a basic implementation of an append-only bulletin board with a file as a storage

.gitignore

@@ -13,3 +13,5 @@
 
 # Dependency directories (remove the comment below to include it)
 # vendor/
+
+.idea

storage/fileStorage.go

@@ -0,0 +1,81 @@
+package storage
+
+import (
+	"bufio"
+	"encoding/json"
+	"io"
+	"os"
+	"sync"
+)
+
+var _ Storage = (*FileStorage)(nil)
+
+type FileStorage struct {
+	sync.Mutex
+	file   *os.File
+	reader *bufio.Reader
+}
+
+func InitFileStorage(filename string) (*FileStorage, error) {
+	var (
+		fs  FileStorage
+		err error
+	)
+	if fs.file, err = os.OpenFile(filename, os.O_APPEND|os.O_CREATE|os.O_RDWR, 0644); err != nil {
+		return nil, err
+	}
+	fs.reader = bufio.NewReader(fs.file)
+	return &fs, nil
+}
+
+func (fs *FileStorage) Post(m Message) error {
+	fs.Lock()
+	defer fs.Unlock()
+
+	data, err := json.Marshal(m)
+	if err != nil {
+		return err
+	}
+	data = append(data, '\n')
+	_, err = fs.file.Write(data)
+	return err
+}
+
+func (fs *FileStorage) GetMessages(offset int) ([]Message, error) {
+	fs.Lock()
+	defer fs.Unlock()
+
+	var (
+		msgs []Message
+		err  error
+		row  []byte
+		data Message
+	)
+	if _, err = fs.file.Seek(0, 0); err != nil {
+		return nil, err
+	}
+	for {
+		row, err = fs.reader.ReadBytes('\n')
+		if err != nil {
+			break
+		}
+
+		if offset > 0 {
+			offset--
+			continue
+		}
+
+		if err = json.Unmarshal(row, &data); err != nil {
+			return nil, err
+		}
+		msgs = append(msgs, data)
+	}
+	if err != io.EOF {
+		return nil, err
+	}
+	return msgs, nil
+}
+
+func (fs *FileStorage) Close() error {
+	return fs.file.Close()
+}

storage/fileStorage_test.go

@@ -0,0 +1,46 @@
+package storage
+
+import (
+	"math/rand"
+	"reflect"
+	"testing"
+	"time"
+)
+
+func randomBytes(n int) []byte {
+	rand.Seed(time.Now().UnixNano())
+	b := make([]byte, n)
+	if _, err := rand.Read(b); err != nil {
+		return nil
+	}
+	return b
+}
+
+func TestFull(t *testing.T) {
+	N := 10
+	offset := 5
+	fs, err := InitFileStorage("test")
+	if err != nil {
+		t.Error(err)
+	}
+	defer fs.Close()
+	msgs := make([]Message, 0, N)
+	for i := 0; i < N; i++ {
+		msg := Message{
+			Data:      randomBytes(10),
+			Signature: randomBytes(10),
+		}
+		msgs = append(msgs, msg)
+		if err = fs.Post(msg); err != nil {
+			t.Error(err)
+		}
+	}
+	offsetMsgs, err := fs.GetMessages(offset)
+	if err != nil {
+		t.Error(err)
+	}
+	expectedOffsetMsgs := msgs[offset:]
+	if !reflect.DeepEqual(offsetMsgs, expectedOffsetMsgs) {
+		t.Errorf("expected messages: %v, actual messages: %v", expectedOffsetMsgs, offsetMsgs)
+	}
+}

storage/types.go

@@ -0,0 +1,12 @@
+package storage
+
+type Message struct {
+	Data      []byte `json:"data"`
+	Signature []byte `json:"signature"`
+}
+
+type Storage interface {
+	Post(message Message) error
+	GetMessages(offset int) ([]Message, error)
+	Close() error
+}