Implement memlog store operations

Implement store operation that get/set key value pairs from
the in-memory store, and log update operations.

This change also include unit tests and store compliance tests.

The addition of the statestore package is split up into multiple
changeset to ease review. The final version of the package can be found
[here](https://github.com/urso/beats/tree/fb-input-v2-combined/libbeat/statestore).

Once finalized, the libbeat/statestore package contains:
- The statestore frontend and interface for use within Beats
- Interfaces for the store backend
- A common set of tests store backends need to support
- a storetest package for testing new features that require a store. The
  testing helpers use map[string]interface{} that can be initialized or
  queried after the test run for validation purposes.
- The default memlog backend + tests

libbeat/statestore/backend/memlog/memlog.go

@@ -54,6 +54,9 @@ type Settings struct {
 	// Checkpoint predicate that can trigger a registry file rotation.  If not
 	// configured, memlog will automatically trigger a checkpoint every 10MB.
 	Checkpoint CheckpointPredicate
+
+	// If set memlog will not check the version of the meta file.
+	IgnoreVersionCheck bool
 }
 
 // CheckpointPredicate is the type for configurable checkpoint checks.
@@ -62,7 +65,7 @@ type CheckpointPredicate func(fileSize uint64) bool
 
 const defaultFileMode os.FileMode = 0600
 
-const defaultBufferSize = 4096
+const defaultBufferSize = 4 * 1024
 
 func defaultCheckpoint(filesize uint64) bool {
 	const limit = 10 * 1 << 20 // set rotation limit to 10MB by default
@@ -110,7 +113,7 @@ func (r *Registry) Access(name string) (backend.Store, error) {
 	home := filepath.Join(r.settings.Root, name)
 	fileMode := r.settings.FileMode
 	bufSz := r.settings.BufferSize
-	store, err := openStore(logger, home, fileMode, bufSz, r.settings.Checkpoint)
+	store, err := openStore(logger, home, fileMode, bufSz, !r.settings.IgnoreVersionCheck, r.settings.Checkpoint)
 	if err != nil {
 		return nil, err
 	}

libbeat/statestore/backend/memlog/memlog_test.go

@@ -0,0 +1,257 @@
+// Licensed to Elasticsearch B.V. under one or more contributor
+// license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright
+// ownership. Elasticsearch B.V. licenses this file to you under
+// the Apache License, Version 2.0 (the "License"); you may
+// not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+package memlog
+
+import (
+	"encoding/json"
+	"io"
+	"io/ioutil"
+	"math"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/elastic/beats/v7/libbeat/logp"
+	"github.com/elastic/beats/v7/libbeat/statestore"
+	"github.com/elastic/beats/v7/libbeat/statestore/backend"
+	"github.com/elastic/beats/v7/libbeat/statestore/internal/storecompliance"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func init() {
+	logp.DevelopmentSetup()
+}
+
+func TestCompliance_Default(t *testing.T) {
+	storecompliance.TestBackendCompliance(t, func(testPath string) (backend.Registry, error) {
+		return New(logp.NewLogger("test"), Settings{Root: testPath})
+	})
+}
+
+func TestCompliance_AlwaysCheckpoint(t *testing.T) {
+	storecompliance.TestBackendCompliance(t, func(testPath string) (backend.Registry, error) {
+		return New(logp.NewLogger("test"), Settings{
+			Root: testPath,
+			Checkpoint: func(filesize uint64) bool {
+				return true
+			},
+		})
+	})
+}
+
+func TestLoadVersion1(t *testing.T) {
+	dataHome := "testdata/1"
+
+	list, err := ioutil.ReadDir(dataHome)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cases := list[:0]
+	for _, info := range list {
+		if info.IsDir() {
+			cases = append(cases, info)
+		}
+	}
+
+	for _, info := range cases {
+		name := filepath.Base(info.Name())
+		t.Run(name, func(t *testing.T) {
+			testLoadVersion1Case(t, filepath.Join(dataHome, info.Name()))
+		})
+	}
+}
+
+func testLoadVersion1Case(t *testing.T, dataPath string) {
+	path, err := ioutil.TempDir("", "")
+	if err != nil {
+		t.Fatalf("Failed to create temporary test directory: %v", err)
+	}
+	defer os.RemoveAll(path)
+
+	t.Logf("Test tmp dir: %v", path)
+
+	if err := copyPath(path, dataPath); err != nil {
+		t.Fatalf("Failed to copy test file to the temporary directory: %v", err)
+	}
+
+	// load expected test results
+	raw, err := ioutil.ReadFile(filepath.Join(path, "expected.json"))
+	if err != nil {
+		t.Fatalf("Failed to load expected.json: %v", err)
+	}
+
+	expected := struct {
+		Txid     uint64
+		Datafile string
+		Entries  map[string]interface{}
+	}{}
+	if err := json.Unmarshal(raw, &expected); err != nil {
+		t.Fatalf("Failed to parse expected.json: %v", err)
+	}
+
+	// load store:
+	store, err := openStore(logp.NewLogger("test"), path, 0660, 4096, true, func(_ uint64) bool {
+		return false
+	})
+	if err != nil {
+		t.Fatalf("Failed to load test store: %v", err)
+	}
+	defer store.Close()
+
+	disk := store.disk
+	disk.removeOldDataFiles()
+
+	// validate store:
+	assert.Equal(t, expected.Txid, disk.nextTxID-1)
+	if expected.Datafile != "" {
+		assert.Equal(t, filepath.Join(path, expected.Datafile), disk.activeDataFile.path)
+	}
+
+	// check all keys in expected are known and do match stored values:
+	func() {
+		for key, val := range expected.Entries {
+			var tmp interface{}
+			err := store.Get(key, &tmp)
+			require.NoError(t, err, "error reading entry (key=%v)", key)
+
+			assert.Equal(t, val, tmp, "failed when checking key '%s'", key)
+		}
+	}()
+
+	// check store does not contain any additional keys
+	func() {
+		err = store.Each(func(key string, val statestore.ValueDecoder) (bool, error) {
+			_, exists := expected.Entries[string(key)]
+			if !exists {
+				t.Errorf("unexpected key: %s", key)
+			}
+			return true, nil
+		})
+		assert.NoError(t, err)
+	}()
+}
+
+func TestTxIDLessEqual(t *testing.T) {
+	cases := map[string]struct {
+		a, b uint64
+		want bool
+	}{
+		"is equal":  {10, 10, true},
+		"is less":   {8, 9, true},
+		"is bigger": {9, 8, false},
+		"is less 0 with integer overflow": {
+			math.MaxUint64 - 2, 0, true,
+		},
+		"is less random value with integer overflow": {
+			math.MaxUint64 - 2, 10, true,
+		},
+		"is less with large ids": {
+			math.MaxUint64 - 10, math.MaxUint64 - 9, true,
+		},
+		"is bigger with large ids": {
+			math.MaxUint64 - 9, math.MaxUint64 - 10, false,
+		},
+	}
+
+	for name, test := range cases {
+		t.Run(name, func(t *testing.T) {
+			got := isTxIDLessEqual(test.a, test.b)
+			if got != test.want {
+				t.Fatalf("%v <= %v should be %v", test.a, test.b, test.want)
+			}
+		})
+	}
+}
+
+func copyPath(to, from string) error {
+	info, err := os.Stat(from)
+	if err != nil {
+		return err
+	}
+
+	if info.IsDir() {
+		return copyDir(to, from)
+	}
+	if info.Mode().IsRegular() {
+		return copyFile(to, from)
+	}
+
+	// ignore other file types
+	return nil
+}
+
+func copyDir(to, from string) error {
+	if !isDir(to) {
+		info, err := os.Stat(from)
+		if err != nil {
+			return err
+		}
+
+		if err := os.MkdirAll(to, info.Mode()); err != nil {
+			return err
+		}
+	}
+
+	list, err := ioutil.ReadDir(from)
+	if err != nil {
+		return err
+	}
+
+	for _, file := range list {
+		name := file.Name()
+		err := copyPath(filepath.Join(to, name), filepath.Join(from, name))
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func copyFile(to, from string) error {
+	in, err := os.Open(from)
+	if err != nil {
+		return err
+	}
+	defer in.Close()
+
+	info, err := in.Stat()
+	if err != nil {
+		return err
+	}
+
+	out, err := os.OpenFile(to, os.O_CREATE|os.O_RDWR|os.O_TRUNC, info.Mode())
+	if err != nil {
+		return err
+	}
+	defer out.Close()
+
+	_, err = io.Copy(out, in)
+	return err
+}
+
+func isDir(path string) bool {
+	info, err := os.Stat(path)
+	return err == nil && info.IsDir()
+}
+
+func isFile(path string) bool {
+	info, err := os.Stat(path)
+	return err == nil && info.Mode().IsRegular()
+}