got != -1 {
+			t.Errorf("int32: got=%d; want=-1", got)
+		}
+		if got := dec(8).Int64(); got != -1 {
+			t.Errorf("int64: got=%d; want=-1", got)
+		}
+	})
+
+	t.Run("InsufficientData", func(t *testing.T) {
+		dec := func(n int) *Decoder {
+			// Make a buffer that's too small and contains arbitrary bytes
+			buf := make([]byte, n-1)
+			for i := 0; i < n-1; i++ {
+				buf[i] = 0xAD
+			}
+
+			// Use t.Cleanup to perform an assertion on this
+			// decoder after the test code is finished with it.
+			d := NewDecoder(buf)
+			t.Cleanup(func() {
+				if err := d.Err(); err == nil || !errors.Is(err, io.EOF) {
+					t.Errorf("(n=%d) expected io.EOF; got=%v", n, err)
+				}
+			})
+			return d
+		}
+
+		dec(2).Uint16()
+		dec(4).Uint32()
+		dec(8).Uint64()
+		dec(pointerSize / 8).Uintptr()
+
+		dec(2).Int16()
+		dec(4).Int32()
+		dec(8).Int64()
+	})
+
+	t.Run("Bytes", func(t *testing.T) {
+		d := NewDecoder([]byte("hello worldasdf"))
+		t.Cleanup(func() {
+			if err := d.Err(); err != nil {
+				t.Fatal(err)
+			}
+		})
+
+		buf := make([]byte, 11)
+		d.Bytes(buf)
+		if got := string(buf); got != "hello world" {
+			t.Errorf("bytes: got=%q; want=%q", got, "hello world")
+		}
+	})
+}