logging/backend.go

package logging

import (
	"fmt"
	"io"
	"os"
	"path/filepath"
)

// Backend is the interface that specifies the methods that a backend must
// implement.
type Backend interface {
	Write(*Record) error
	SetFormatter(Formatter)
	SetLevel(Level)
	Level() Level
	Reopen() error
	Close() error
}

//
// Backend to write in file-like objects
//

var _ Backend = &FileBackend{}

// FileBackend is a backend that writes to a file.
type FileBackend struct {
	formatter Formatter
	l         io.Writer
	filepath  string
	level     Level
}

// NewStdoutBackend creates a new backend to write the logs on the standard
// output.
func NewStdoutBackend() *FileBackend {
	return &FileBackend{
		l:         os.Stdout,
		formatter: defaultFormatter,
	}
}

// NewStderrBackend creates a new backend to write the logs on the error output.
func NewStderrBackend() *FileBackend {
	return &FileBackend{
		l:         os.Stderr,
		formatter: defaultFormatter,
	}
}

// NewFileBackend creates a new backend to write the logs in a given file.
func NewFileBackend(filename string) (*FileBackend, error) {
	fd, err := os.OpenFile(filepath.Clean(filename),
		os.O_WRONLY|os.O_APPEND|os.O_CREATE, 0o600)
	if err != nil {
		return nil, fmt.Errorf("cannot open log file %s: %w", filename, err)
	}

	b := &FileBackend{
		l:         fd,
		formatter: defaultFormatter,
		filepath:  filename,
	}

	return b, nil
}

// NewIoBackend creates a new backend to write the logs in a given io.Writer.
func NewIoBackend(buf io.Writer) *FileBackend {
	return &FileBackend{
		l:         buf,
		formatter: defaultFormatter,
	}
}

func (b FileBackend) Write(r *Record) error {
	text := b.formatter(r)

	_, err := io.WriteString(b.l, text)
	if err != nil {
		return fmt.Errorf("cannot write logs: %w", err)
	}

	return nil
}

// SetLevel changes the log level of the backend.
func (b *FileBackend) SetLevel(l Level) {
	b.level = l
}

// Level returns the log level set for this backend.
func (b *FileBackend) Level() Level {
	return b.level
}

// SetFormatter defines the formatter for this backend.
func (b *FileBackend) SetFormatter(f Formatter) {
	b.formatter = f
}

// Reopen closes and reopens the file it writes to. It should be used after log
// rotation.
func (b *FileBackend) Reopen() error {
	if err := b.Close(); err != nil {
		return err
	}

	fd, err := os.OpenFile(b.filepath, os.O_WRONLY|os.O_APPEND|os.O_CREATE, 0o600)
	if err != nil {
		return fmt.Errorf("cannot open log file %s: %w", b.filepath, err)
	}

	b.l = fd

	return nil
}

// Close closes the underlying file used by the backend.
func (b *FileBackend) Close() error {
	if b.filepath == "" {
		return nil
	}

	if c, ok := b.l.(io.Closer); ok {
		if err := c.Close(); err != nil {
			return fmt.Errorf("cannot close log file: %w", err)
		}
	}

	return nil
}

//
// Noop Backend
//

var _ Backend = &NoopBackend{}

// NoopBackend does nothing and discards all log entries without writing them anywhere.
type NoopBackend struct{}

// NewNoopBackend creates a noop backend.
func NewNoopBackend() (*NoopBackend, error) {
	return &NoopBackend{}, nil
}

// Write is a noop.
func (*NoopBackend) Write(_ *Record) error {
	return nil
}

// SetFormatter is a noop.
func (*NoopBackend) SetFormatter(_ Formatter) {}

// SetLevel is a noop.
func (*NoopBackend) SetLevel(_ Level) {}

// Level always returns DefeultLevel.
func (*NoopBackend) Level() Level {
	return DefaultLevel
}

// Reopen is a noop.
func (*NoopBackend) Reopen() error {
	return nil
}

// Close is a noop.
func (*NoopBackend) Close() error {
	return nil
}