// Copyright (C) 2014 The Syncthing Authors.
//
// This program is free software: you can redistribute it and/or modify it
// 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.
//
// This program 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 along
// with this program. If not, see <http://www.gnu.org/licenses/>.

package model

import (
	"io"
	"os"
	"path/filepath"
	"sync"

	"github.com/syncthing/protocol"
	"github.com/syncthing/syncthing/internal/db"
)

// A sharedPullerState is kept for each file that is being synced and is kept
// updated along the way.
type sharedPullerState struct {
	// Immutable, does not require locking
	file     protocol.FileInfo
	folder   string
	tempName string
	realName string
	reused   int // Number of blocks reused from temporary file

	// Mutable, must be locked for access
	err        error      // The first error we hit
	fd         *os.File   // The fd of the temp file
	copyTotal  int        // Total number of copy actions for the whole job
	pullTotal  int        // Total number of pull actions for the whole job
	copyOrigin int        // Number of blocks copied from the original file
	copyNeeded int        // Number of copy actions still pending
	pullNeeded int        // Number of block pulls still pending
	mut        sync.Mutex // Protects the above
}

// A momentary state representing the progress of the puller
type pullerProgress struct {
	Total               int
	Reused              int
	CopiedFromOrigin    int
	CopiedFromElsewhere int
	Pulled              int
	Pulling             int
	BytesDone           int64
	BytesTotal          int64
}

// A lockedWriterAt synchronizes WriteAt calls with an external mutex.
// WriteAt() is goroutine safe by itself, but not against for example Close().
type lockedWriterAt struct {
	mut *sync.Mutex
	wr  io.WriterAt
}

func (w lockedWriterAt) WriteAt(p []byte, off int64) (n int, err error) {
	w.mut.Lock()
	defer w.mut.Unlock()
	return w.wr.WriteAt(p, off)
}

// tempFile returns the fd for the temporary file, reusing an open fd
// or creating the file as necessary.
func (s *sharedPullerState) tempFile() (io.WriterAt, error) {
	s.mut.Lock()
	defer s.mut.Unlock()

	// If we've already hit an error, return early
	if s.err != nil {
		return nil, s.err
	}

	// If the temp file is already open, return the file descriptor
	if s.fd != nil {
		return lockedWriterAt{&s.mut, s.fd}, nil
	}

	// Ensure that the parent directory is writable. This is
	// osutil.InWritableDir except we need to do more stuff so we duplicate it
	// here.
	dir := filepath.Dir(s.tempName)
	if info, err := os.Stat(dir); err != nil {
		s.failLocked("dst stat dir", err)
		return nil, err
	} else if info.Mode()&0200 == 0 {
		err := os.Chmod(dir, 0755)
		if err == nil {
			defer func() {
				err := os.Chmod(dir, info.Mode().Perm())
				if err != nil {
					panic(err)
				}
			}()
		}
	}

	// Attempt to create the temp file
	flags := os.O_WRONLY
	if s.reused == 0 {
		flags |= os.O_CREATE | os.O_EXCL
	} else {
		// With sufficiently bad luck when exiting or crashing, we may have
		// had time to chmod the temp file to read only state but not yet
		// moved it to it's final name. This leaves us with a read only temp
		// file that we're going to try to reuse. To handle that, we need to
		// make sure we have write permissions on the file before opening it.
		err := os.Chmod(s.tempName, 0644)
		if err != nil {
			s.failLocked("dst create chmod", err)
			return nil, err
		}
	}
	fd, err := os.OpenFile(s.tempName, flags, 0644)
	if err != nil {
		s.failLocked("dst create", err)
		return nil, err
	}

	// Same fd will be used by all writers
	s.fd = fd

	return lockedWriterAt{&s.mut, s.fd}, nil
}

// sourceFile opens the existing source file for reading
func (s *sharedPullerState) sourceFile() (*os.File, error) {
	s.mut.Lock()
	defer s.mut.Unlock()

	// If we've already hit an error, return early
	if s.err != nil {
		return nil, s.err
	}

	// Attempt to open the existing file
	fd, err := os.Open(s.realName)
	if err != nil {
		s.failLocked("src open", err)
		return nil, err
	}

	return fd, nil
}

// earlyClose prints a warning message composed of the context and
// error, and marks the sharedPullerState as failed. Is a no-op when called on
// an already failed state.
func (s *sharedPullerState) fail(context string, err error) {
	s.mut.Lock()
	defer s.mut.Unlock()

	s.failLocked(context, err)
}

func (s *sharedPullerState) failLocked(context string, err error) {
	if s.err != nil {
		return
	}

	l.Infof("Puller (folder %q, file %q): %s: %v", s.folder, s.file.Name, context, err)
	s.err = err
}

func (s *sharedPullerState) failed() error {
	s.mut.Lock()
	defer s.mut.Unlock()

	return s.err
}

func (s *sharedPullerState) copyDone() {
	s.mut.Lock()
	s.copyNeeded--
	if debug {
		l.Debugln("sharedPullerState", s.folder, s.file.Name, "copyNeeded ->", s.copyNeeded)
	}
	s.mut.Unlock()
}

func (s *sharedPullerState) copiedFromOrigin() {
	s.mut.Lock()
	s.copyOrigin++
	s.mut.Unlock()
}

func (s *sharedPullerState) pullStarted() {
	s.mut.Lock()
	s.copyTotal--
	s.copyNeeded--
	s.pullTotal++
	s.pullNeeded++
	if debug {
		l.Debugln("sharedPullerState", s.folder, s.file.Name, "pullNeeded start ->", s.pullNeeded)
	}
	s.mut.Unlock()
}

func (s *sharedPullerState) pullDone() {
	s.mut.Lock()
	s.pullNeeded--
	if debug {
		l.Debugln("sharedPullerState", s.folder, s.file.Name, "pullNeeded done ->", s.pullNeeded)
	}
	s.mut.Unlock()
}

// finalClose atomically closes and returns closed status of a file. A true
// first return value means the file was closed and should be finished, with
// the error indicating the success or failure of the close. A false first
// return value indicates the file is not ready to be closed, or is already
// closed and should in either case not be finished off now.
func (s *sharedPullerState) finalClose() (bool, error) {
	s.mut.Lock()
	defer s.mut.Unlock()

	if s.pullNeeded+s.copyNeeded != 0 && s.err == nil {
		// Not done yet.
		return false, nil
	}

	if fd := s.fd; fd != nil {
		s.fd = nil
		return true, fd.Close()
	}
	return false, nil
}

// Returns the momentarily progress for the puller
func (s *sharedPullerState) Progress() *pullerProgress {
	s.mut.Lock()
	defer s.mut.Unlock()
	total := s.reused + s.copyTotal + s.pullTotal
	done := total - s.copyNeeded - s.pullNeeded
	return &pullerProgress{
		Total:               total,
		Reused:              s.reused,
		CopiedFromOrigin:    s.copyOrigin,
		CopiedFromElsewhere: s.copyTotal - s.copyNeeded - s.copyOrigin,
		Pulled:              s.pullTotal - s.pullNeeded,
		Pulling:             s.pullNeeded,
		BytesTotal:          db.BlocksToSize(total),
		BytesDone:           db.BlocksToSize(done),
	}
}