Pull request: replace agherr with golibs' errors

Merge in DNS/adguard-home from golibs-errors to master

Squashed commit of the following:

commit 5aba278a31c5a213bd9e08273ce7277c57713b22
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon May 24 17:05:18 2021 +0300

    all: imp code

commit f447eb875b81779fa9e391d98c31c1eeba7ef323
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon May 24 15:33:45 2021 +0300

    replace agherr with golibs' errors

internal/aghio/limitedreader_test.go

@@ -0,0 +1,108 @@
+package aghio
+
+import (
+	"fmt"
+	"io"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestLimitReader(t *testing.T) {
+	testCases := []struct {
+		want error
+		name string
+		n    int64
+	}{{
+		want: nil,
+		name: "positive",
+		n:    1,
+	}, {
+		want: nil,
+		name: "zero",
+		n:    0,
+	}, {
+		want: fmt.Errorf("aghio: invalid n in LimitReader: -1"),
+		name: "negative",
+		n:    -1,
+	}}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			_, err := LimitReader(nil, tc.n)
+			assert.Equal(t, tc.want, err)
+		})
+	}
+}
+
+func TestLimitedReader_Read(t *testing.T) {
+	testCases := []struct {
+		err   error
+		name  string
+		rStr  string
+		limit int64
+		want  int
+	}{{
+		err:   nil,
+		name:  "perfectly_match",
+		rStr:  "abc",
+		limit: 3,
+		want:  3,
+	}, {
+		err:   io.EOF,
+		name:  "eof",
+		rStr:  "",
+		limit: 3,
+		want:  0,
+	}, {
+		err: &LimitReachedError{
+			Limit: 0,
+		},
+		name:  "limit_reached",
+		rStr:  "abc",
+		limit: 0,
+		want:  0,
+	}, {
+		err:   nil,
+		name:  "truncated",
+		rStr:  "abc",
+		limit: 2,
+		want:  2,
+	}}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			readCloser := io.NopCloser(strings.NewReader(tc.rStr))
+			buf := make([]byte, tc.limit+1)
+
+			lreader, err := LimitReader(readCloser, tc.limit)
+			require.NoError(t, err)
+
+			n, err := lreader.Read(buf)
+			require.Equal(t, tc.err, err)
+			assert.Equal(t, tc.want, n)
+		})
+	}
+}
+
+func TestLimitedReader_LimitReachedError(t *testing.T) {
+	testCases := []struct {
+		err  error
+		name string
+		want string
+	}{{
+		err: &LimitReachedError{
+			Limit: 0,
+		},
+		name: "simplest",
+		want: "attempted to read more than 0 bytes",
+	}}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			assert.Equal(t, tc.want, tc.err.Error())
+		})
+	}
+}