fix: handle with read the empty file

core/filex/file.go

@@ -35,6 +35,7 @@ func firstLine(file *os.File) (string, error) {
 	for {
 		buf := make([]byte, bufSize)
 		n, err := file.ReadAt(buf, offset)
+
 		if err != nil && err != io.EOF {
 			return "", err
 		}
@@ -45,6 +46,10 @@ func firstLine(file *os.File) (string, error) {
 			}
 		}
 
+		if err == io.EOF {
+			return string(append(first, buf[:n]...)), nil
+		}
+
 		first = append(first, buf[:n]...)
 		offset += bufSize
 	}
@@ -56,31 +61,44 @@ func lastLine(filename string, file *os.File) (string, error) {
 		return "", err
 	}
 
+	bf := int64(bufSize)
 	var last []byte
 	offset := info.Size()
 	for {
-		offset -= bufSize
-		if offset < 0 {
+		if offset < bufSize {
+			bf = offset
 			offset = 0
+		} else {
+			offset -= bf
 		}
-		buf := make([]byte, bufSize)
+
+		buf := make([]byte, bf)
 		n, err := file.ReadAt(buf, offset)
 		if err != nil && err != io.EOF {
 			return "", err
 		}
 
+		if n == 0 {
+			return "", nil
+		}
+
 		if buf[n-1] == '\n' {
 			buf = buf[:n-1]
 			n--
 		} else {
 			buf = buf[:n]
 		}
+
 		for n--; n >= 0; n-- {
 			if buf[n] == '\n' {
 				return string(append(buf[n+1:], last...)), nil
 			}
 		}
 
 		last = append(buf, last...)
+
+		if offset == 0 {
+			return string(last), nil
+		}
 	}
 }

core/filex/file_test.go

@@ -52,6 +52,7 @@ last line`
 second line
 last line
 `
+	emptyContent = ``
 )
 
 func TestFirstLine(t *testing.T) {
@@ -79,6 +80,26 @@ func TestFirstLineError(t *testing.T) {
 	assert.Error(t, err)
 }
 
+func TestFirstLineEmptyFile(t *testing.T) {
+	filename, err := fs.TempFilenameWithText(emptyContent)
+	assert.Nil(t, err)
+	defer os.Remove(filename)
+
+	val, err := FirstLine(filename)
+	assert.Nil(t, err)
+	assert.Equal(t, "", val)
+}
+
+func TestFirstLineWithoutNewline(t *testing.T) {
+	filename, err := fs.TempFilenameWithText(longLine)
+	assert.Nil(t, err)
+	defer os.Remove(filename)
+
+	val, err := FirstLine(filename)
+	assert.Nil(t, err)
+	assert.Equal(t, longLine, val)
+}
+
 func TestLastLine(t *testing.T) {
 	filename, err := fs.TempFilenameWithText(text)
 	assert.Nil(t, err)
@@ -99,6 +120,16 @@ func TestLastLineWithLastNewline(t *testing.T) {
 	assert.Equal(t, longLine, val)
 }
 
+func TestLastLineWithoutLastNewline(t *testing.T) {
+	filename, err := fs.TempFilenameWithText(longLine)
+	assert.Nil(t, err)
+	defer os.Remove(filename)
+
+	val, err := LastLine(filename)
+	assert.Nil(t, err)
+	assert.Equal(t, longLine, val)
+}
+
 func TestLastLineShort(t *testing.T) {
 	filename, err := fs.TempFilenameWithText(shortText)
 	assert.Nil(t, err)
@@ -123,3 +154,13 @@ func TestLastLineError(t *testing.T) {
 	_, err := LastLine("/tmp/does-not-exist")
 	assert.Error(t, err)
 }
+
+func TestLastLineEmptyFile(t *testing.T) {
+	filename, err := fs.TempFilenameWithText(emptyContent)
+	assert.Nil(t, err)
+	defer os.Remove(filename)
+
+	val, err := LastLine(filename)
+	assert.Nil(t, err)
+	assert.Equal(t, "", val)
+}