A Go package for implementing a FUSE file system (fork with 'almost' zero-copy read/write)
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fusego/mount_config.go

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9.8 KiB

// 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 fuse
import (
"context"
"fmt"
"log"
"runtime"
"strings"
)
// Optional configuration accepted by Mount.
type MountConfig struct {
// The context from which every op read from the connetion by the sever
// should inherit. If nil, context.Background() will be used.
OpContext context.Context
// If non-empty, the name of the file system as displayed by e.g. `mount`.
// This is important because the `umount` command requires root privileges if
// it doesn't agree with /etc/fstab.
FSName string
// Mount the file system in read-only mode. File modes will appear as normal,
// but opening a file for writing and metadata operations like chmod,
// chtimes, etc. will fail.
ReadOnly bool
// A logger to use for logging errors. All errors are logged, with the
// exception of a few blacklisted errors that are expected. If nil, no error
// logging is performed.
ErrorLogger *log.Logger
// A logger to use for logging debug information. If nil, no debug logging is
// performed.
DebugLogger *log.Logger
// Linux only. OS X always behaves as if writeback caching is disabled.
//
// By default on Linux we allow the kernel to perform writeback caching
// (cf. http://goo.gl/LdZzo1):
//
// * When the user calls write(2), the kernel sticks the user's data into
// its page cache. Only later does it call through to the file system,
// potentially after coalescing multiple small user writes.
//
// * The file system may receive multiple write ops from the kernel
// concurrently if there is a lot of page cache data to flush.
//
// * Write performance may be significantly improved due to the user and
// the kernel not waiting for serial round trips to the file system. This
// is especially true if the user makes tiny writes.
//
// * close(2) (and anything else calling f_op->flush) causes all dirty
// pages to be written out before it proceeds to send a FlushFileOp
// (cf. https://goo.gl/TMrY6X).
//
// * Similarly, close(2) causes the kernel to send a setattr request
// filling in the mtime if any dirty pages were flushed, since the time
// at which the pages were written to the file system can't be trusted.
//
// * close(2) (and anything else calling f_op->flush) writes out all dirty
// pages, then sends a setattr request with an appropriate mtime for
// those writes if there were any, and only then proceeds to send a
// flush.
//
// Code walk:
//
// * (https://goo.gl/zTIZQ9) fuse_flush calls write_inode_now before
// calling the file system. The latter eventually calls into
// __writeback_single_inode.
//
// * (https://goo.gl/L7Z2w5) __writeback_single_inode calls
// do_writepages, which writes out any dirty pages.
//
// * (https://goo.gl/DOPgla) __writeback_single_inode later calls
// write_inode, which calls into the superblock op struct's write_inode
// member. For fuse, this is fuse_write_inode
// (cf. https://goo.gl/eDSKOX).
//
// * (https://goo.gl/PbkGA1) fuse_write_inode calls fuse_flush_times.
//
// * (https://goo.gl/ig8x9V) fuse_flush_times sends a setttr request
// for setting the inode's mtime.
//
// However, this brings along some caveats:
//
// * The file system must handle SetInodeAttributesOp or close(2) will fail,
// due to the call chain into fuse_flush_times listed above.
//
// * The kernel caches mtime and ctime regardless of whether the file
// system tells it to do so, disregarding the result of further getattr
// requests (cf. https://goo.gl/3ZZMUw, https://goo.gl/7WtQUp). It
// appears this may be true of the file size, too. Writeback caching may
// therefore not be suitable for file systems where these attributes can
// spontaneously change for reasons the kernel doesn't observe. See
// http://goo.gl/V5WQCN for more discussion.
//
// Setting DisableWritebackCaching disables this behavior. Instead the file
// system is called one or more times for each write(2), and the user's
// syscall doesn't return until the file system returns.
DisableWritebackCaching bool
// OS X only.
//
// Normally on OS X we mount with the novncache option
// (cf. http://goo.gl/1pTjuk), which disables entry caching in the kernel.
// This is because osxfuse does not honor the entry expiration values we
// return to it, instead caching potentially forever (cf.
// http://goo.gl/8yR0Ie), and it is probably better to fail to cache than to
// cache for too long, since the latter is more likely to hide consistency
// bugs that are difficult to detect and diagnose.
//
// This field disables the use of novncache, restoring entry caching. Beware:
// the value of ChildInodeEntry.EntryExpiration is ignored by the kernel, and
// entries will be cached for an arbitrarily long time.
EnableVnodeCaching bool
// Linux only.
//
// Linux 4.20 introduced caching symlink targets in the page cache:
// https://github.com/torvalds/linux/commit/5571f1e65486be025f73fa6aa30fb03725d362a2
//
// This is not enabled by default because the old behavior masked a bug:
// file systems could return any size in the inode attributes of
// symlinks. After enabling caching, the specified size caps the symlink
// target.
EnableSymlinkCaching bool
// Linux only.
//
// Tell the kernel to treat returning -ENOSYS on OpenFile as not needing
// OpenFile calls at all (Linux >= 3.16):
EnableNoOpenSupport bool
// Linux only.
//
// Tell the kernel to treat returning -ENOSYS on OpenDir as not needing
// OpenDir calls at all (Linux >= 5.1):
EnableNoOpendirSupport bool
// Disable FUSE default permissions.
// This is useful for situations where the backing data store (e.g., S3) doesn't
// actually utilise any form of qualifiable UNIX permissions.
DisableDefaultPermissions bool
// Use vectored reads.
// Vectored read allows file systems to avoid memory copying overhead if
// the data is already in memory when they return it to FUSE.
// When turned on, ReadFileOp.Dst is always nil and the FS must return data
// being read from the file as a list of slices in ReadFileOp.Data.
UseVectoredRead bool
// OS X only.
//
// The name of the mounted volume, as displayed in the Finder. If empty, a
// default name involving the string 'osxfuse' is used.
VolumeName string
// Additional key=value options to pass unadulterated to the underlying mount
// command. See `man 8 mount`, the fuse documentation, etc. for
// system-specific information.
//
// For expert use only! May invalidate other guarantees made in the
// documentation for this package.
Options map[string]string
// Sets the filesystem type (third field in /etc/mtab). /etc/mtab and
// /proc/mounts will show the filesystem type as fuse.<Subtype>.
// If not set, /proc/mounts will show the filesystem type as fuse/fuseblk.
Subtype string
// Flag to enable async reads that are received from
// the kernel
EnableAsyncReads bool
}
// Create a map containing all of the key=value mount options to be given to
// the mount helper.
func (c *MountConfig) toMap() (opts map[string]string) {
isDarwin := runtime.GOOS == "darwin"
opts = make(map[string]string)
// Enable permissions checking in the kernel. See the comments on
// InodeAttributes.Mode.
if !c.DisableDefaultPermissions {
opts["default_permissions"] = ""
}
// HACK(jacobsa): Work around what appears to be a bug in systemd v219, as
// shipped in Ubuntu 15.04, where it automatically unmounts any file system
// that doesn't set an explicit name.
//
// When Ubuntu contains systemd v220, this workaround should be removed and
// the systemd bug reopened if the problem persists.
//
// Cf. https://github.com/bazil/fuse/issues/89
// Cf. https://bugs.freedesktop.org/show_bug.cgi?id=90907
fsname := c.FSName
if runtime.GOOS == "linux" && fsname == "" {
fsname = "some_fuse_file_system"
}
// Special file system name?
if fsname != "" {
opts["fsname"] = fsname
}
subtype := c.Subtype
if subtype != "" {
opts["subtype"] = subtype
}
// Read only?
if c.ReadOnly {
opts["ro"] = ""
}
// Handle OS X options.
if isDarwin {
if !c.EnableVnodeCaching {
opts["novncache"] = ""
}
if c.VolumeName != "" {
// Cf. https://github.com/osxfuse/osxfuse/wiki/Mount-options#volname
opts["volname"] = c.VolumeName
}
}
// OS X: disable the use of "Apple Double" (._foo and .DS_Store) files, which
// just add noise to debug output and can have significant cost on
// network-based file systems.
//
// Cf. https://github.com/osxfuse/osxfuse/wiki/Mount-options
if isDarwin {
opts["noappledouble"] = ""
}
// Last but not least: other user-supplied options.
for k, v := range c.Options {
opts[k] = v
}
return opts
}
func escapeOptionsKey(s string) (res string) {
res = s
res = strings.Replace(res, `\`, `\\`, -1)
res = strings.Replace(res, `,`, `\,`, -1)
return res
}
func mapToOptionsString(opts map[string]string) string {
var components []string
for k, v := range opts {
k = escapeOptionsKey(k)
component := k
if v != "" {
component = fmt.Sprintf("%s=%s", k, v)
}
components = append(components, component)
}
return strings.Join(components, ",")
}
// Create an options string suitable for passing to the mount helper.
func (c *MountConfig) toOptionsString() string {
return mapToOptionsString(c.toMap())
}