// Copyright 2020 The gVisor Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package overlay

import (
	"sync/atomic"

	"gvisor.dev/gvisor/pkg/abi/linux"
	"gvisor.dev/gvisor/pkg/context"
	"gvisor.dev/gvisor/pkg/fspath"
	"gvisor.dev/gvisor/pkg/sentry/vfs"
	"gvisor.dev/gvisor/pkg/sync"
	"gvisor.dev/gvisor/pkg/syserror"
)

func (d *dentry) isDir() bool {
	return atomic.LoadUint32(&d.mode)&linux.S_IFMT == linux.S_IFDIR
}

// Preconditions: d.dirMu must be locked. d.isDir().
func (d *dentry) collectWhiteoutsForRmdirLocked(ctx context.Context) (map[string]bool, error) {
	vfsObj := d.fs.vfsfs.VirtualFilesystem()
	var readdirErr error
	whiteouts := make(map[string]bool)
	var maybeWhiteouts []string
	d.iterLayers(func(layerVD vfs.VirtualDentry, isUpper bool) bool {
		layerFD, err := vfsObj.OpenAt(ctx, d.fs.creds, &vfs.PathOperation{
			Root:  layerVD,
			Start: layerVD,
		}, &vfs.OpenOptions{
			Flags: linux.O_RDONLY | linux.O_DIRECTORY,
		})
		if err != nil {
			readdirErr = err
			return false
		}
		defer layerFD.DecRef()

		// Reuse slice allocated for maybeWhiteouts from a previous layer to
		// reduce allocations.
		maybeWhiteouts = maybeWhiteouts[:0]
		if err := layerFD.IterDirents(ctx, vfs.IterDirentsCallbackFunc(func(dirent vfs.Dirent) error {
			if dirent.Name == "." || dirent.Name == ".." {
				return nil
			}
			if _, ok := whiteouts[dirent.Name]; ok {
				// This file has been whited-out in a previous layer.
				return nil
			}
			if dirent.Type == linux.DT_CHR {
				// We have to determine if this is a whiteout, which doesn't
				// count against the directory's emptiness. However, we can't
				// do so while holding locks held by layerFD.IterDirents().
				maybeWhiteouts = append(maybeWhiteouts, dirent.Name)
				return nil
			}
			// Non-whiteout file in the directory prevents rmdir.
			return syserror.ENOTEMPTY
		})); err != nil {
			readdirErr = err
			return false
		}

		for _, maybeWhiteoutName := range maybeWhiteouts {
			stat, err := vfsObj.StatAt(ctx, d.fs.creds, &vfs.PathOperation{
				Root:  layerVD,
				Start: layerVD,
				Path:  fspath.Parse(maybeWhiteoutName),
			}, &vfs.StatOptions{})
			if err != nil {
				readdirErr = err
				return false
			}
			if stat.RdevMajor != 0 || stat.RdevMinor != 0 {
				// This file is a real character device, not a whiteout.
				readdirErr = syserror.ENOTEMPTY
				return false
			}
			whiteouts[maybeWhiteoutName] = isUpper
		}
		// Continue iteration since we haven't found any non-whiteout files in
		// this directory yet.
		return true
	})
	return whiteouts, readdirErr
}

type directoryFD struct {
	fileDescription
	vfs.DirectoryFileDescriptionDefaultImpl
	vfs.DentryMetadataFileDescriptionImpl

	mu      sync.Mutex
	off     int64
	dirents []vfs.Dirent
}

// Release implements vfs.FileDescriptionImpl.Release.
func (fd *directoryFD) Release() {
}

// IterDirents implements vfs.FileDescriptionImpl.IterDirents.
func (fd *directoryFD) IterDirents(ctx context.Context, cb vfs.IterDirentsCallback) error {
	fd.mu.Lock()
	defer fd.mu.Unlock()

	d := fd.dentry()
	if fd.dirents == nil {
		ds, err := d.getDirents(ctx)
		if err != nil {
			return err
		}
		fd.dirents = ds
	}

	for fd.off < int64(len(fd.dirents)) {
		if err := cb.Handle(fd.dirents[fd.off]); err != nil {
			return err
		}
		fd.off++
	}
	return nil
}

// Preconditions: d.isDir().
func (d *dentry) getDirents(ctx context.Context) ([]vfs.Dirent, error) {
	d.fs.renameMu.RLock()
	defer d.fs.renameMu.RUnlock()
	d.dirMu.Lock()
	defer d.dirMu.Unlock()

	if d.dirents != nil {
		return d.dirents, nil
	}

	parent := genericParentOrSelf(d)
	dirents := []vfs.Dirent{
		{
			Name:    ".",
			Type:    linux.DT_DIR,
			Ino:     d.ino,
			NextOff: 1,
		},
		{
			Name:    "..",
			Type:    uint8(atomic.LoadUint32(&parent.mode) >> 12),
			Ino:     parent.ino,
			NextOff: 2,
		},
	}

	// Merge dirents from all layers comprising this directory.
	vfsObj := d.fs.vfsfs.VirtualFilesystem()
	var readdirErr error
	prevDirents := make(map[string]struct{})
	var maybeWhiteouts []vfs.Dirent
	d.iterLayers(func(layerVD vfs.VirtualDentry, isUpper bool) bool {
		layerFD, err := vfsObj.OpenAt(ctx, d.fs.creds, &vfs.PathOperation{
			Root:  layerVD,
			Start: layerVD,
		}, &vfs.OpenOptions{
			Flags: linux.O_RDONLY | linux.O_DIRECTORY,
		})
		if err != nil {
			readdirErr = err
			return false
		}
		defer layerFD.DecRef()

		// Reuse slice allocated for maybeWhiteouts from a previous layer to
		// reduce allocations.
		maybeWhiteouts = maybeWhiteouts[:0]
		if err := layerFD.IterDirents(ctx, vfs.IterDirentsCallbackFunc(func(dirent vfs.Dirent) error {
			if dirent.Name == "." || dirent.Name == ".." {
				return nil
			}
			if _, ok := prevDirents[dirent.Name]; ok {
				// This file is hidden by, or merged with, another file with
				// the same name in a previous layer.
				return nil
			}
			prevDirents[dirent.Name] = struct{}{}
			if dirent.Type == linux.DT_CHR {
				// We can't determine if this file is a whiteout while holding
				// locks held by layerFD.IterDirents().
				maybeWhiteouts = append(maybeWhiteouts, dirent)
				return nil
			}
			dirent.NextOff = int64(len(dirents) + 1)
			dirents = append(dirents, dirent)
			return nil
		})); err != nil {
			readdirErr = err
			return false
		}

		for _, dirent := range maybeWhiteouts {
			stat, err := vfsObj.StatAt(ctx, d.fs.creds, &vfs.PathOperation{
				Root:  layerVD,
				Start: layerVD,
				Path:  fspath.Parse(dirent.Name),
			}, &vfs.StatOptions{})
			if err != nil {
				readdirErr = err
				return false
			}
			if stat.RdevMajor == 0 && stat.RdevMinor == 0 {
				// This file is a whiteout; don't emit a dirent for it.
				continue
			}
			dirent.NextOff = int64(len(dirents) + 1)
			dirents = append(dirents, dirent)
		}
		return true
	})
	if readdirErr != nil {
		return nil, readdirErr
	}

	// Cache dirents for future directoryFDs.
	d.dirents = dirents
	return dirents, nil
}

// Seek implements vfs.FileDescriptionImpl.Seek.
func (fd *directoryFD) Seek(ctx context.Context, offset int64, whence int32) (int64, error) {
	fd.mu.Lock()
	defer fd.mu.Unlock()

	switch whence {
	case linux.SEEK_SET:
		if offset < 0 {
			return 0, syserror.EINVAL
		}
		if offset == 0 {
			// Ensure that the next call to fd.IterDirents() calls
			// fd.dentry().getDirents().
			fd.dirents = nil
		}
		fd.off = offset
		return fd.off, nil
	case linux.SEEK_CUR:
		offset += fd.off
		if offset < 0 {
			return 0, syserror.EINVAL
		}
		// Don't clear fd.dirents in this case, even if offset == 0.
		fd.off = offset
		return fd.off, nil
	default:
		return 0, syserror.EINVAL
	}
}