Pull request: dhcpd: fix ip ranges

Updates #2541.

Squashed commit of the following:

commit c81299991876f48836d24872d9145331a0bc9e6e
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Mar 16 18:10:07 2021 +0300

    agherr: imp docs

commit f43a5f5cde0ea16dd38dd533e16e415a1d306cb2
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Mar 16 17:35:59 2021 +0300

    all: imp err handling, fix code

commit ed26ad0ff53882725f7747264f8094e6fb9b0423
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Mar 16 12:24:17 2021 +0300

    dhcpd: fix ip ranges

internal/agherr/agherr_test.go

@@ -6,30 +6,32 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestError_Error(t *testing.T) {
 	testCases := []struct {
+		err  error
 		name string
 		want string
-		err  error
 	}{{
+		err:  Many("a"),
 		name: "simple",
 		want: "a",
-		err:  Many("a"),
 	}, {
+		err:  Many("a", errors.New("b")),
 		name: "wrapping",
 		want: "a: b",
-		err:  Many("a", errors.New("b")),
 	}, {
+		err:  Many("a", errors.New("b"), errors.New("c"), errors.New("d")),
 		name: "wrapping several",
 		want: "a: b (hidden: c, d)",
-		err:  Many("a", errors.New("b"), errors.New("c"), errors.New("d")),
 	}, {
+		err:  Many("a", Many("b", errors.New("c"), errors.New("d"))),
 		name: "wrapping wrapper",
 		want: "a: b: c (hidden: d)",
-		err:  Many("a", Many("b", errors.New("c"), errors.New("d"))),
 	}}
+
 	for _, tc := range testCases {
 		assert.Equal(t, tc.want, tc.err.Error(), tc.name)
 	}
@@ -43,33 +45,78 @@ func TestError_Unwrap(t *testing.T) {
 		errWrapped
 		errNil
 	)
+
 	errs := []error{
 		errSimple:  errors.New("a"),
 		errWrapped: fmt.Errorf("err: %w", errors.New("nested")),
 		errNil:     nil,
 	}
+
 	testCases := []struct {
-		name    string
 		want    error
 		wrapped error
+		name    string
 	}{{
-		name:    "simple",
 		want:    errs[errSimple],
 		wrapped: Many("a", errs[errSimple]),
+		name:    "simple",
 	}, {
-		name:    "nested",
 		want:    errs[errWrapped],
 		wrapped: Many("b", errs[errWrapped]),
+		name:    "nested",
 	}, {
-		name:    "nil passed",
 		want:    errs[errNil],
 		wrapped: Many("c", errs[errNil]),
+		name:    "nil passed",
 	}, {
-		name:    "nil not passed",
 		want:    nil,
 		wrapped: Many("d"),
+		name:    "nil not passed",
 	}}
+
 	for _, tc := range testCases {
 		assert.Equal(t, tc.want, errors.Unwrap(tc.wrapped), tc.name)
 	}
 }
+
+func TestAnnotate(t *testing.T) {
+	const s = "1234"
+	const wantMsg = `bad string "1234": test`
+
+	// Don't use const, because we can't take a pointer of a constant.
+	var errTest error = Error("test")
+
+	t.Run("nil", func(t *testing.T) {
+		var errPtr *error
+		assert.NotPanics(t, func() {
+			Annotate("bad string %q: %w", errPtr, s)
+		})
+	})
+
+	t.Run("non_nil", func(t *testing.T) {
+		errPtr := &errTest
+		assert.NotPanics(t, func() {
+			Annotate("bad string %q: %w", errPtr, s)
+		})
+
+		require.NotNil(t, errPtr)
+
+		err := *errPtr
+		require.NotNil(t, err)
+
+		assert.Equal(t, wantMsg, err.Error())
+	})
+
+	t.Run("defer", func(t *testing.T) {
+		f := func() (err error) {
+			defer Annotate("bad string %q: %w", &errTest, s)
+
+			return errTest
+		}
+
+		err := f()
+		require.NotNil(t, err)
+
+		assert.Equal(t, wantMsg, err.Error())
+	})
+}