// Copyright 2018 Google Inc.
//
// 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 fs

import (
	"bytes"
	"fmt"
	"sync"
	"sync/atomic"

	"gvisor.googlesource.com/gvisor/pkg/refs"
)

// DirentOperations provide file systems greater control over how long a Dirent stays pinned
// in core. Implementations must not take Dirent.mu.
type DirentOperations interface {
	// Revalidate returns true if the Dirent is stale and its InodeOperations needs to be reloaded. Revalidate
	// will never be called on a Dirent that is mounted.
	Revalidate(dirent *Dirent) bool

	// Keep returns true if the Dirent should be kept in memory for as long as possible
	// beyond any active references.
	Keep(dirent *Dirent) bool
}

// MountSourceOperations contains filesystem specific operations.
type MountSourceOperations interface {
	// TODO: Add:
	//
	// StatFS() (Info, error)
	// BlockSize() int64
	// FS() Filesystem

	// DirentOperations provide optional extra management of Dirents.
	DirentOperations

	// Destroy destroys the MountSource.
	Destroy()

	// Below are MountSourceOperations that do not conform to Linux.

	// ResetInodeMappings clears all mappings of Inodes before SaveInodeMapping
	// is called.
	ResetInodeMappings()

	// SaveInodeMappings is called during saving to store, for each reachable
	// Inode in the mounted filesystem, a mapping of Inode.StableAttr.InodeID
	// to the Inode's path relative to its mount point. If an Inode is
	// reachable at more than one path due to hard links, it is unspecified
	// which path is mapped. Filesystems that do not use this information to
	// restore inodes can make SaveInodeMappings a no-op.
	SaveInodeMapping(inode *Inode, path string)
}

// InodeMappings defines a fmt.Stringer MountSource Inode mappings.
type InodeMappings map[uint64]string

// String implements fmt.Stringer.String.
func (i InodeMappings) String() string {
	var mappingsBuf bytes.Buffer
	mappingsBuf.WriteString("\n")
	for ino, name := range i {
		mappingsBuf.WriteString(fmt.Sprintf("\t%q\t\tinode number %d\n", name, ino))
	}
	return mappingsBuf.String()
}

// MountSource represents a source of file objects.
//
// MountSource corresponds to struct super_block in Linux.
//
// A mount source may represent a physical device (or a partition of a physical
// device) or a virtual source of files such as procfs for a specific PID
// namespace. There should be only one mount source per logical device. E.g.
// there should be only procfs mount source for a given PID namespace.
//
// A mount source represents files as inodes. Every inode belongs to exactly
// one mount source. Each file object may only be represented using one inode
// object in a sentry instance.
//
// This is an amalgamation of structs super_block, vfsmount, and mount, while
// MountSourceOperations is akin to struct super_operations.
//
// Hence, mount source also contains common mounted file system state, such as
// mount flags, the root Dirent, and children mounts. For now, this
// amalgamation implies that a mount source cannot be shared by multiple mounts
// (e.g. cannot be mounted at different locations).
//
// TODO: Move mount-specific information out of MountSource.
type MountSource struct {
	refs.AtomicRefCount

	// MountSourceOperations defines filesystem specific behavior.
	MountSourceOperations

	// Filesystem is the filesystem backing the mount. Can be nil if there
	// is no filesystem backing the mount.
	Filesystem Filesystem

	// Flags are the flags that this filesystem was mounted with.
	Flags MountSourceFlags

	// fscache keeps Dirents pinned beyond application references to them.
	// It must be flushed before kernel.SaveTo.
	fscache *DirentCache `state:"nosave"`

	// direntRefs is the sum of references on all Dirents in this MountSource.
	//
	// direntRefs is increased when a Dirent in MountSource is IncRef'd, and
	// decreased when a Dirent in MountSource is DecRef'd.
	//
	// To cleanly unmount a MountSource, one must check that no direntRefs are
	// held anymore. To check, one must hold root.parent.dirMu of the
	// MountSource's root Dirent before reading direntRefs to prevent further
	// walks to Dirents in this MountSource.
	//
	// direntRefs must be atomically changed.
	direntRefs uint64

	// mu protects the fields below, which are set by the MountNamespace
	// during MountSource/Unmount.
	mu sync.Mutex `state:"nosave"`

	// id is a unique id for this mount.
	id uint64

	// root is the root Dirent of this mount.
	root *Dirent

	// parent is the parent MountSource, or nil if this MountSource is the root.
	parent *MountSource

	// children are the child MountSources of this MountSource.
	children map[*MountSource]struct{}
}

// defaultDirentCacheSize is the number of Dirents that the VFS can hold an extra
// reference on.
const defaultDirentCacheSize uint64 = 1000

// NewMountSource returns a new MountSource. Filesystem may be nil if there is no
// filesystem backing the mount.
func NewMountSource(mops MountSourceOperations, filesystem Filesystem, flags MountSourceFlags) *MountSource {
	return &MountSource{
		MountSourceOperations: mops,
		Flags:      flags,
		Filesystem: filesystem,
		fscache:    NewDirentCache(defaultDirentCacheSize),
		children:   make(map[*MountSource]struct{}),
	}
}

// Parent returns the parent mount, or nil if this mount is the root.
func (msrc *MountSource) Parent() *MountSource {
	msrc.mu.Lock()
	defer msrc.mu.Unlock()
	return msrc.parent
}

// ID returns the ID of this mount.
func (msrc *MountSource) ID() uint64 {
	msrc.mu.Lock()
	defer msrc.mu.Unlock()
	return msrc.id
}

// Children returns the (immediate) children of this MountSource.
func (msrc *MountSource) Children() []*MountSource {
	msrc.mu.Lock()
	defer msrc.mu.Unlock()

	ms := make([]*MountSource, 0, len(msrc.children))
	for c := range msrc.children {
		ms = append(ms, c)
	}
	return ms
}

// Submounts returns all mounts that are descendants of this mount.
func (msrc *MountSource) Submounts() []*MountSource {
	var ms []*MountSource
	for _, c := range msrc.Children() {
		ms = append(ms, c)
		ms = append(ms, c.Submounts()...)
	}
	return ms
}

// Root returns the root dirent of this mount.
func (msrc *MountSource) Root() *Dirent {
	msrc.mu.Lock()
	defer msrc.mu.Unlock()
	return msrc.root
}

// DirentRefs returns the current mount direntRefs.
func (msrc *MountSource) DirentRefs() uint64 {
	return atomic.LoadUint64(&msrc.direntRefs)
}

// IncDirentRefs increases direntRefs.
func (msrc *MountSource) IncDirentRefs() {
	atomic.AddUint64(&msrc.direntRefs, 1)
}

// DecDirentRefs decrements direntRefs.
func (msrc *MountSource) DecDirentRefs() {
	if atomic.AddUint64(&msrc.direntRefs, ^uint64(0)) == ^uint64(0) {
		panic("Decremented zero mount reference direntRefs")
	}
}

func (msrc *MountSource) destroy() {
	if c := msrc.DirentRefs(); c != 0 {
		panic(fmt.Sprintf("MountSource with non-zero direntRefs is being destroyed: %d", c))
	}
	msrc.MountSourceOperations.Destroy()
}

// DecRef drops a reference on the MountSource.
func (msrc *MountSource) DecRef() {
	msrc.DecRefWithDestructor(msrc.destroy)
}

// FlushDirentRefs drops all references held by the MountSource on Dirents.
func (msrc *MountSource) FlushDirentRefs() {
	msrc.fscache.Invalidate()
}

// NewCachingMountSource returns a generic mount that will cache dirents
// aggressively. Filesystem may be nil if there is no backing filesystem.
func NewCachingMountSource(filesystem Filesystem, flags MountSourceFlags) *MountSource {
	return NewMountSource(&SimpleMountSourceOperations{
		keep: true,
	}, filesystem, flags)
}

// NewNonCachingMountSource returns a generic mount that will never cache dirents.
// Filesystem may be nil if there is no backing filesystem.
func NewNonCachingMountSource(filesystem Filesystem, flags MountSourceFlags) *MountSource {
	return NewMountSource(&SimpleMountSourceOperations{
		keep: false,
	}, filesystem, flags)
}

// SimpleMountSourceOperations implements MountSourceOperations.
type SimpleMountSourceOperations struct {
	keep bool
}

// Revalidate implements MountSourceOperations.Revalidate.
func (*SimpleMountSourceOperations) Revalidate(*Dirent) bool {
	return false
}

// Keep implements MountSourceOperations.Keep.
func (smo *SimpleMountSourceOperations) Keep(*Dirent) bool {
	return smo.keep
}

// ResetInodeMappings implements MountSourceOperations.ResetInodeMappings.
func (*SimpleMountSourceOperations) ResetInodeMappings() {}

// SaveInodeMapping implements MountSourceOperations.SaveInodeMapping.
func (*SimpleMountSourceOperations) SaveInodeMapping(*Inode, string) {}

// Destroy implements MountSourceOperations.Destroy.
func (*SimpleMountSourceOperations) Destroy() {}

// Info defines attributes of a filesystem.
type Info struct {
	// Type is the filesystem type magic value.
	Type uint64

	// TotalBlocks is the total data blocks in the filesystem.
	TotalBlocks uint64

	// FreeBlocks is the number of free blocks available.
	FreeBlocks uint64

	// TotalFiles is the total file nodes in the filesystem.
	TotalFiles uint64

	// FreeFiles is the number of free file nodes.
	FreeFiles uint64
}