fusego/fuseops/ops.go

// Copyright 2015 Google Inc. All Rights Reserved.
//
// 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 fuseops contains implementations of the fuse.Op interface that may
// be returned by fuse.Connection.ReadOp. See documentation in that package for
// more.
package fuseops

import (
	"os"
	"time"

	"github.com/jacobsa/bazilfuse"
)

// This method is called once when mounting the file system. It must succeed
// in order for the mount to succeed.
type InitOp struct {
}

////////////////////////////////////////////////////////////////////////
// Inodes
////////////////////////////////////////////////////////////////////////

// Look up a child by name within a parent directory. The kernel calls this
// when resolving user paths to dentry structs, which are then cached.
type LookUpInodeOp struct {
}

// Refresh the attributes for an inode whose ID was previously returned by
// LookUpInode. The kernel calls this when the FUSE VFS layer's cache of
// inode attributes is stale. This is controlled by the AttributesExpiration
// field of responses to LookUp, etc.
type GetInodeAttributesOp struct {
}

// Change attributes for an inode.
//
// The kernel calls this for obvious cases like chmod(2), and for less
// obvious cases like ftrunctate(2).
type SetInodeAttributesOp struct {
}

// Forget an inode ID previously issued (e.g. by LookUpInode or MkDir). The
// kernel calls this when removing an inode from its internal caches.
type ForgetInodeOp struct {
}

////////////////////////////////////////////////////////////////////////
// Inode creation
////////////////////////////////////////////////////////////////////////

// Create a directory inode as a child of an existing directory inode. The
// kernel sends this in response to a mkdir(2) call.
//
// The kernel appears to verify the name doesn't already exist (mkdir calls
// mkdirat calls user_path_create calls filename_create, which verifies:
// http://goo.gl/FZpLu5). But volatile file systems and paranoid non-volatile
// file systems should check for the reasons described below on CreateFile.
type MkDirOp struct {
}

// Create a file inode and open it.
//
// The kernel calls this method when the user asks to open a file with the
// O_CREAT flag and the kernel has observed that the file doesn't exist. (See
// for example lookup_open, http://goo.gl/PlqE9d).
//
// However it's impossible to tell for sure that all kernels make this check
// in all cases and the official fuse documentation is less than encouraging
// (" the file does not exist, first create it with the specified mode, and
// then open it"). Therefore file systems would be smart to be paranoid and
// check themselves, returning EEXIST when the file already exists. This of
// course particularly applies to file systems that are volatile from the
// kernel's point of view.
type CreateFileOp struct {
}

////////////////////////////////////////////////////////////////////////
// Unlinking
////////////////////////////////////////////////////////////////////////

// Unlink a directory from its parent. Because directories cannot have a link
// count above one, this means the directory inode should be deleted as well
// once the kernel calls ForgetInode.
//
// The file system is responsible for checking that the directory is empty.
//
// Sample implementation in ext2: ext2_rmdir (http://goo.gl/B9QmFf)
type RmDirOp struct {
}

// Unlink a file from its parent. If this brings the inode's link count to
// zero, the inode should be deleted once the kernel calls ForgetInode. It
// may still be referenced before then if a user still has the file open.
//
// Sample implementation in ext2: ext2_unlink (http://goo.gl/hY6r6C)
type UnlinkOp struct {
}

////////////////////////////////////////////////////////////////////////
// Directory handles
////////////////////////////////////////////////////////////////////////

// Open a directory inode.
//
// On Linux the kernel calls this method when setting up a struct file for a
// particular inode with type directory, usually in response to an open(2)
// call from a user-space process. On OS X it may not be called for every
// open(2) (cf. https://github.com/osxfuse/osxfuse/issues/199).
type OpenDirOp struct {
}

// Read entries from a directory previously opened with OpenDir.
type ReadDirOp struct {
}

// Release a previously-minted directory handle. The kernel calls this when
// there are no more references to an open directory: all file descriptors
// are closed and all memory mappings are unmapped.
//
// The kernel guarantees that the handle ID will not be used in further calls
// to the file system (unless it is reissued by the file system).
type ReleaseDirHandleOp struct {
}

////////////////////////////////////////////////////////////////////////
// File handles
////////////////////////////////////////////////////////////////////////

// Open a file inode.
//
// On Linux the kernel calls this method when setting up a struct file for a
// particular inode with type file, usually in response to an open(2) call
// from a user-space process. On OS X it may not be called for every open(2)
// (cf.https://github.com/osxfuse/osxfuse/issues/199).
type OpenFileOp struct {
}

// Read data from a file previously opened with CreateFile or OpenFile.
//
// Note that this method is not called for every call to read(2) by the end
// user; some reads may be served by the page cache. See notes on Write for
// more.
type ReadFileOp struct {
}

// Write data to a file previously opened with CreateFile or OpenFile.
//
// When the user writes data using write(2), the write goes into the page
// cache and the page is marked dirty. Later the kernel may write back the
// page via the FUSE VFS layer, causing this method to be called:
//
//  *  The kernel calls address_space_operations::writepage when a dirty page
//     needs to be written to backing store (cf. http://goo.gl/Ezbewg). Fuse
//     sets this to fuse_writepage (cf. http://goo.gl/IeNvLT).
//
//  *  (http://goo.gl/Eestuy) fuse_writepage calls fuse_writepage_locked.
//
//  *  (http://goo.gl/RqYIxY) fuse_writepage_locked makes a write request to
//     the userspace server.
//
// Note that writes *will* be received before a call to Flush when closing
// the file descriptor to which they were written:
//
//  *  (http://goo.gl/PheZjf) fuse_flush calls write_inode_now, which appears
//     to start a writeback in the background (it talks about a "flusher
//     thread").
//
//  *  (http://goo.gl/1IiepM) fuse_flush then calls fuse_sync_writes, which
//     "[waits] for all pending writepages on the inode to finish".
//
//  *  (http://goo.gl/zzvxWv) Only then does fuse_flush finally send the
//     flush request.
//
type WriteFileOp struct {
}

// Synchronize the current contents of an open file to storage.
//
// vfs.txt documents this as being called for by the fsync(2) system call
// (cf. http://goo.gl/j9X8nB). Code walk for that case:
//
//  *  (http://goo.gl/IQkWZa) sys_fsync calls do_fsync, calls vfs_fsync, calls
//     vfs_fsync_range.
//
//  *  (http://goo.gl/5L2SMy) vfs_fsync_range calls f_op->fsync.
//
// Note that this is also called by fdatasync(2) (cf. http://goo.gl/01R7rF),
// and may be called for msync(2) with the MS_SYNC flag (see the notes on
// FlushFile).
//
// See also: FlushFile, which may perform a similar purpose when closing a
// file (but which is not used in "real" file systems).
type SyncFileOp struct {
}

// Flush the current state of an open file to storage upon closing a file
// descriptor.
//
// vfs.txt documents this as being called for each close(2) system call (cf.
// http://goo.gl/FSkbrq). Code walk for that case:
//
//  *  (http://goo.gl/e3lv0e) sys_close calls __close_fd, calls filp_close.
//  *  (http://goo.gl/nI8fxD) filp_close calls f_op->flush (fuse_flush).
//
// But note that this is also called in other contexts where a file
// descriptor is closed, such as dup2(2) (cf. http://goo.gl/NQDvFS). In the
// case of close(2), a flush error is returned to the user. For dup2(2), it
// is not.
//
// One potentially significant case where this may not be called is mmap'd
// files, where the behavior is complicated:
//
//  *  munmap(2) does not cause flushes (cf. http://goo.gl/j8B9g0).
//
//  *  On OS X, if a user modifies a mapped file via the mapping before
//     closing the file with close(2), the WriteFile calls for the
//     modifications may not be received before the FlushFile request for the
//     close(2) (cf. http://goo.gl/kVmNcx).
//
//  *  However, even on OS X you can arrange for writes via a mapping to be
//     flushed by calling msync(2) followed by close(2). On OS X msync(2)
//     will cause a WriteFile to go through and close(2) will cause a
//     FlushFile as usual (cf. http://goo.gl/kVmNcx). On Linux, msync(2) does
//     nothing unless you set the MS_SYNC flag, in which case it causes a
//     SyncFile (cf. http://goo.gl/P3mErk).
//
// In summary: if you make data durable in both FlushFile and SyncFile, then
// your users can get safe behavior from mapped files by calling msync(2)
// with MS_SYNC, followed by munmap(2), followed by close(2). On Linux, the
// msync(2) appears to be optional because close(2) implies dirty page
// writeback (cf. http://goo.gl/HyzLTT).
//
// Because of cases like dup2(2), calls to FlushFile are not necessarily one
// to one with calls to OpenFile. They should not be used for reference
// counting, and the handle must remain valid even after the method is called
// (use ReleaseFileHandle to dispose of it).
//
// Typical "real" file systems do not implement this, presumably relying on
// the kernel to write out the page cache to the block device eventually.
// They can get away with this because a later open(2) will see the same
// data. A file system that writes to remote storage however probably wants
// to at least schedule a real flush, and maybe do it immediately in order to
// return any errors that occur.
type FlushFileOp struct {
}

// Release a previously-minted file handle. The kernel calls this when there
// are no more references to an open file: all file descriptors are closed
// and all memory mappings are unmapped.
//
// The kernel guarantees that the handle ID will not be used in further calls
// to the file system (unless it is reissued by the file system).
type ReleaseFileHandleOp struct {
}

////////////////////////////////////////////////////////////////////////
// Requests and responses
////////////////////////////////////////////////////////////////////////

type InitRequest struct {
	Header RequestHeader
}

type InitResponse struct {
}

type LookUpInodeRequest struct {
	Header RequestHeader

	// The ID of the directory inode to which the child belongs.
	Parent InodeID

	// The name of the child of interest, relative to the parent. For example, in
	// this directory structure:
	//
	//     foo/
	//         bar/
	//             baz
	//
	// the file system may receive a request to look up the child named "bar" for
	// the parent foo/.
	Name string
}

type LookUpInodeResponse struct {
	Entry ChildInodeEntry
}

type GetInodeAttributesRequest struct {
	Header RequestHeader

	// The inode of interest.
	Inode InodeID
}

type GetInodeAttributesResponse struct {
	// Attributes for the inode, and the time at which they should expire. See
	// notes on ChildInodeEntry.AttributesExpiration for more.
	Attributes           InodeAttributes
	AttributesExpiration time.Time
}

type SetInodeAttributesRequest struct {
	Header RequestHeader

	// The inode of interest.
	Inode InodeID

	// The attributes to modify, or nil for attributes that don't need a change.
	Size  *uint64
	Mode  *os.FileMode
	Atime *time.Time
	Mtime *time.Time
}

type SetInodeAttributesResponse struct {
	// The new attributes for the inode, and the time at which they should
	// expire. See notes on ChildInodeEntry.AttributesExpiration for more.
	Attributes           InodeAttributes
	AttributesExpiration time.Time
}

type ForgetInodeRequest struct {
	Header RequestHeader

	// The inode to be forgotten. The kernel guarantees that the node ID will not
	// be used in further calls to the file system (unless it is reissued by the
	// file system).
	ID InodeID
}

type ForgetInodeResponse struct {
}

type MkDirRequest struct {
	Header RequestHeader

	// The ID of parent directory inode within which to create the child.
	Parent InodeID

	// The name of the child to create, and the mode with which to create it.
	Name string
	Mode os.FileMode
}

type MkDirResponse struct {
	// Information about the inode that was created.
	Entry ChildInodeEntry
}

type CreateFileRequest struct {
	Header RequestHeader

	// The ID of parent directory inode within which to create the child file.
	Parent InodeID

	// The name of the child to create, and the mode with which to create it.
	Name string
	Mode os.FileMode

	// Flags for the open operation.
	Flags bazilfuse.OpenFlags
}

type CreateFileResponse struct {
	// Information about the inode that was created.
	Entry ChildInodeEntry

	// An opaque ID that will be echoed in follow-up calls for this file using
	// the same struct file in the kernel. In practice this usually means
	// follow-up calls using the file descriptor returned by open(2).
	//
	// The handle may be supplied in future calls to methods like ReadFile that
	// accept a file handle. The file system must ensure this ID remains valid
	// until a later call to ReleaseFileHandle.
	Handle HandleID
}

type RmDirRequest struct {
	Header RequestHeader

	// The ID of parent directory inode, and the name of the directory being
	// removed within it.
	Parent InodeID
	Name   string
}

type UnlinkResponse struct {
}

type UnlinkRequest struct {
	Header RequestHeader

	// The ID of parent directory inode, and the name of the file being removed
	// within it.
	Parent InodeID
	Name   string
}

type RmDirResponse struct {
}

type OpenDirRequest struct {
	Header RequestHeader

	// The ID of the inode to be opened.
	Inode InodeID

	// Mode and options flags.
	Flags bazilfuse.OpenFlags
}

type OpenDirResponse struct {
	// An opaque ID that will be echoed in follow-up calls for this directory
	// using the same struct file in the kernel. In practice this usually means
	// follow-up calls using the file descriptor returned by open(2).
	//
	// The handle may be supplied in future calls to methods like ReadDir that
	// accept a directory handle. The file system must ensure this ID remains
	// valid until a later call to ReleaseDirHandle.
	Handle HandleID
}

type ReadDirRequest struct {
	Header RequestHeader

	// The directory inode that we are reading, and the handle previously
	// returned by OpenDir when opening that inode.
	Inode  InodeID
	Handle HandleID

	// The offset within the directory at which to read.
	//
	// Warning: this field is not necessarily a count of bytes. Its legal values
	// are defined by the results returned in ReadDirResponse. See the notes
	// below and the notes on that struct.
	//
	// In the Linux kernel this ultimately comes from file::f_pos, which starts
	// at zero and is set by llseek and by the final consumed result returned by
	// each call to ReadDir:
	//
	//  *  (http://goo.gl/2nWJPL) iterate_dir, which is called by getdents(2) and
	//     readdir(2), sets dir_context::pos to file::f_pos before calling
	//     f_op->iterate, and then does the opposite assignment afterward.
	//
	//  *  (http://goo.gl/rTQVSL) fuse_readdir, which implements iterate for fuse
	//     directories, passes dir_context::pos as the offset to fuse_read_fill,
	//     which passes it on to user-space. fuse_readdir later calls
	//     parse_dirfile with the same context.
	//
	//  *  (http://goo.gl/vU5ukv) For each returned result (except perhaps the
	//     last, which may be truncated by the page boundary), parse_dirfile
	//     updates dir_context::pos with fuse_dirent::off.
	//
	// It is affected by the Posix directory stream interfaces in the following
	// manner:
	//
	//  *  (http://goo.gl/fQhbyn, http://goo.gl/ns1kDF) opendir initially causes
	//     filepos to be set to zero.
	//
	//  *  (http://goo.gl/ezNKyR, http://goo.gl/xOmDv0) readdir allows the user
	//     to iterate through the directory one entry at a time. As each entry is
	//     consumed, its d_off field is stored in __dirstream::filepos.
	//
	//  *  (http://goo.gl/WEOXG8, http://goo.gl/rjSXl3) telldir allows the user
	//     to obtain the d_off field from the most recently returned entry.
	//
	//  *  (http://goo.gl/WG3nDZ, http://goo.gl/Lp0U6W) seekdir allows the user
	//     to seek backward to an offset previously returned by telldir. It
	//     stores the new offset in filepos, and calls llseek to update the
	//     kernel's struct file.
	//
	//  *  (http://goo.gl/gONQhz, http://goo.gl/VlrQkc) rewinddir allows the user
	//     to go back to the beginning of the directory, obtaining a fresh view.
	//     It updates filepos and calls llseek to update the kernel's struct
	//     file.
	//
	// Unfortunately, FUSE offers no way to intercept seeks
	// (http://goo.gl/H6gEXa), so there is no way to cause seekdir or rewinddir
	// to fail. Additionally, there is no way to distinguish an explicit
	// rewinddir followed by readdir from the initial readdir, or a rewinddir
	// from a seekdir to the value returned by telldir just after opendir.
	//
	// Luckily, Posix is vague about what the user will see if they seek
	// backwards, and requires the user not to seek to an old offset after a
	// rewind. The only requirement on freshness is that rewinddir results in
	// something that looks like a newly-opened directory. So FUSE file systems
	// may e.g. cache an entire fresh listing for each ReadDir with a zero
	// offset, and return array offsets into that cached listing.
	Offset DirOffset

	// The maximum number of bytes to return in ReadDirResponse.Data. A smaller
	// number is acceptable.
	Size int
}

type ReadDirResponse struct {
	// A buffer consisting of a sequence of FUSE directory entries in the format
	// generated by fuse_add_direntry (http://goo.gl/qCcHCV), which is consumed
	// by parse_dirfile (http://goo.gl/2WUmD2). Use fuseutil.AppendDirent to
	// generate this data.
	//
	// The buffer must not exceed the length specified in ReadDirRequest.Size. It
	// is okay for the final entry to be truncated; parse_dirfile copes with this
	// by ignoring the partial record.
	//
	// Each entry returned exposes a directory offset to the user that may later
	// show up in ReadDirRequest.Offset. See notes on that field for more
	// information.
	//
	// An empty buffer indicates the end of the directory has been reached.
	Data []byte
}

type ReleaseDirHandleRequest struct {
	Header RequestHeader

	// The handle ID to be released. The kernel guarantees that this ID will not
	// be used in further calls to the file system (unless it is reissued by the
	// file system).
	Handle HandleID
}

type ReleaseDirHandleResponse struct {
}

type OpenFileRequest struct {
	Header RequestHeader

	// The ID of the inode to be opened.
	Inode InodeID

	// Mode and options flags.
	Flags bazilfuse.OpenFlags
}

type OpenFileResponse struct {
	// An opaque ID that will be echoed in follow-up calls for this file using
	// the same struct file in the kernel. In practice this usually means
	// follow-up calls using the file descriptor returned by open(2).
	//
	// The handle may be supplied in future calls to methods like ReadFile that
	// accept a file handle. The file system must ensure this ID remains valid
	// until a later call to ReleaseFileHandle.
	Handle HandleID
}

type ReadFileRequest struct {
	Header RequestHeader

	// The file inode that we are reading, and the handle previously returned by
	// CreateFile or OpenFile when opening that inode.
	Inode  InodeID
	Handle HandleID

	// The range of the file to read.
	//
	// The FUSE documentation requires that exactly the number of bytes be
	// returned, except in the case of EOF or error (http://goo.gl/ZgfBkF). This
	// appears to be because it uses file mmapping machinery
	// (http://goo.gl/SGxnaN) to read a page at a time. It appears to understand
	// where EOF is by checking the inode size (http://goo.gl/0BkqKD), returned
	// by a previous call to LookUpInode, GetInodeAttributes, etc.
	Offset int64
	Size   int
}

type ReadFileResponse struct {
	// The data read. If this is less than the requested size, it indicates EOF.
	// An error should not be returned in this case.
	Data []byte
}

type WriteFileRequest struct {
	Header RequestHeader

	// The file inode that we are modifying, and the handle previously returned
	// by CreateFile or OpenFile when opening that inode.
	Inode  InodeID
	Handle HandleID

	// The offset at which to write the data below.
	//
	// The man page for pwrite(2) implies that aside from changing the file
	// handle's offset, using pwrite is equivalent to using lseek(2) and then
	// write(2). The man page for lseek(2) says the following:
	//
	// "The lseek() function allows the file offset to be set beyond the end of
	// the file (but this does not change the size of the file). If data is later
	// written at this point, subsequent reads of the data in the gap (a "hole")
	// return null bytes (aq\0aq) until data is actually written into the gap."
	//
	// It is therefore reasonable to assume that the kernel is looking for
	// the following semantics:
	//
	// *   If the offset is less than or equal to the current size, extend the
	//     file as necessary to fit any data that goes past the end of the file.
	//
	// *   If the offset is greater than the current size, extend the file
	//     with null bytes until it is not, then do the above.
	//
	Offset int64

	// The data to write.
	//
	// The FUSE documentation requires that exactly the number of bytes supplied
	// be written, except on error (http://goo.gl/KUpwwn). This appears to be
	// because it uses file mmapping machinery (http://goo.gl/SGxnaN) to write a
	// page at a time.
	Data []byte
}

type WriteFileResponse struct {
}

type SyncFileRequest struct {
	Header RequestHeader

	// The file and handle being sync'd.
	Inode  InodeID
	Handle HandleID
}

type SyncFileResponse struct {
}

type FlushFileRequest struct {
	Header RequestHeader

	// The file and handle being flushed.
	Inode  InodeID
	Handle HandleID
}

type FlushFileResponse struct {
}

type ReleaseFileHandleRequest struct {
	Header RequestHeader

	// The handle ID to be released. The kernel guarantees that this ID will not
	// be used in further calls to the file system (unless it is reissued by the
	// file system).
	Handle HandleID
}

type ReleaseFileHandleResponse struct {
}