av/device/file/file.go

/*
DESCRIPTION
  file.go provides an implementation of the AVDevice interface for media files.

AUTHORS
  Saxon A. Nelson-Milton <saxon@ausocean.org>

LICENSE
  Copyright (C) 2019 the Australian Ocean Lab (AusOcean)

  It is free software: you can redistribute it and/or modify them
  under the terms of the GNU General Public License as published by the
  Free Software Foundation, either version 3 of the License, or (at your
  option) any later version.

  It is distributed in the hope that it will be useful, but WITHOUT
  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
  for more details.

  You should have received a copy of the GNU General Public License
  in gpl.txt.  If not, see http://www.gnu.org/licenses.
*/

// Package file provides an implementation of AVDevice for files.
package file

import (
	"errors"
	"fmt"
	"io"
	"os"
	"sync"

	"bitbucket.org/ausocean/av/revid/config"
	"bitbucket.org/ausocean/utils/logging"
)

// AVFile is an implementation of the AVDevice interface for a file containg
// audio or video data.
type AVFile struct {
	f         *os.File
	path      string
	loop      bool
	isRunning bool
	log       logging.Logger
	set       bool
	mu        sync.Mutex
}

// NewAVFile returns a new AVFile.
func New(l logging.Logger) *AVFile { return &AVFile{log: l} }

// NewWith returns a new AVFile with required params provided i.e. the Set
// method does not need to be called.
func NewWith(l logging.Logger, path string, loop bool) *AVFile {
	return &AVFile{log: l, path: path, loop: loop, set: true}
}

// Name returns the name of the device.
func (m *AVFile) Name() string {
	return "File"
}

// Set simply sets the AVFile's config to the passed config.
func (m *AVFile) Set(c config.Config) error {
	m.path = c.InputPath
	m.loop = c.Loop
	m.set = true
	return nil
}

// Start will open the file at the location of the InputPath field of the
// config struct.
func (m *AVFile) Start() error {
	m.mu.Lock()
	defer m.mu.Unlock()
	var err error
	if !m.set {
		return errors.New("AVFile has not been set with config")
	}
	m.f, err = os.Open(m.path)
	if err != nil {
		return fmt.Errorf("could not open media file: %w", err)
	}
	m.isRunning = true
	return nil
}

// Stop will close the file such that any further reads will fail.
func (m *AVFile) Stop() error {
	m.mu.Lock()
	defer m.mu.Unlock()
	err := m.f.Close()
	if err == nil {
		m.isRunning = false
		return nil
	}
	return err
}

// Read implements io.Reader. If start has not been called, or Start has been
// called and Stop has since been called, an error is returned.
func (m *AVFile) Read(p []byte) (int, error) {
	m.mu.Lock()
	defer m.mu.Unlock()
	if m.f == nil {
		return 0, errors.New("AV file is closed, AVFile not started")
	}

	n, err := m.f.Read(p)
	if err != nil && err != io.EOF {
		return n, err
	}

	if (n < len(p) || err == io.EOF) && m.loop {
		m.log.Info("looping input file")
		// In the case that we reach end of file but loop is true, we want to
		// seek to start and keep reading from there.
		_, err = m.f.Seek(0, io.SeekStart)
		if err != nil {
			return 0, fmt.Errorf("could not seek to start of file for input loop: %w", err)
		}

		// Now that we've seeked to start, let's try reading again.
		n, err = m.f.Read(p)
		if err != nil {
			return n, fmt.Errorf("could not read after start seek: %w", err)
		}
	}
	return n, err
}

// IsRunning is used to determine if the AVFile device is running.
func (m *AVFile) IsRunning() bool {
	m.mu.Lock()
	defer m.mu.Unlock()
	return m.f != nil && m.isRunning
}