331 lines
12 KiB
Go
331 lines
12 KiB
Go
// Copyright 2015 Google Inc. All Rights Reserved.
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// Author: jacobsa@google.com (Aaron Jacobs)
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package fuse
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import (
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"time"
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bazilfuse "bazil.org/fuse"
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"golang.org/x/net/context"
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)
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// An interface that must be implemented by file systems to be mounted with
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// FUSE. See also the comments on request and response structs.
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//
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// Not all methods need to have interesting implementations. Embed a field of
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// type fuseutil.NotImplementedFileSystem to inherit defaults that return
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// ENOSYS to the kernel.
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//
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// Must be safe for concurrent access via all methods.
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type FileSystem interface {
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// Look up a child by name within a parent directory. The kernel calls this
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// when resolving user paths to dentry structs, which are then cached.
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LookUpInode(
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ctx context.Context,
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req *LookUpInodeRequest) (*LookUpInodeResponse, error)
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// Forget an inode ID previously issued (e.g. by LookUpInode). The kernel
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// calls this when removing an inode from its internal caches.
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ForgetInode(
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ctx context.Context,
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req *ForgetInodeRequest) (*ForgetInodeResponse, error)
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// Open a directory inode. The kernel calls this method when setting up a
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// struct file for a particular inode with type directory, usually in
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// response to an open(2) call from a user-space process.
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OpenDir(
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ctx context.Context,
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req *OpenDirRequest) (*OpenDirResponse, error)
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// XXX: Comments
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ReadDir(
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ctx context.Context,
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req *ReadDirRequest) (*ReadDirResponse, error)
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// Release a previously-minted handle. The kernel calls this when there are
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// no more references to an open file: all file descriptors are closed and
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// all memory mappings are unmapped.
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//
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// The kernel guarantees that the handle ID will not be used in further calls
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// to the file system (unless it is reissued by the file system).
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ReleaseHandle(
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ctx context.Context,
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req *ReleaseHandleRequest) (*ReleaseHandleResponse, error)
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}
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////////////////////////////////////////////////////////////////////////
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// Simple types
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////////////////////////////////////////////////////////////////////////
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// A 64-bit number used to uniquely identify a file or directory in the file
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// system. File systems may mint inode IDs with any value except for
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// RootInodeID.
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//
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// This corresponds to struct inode::i_no in the VFS layer.
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// (Cf. http://goo.gl/tvYyQt)
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type InodeID uint64
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// A distinguished inode ID that identifies the root of the file system, e.g.
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// in a request to OpenDir or LookUpInode. Unlike all other inode IDs, which
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// are minted by the file system, the FUSE VFS layer may send a request for
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// this ID without the file system ever having referenced it in a previous
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// response.
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const RootInodeID InodeID = InodeID(bazilfuse.RootID)
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// Attributes for a file or directory inode. Corresponds to struct inode (cf.
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// http://goo.gl/tvYyQt).
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type InodeAttributes struct {
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// The size of the file in bytes.
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Size uint64
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}
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// A generation number for an inode. Irrelevant for file systems that won't be
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// exported over NFS. For those that will and that reuse inode IDs when they
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// become free, the generation number must change when an ID is reused.
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//
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// This corresponds to struct inode::i_generation in the VFS layer.
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// (Cf. http://goo.gl/tvYyQt)
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//
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// Some related reading:
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//
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// http://fuse.sourceforge.net/doxygen/structfuse__entry__param.html
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// http://stackoverflow.com/q/11071996/1505451
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// http://goo.gl/CqvwyX
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// http://julipedia.meroh.net/2005/09/nfs-file-handles.html
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// http://goo.gl/wvo3MB
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//
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type GenerationNumber uint64
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// An opaque 64-bit number used to identify a particular open handle to a file
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// or directory.
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//
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// This corresponds to fuse_file_info::fh.
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type HandleID uint64
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// XXX: Comments for all
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type Dirent struct {
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Inode InodeID
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Offset DirOffset
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Name string
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Type DirentType
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}
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////////////////////////////////////////////////////////////////////////
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// Requests and responses
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////////////////////////////////////////////////////////////////////////
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type LookUpInodeRequest struct {
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// The ID of the directory inode to which the child belongs.
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Parent InodeID
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// The name of the child of interest, relative to the parent. For example, in
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// this directory structure:
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//
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// foo/
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// bar/
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// baz
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//
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// the file system may receive a request to look up the child named "bar" for
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// the parent foo/.
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Name string
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}
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type LookUpInodeResponse struct {
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// The ID of the child inode. The file system must ensure that the returned
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// inode ID remains valid until a later call to ForgetInode.
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Child InodeID
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// A generation number for this incarnation of the inode with the given ID.
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// See comments on type GenerationNumber for more.
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Generation GenerationNumber
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// Current ttributes for the child inode.
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Attributes InodeAttributes
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// The FUSE VFS layer in the kernel maintains a cache of file attributes,
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// used whenever up to date information about size, mode, etc. is needed.
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//
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// For example, this is the abridged call chain for fstat(2):
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//
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// * (http://goo.gl/tKBH1p) fstat calls vfs_fstat.
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// * (http://goo.gl/3HeITq) vfs_fstat eventuall calls vfs_getattr_nosec.
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// * (http://goo.gl/DccFQr) vfs_getattr_nosec calls i_op->getattr.
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// * (http://goo.gl/dpKkst) fuse_getattr calls fuse_update_attributes.
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// * (http://goo.gl/yNlqPw) fuse_update_attributes uses the values in the
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// struct inode if allowed, otherwise calling out to the user-space code.
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//
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// In addition to obvious cases like fstat, this is also used in more subtle
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// cases like updating size information before seeking (http://goo.gl/2nnMFa)
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// or reading (http://goo.gl/FQSWs8).
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//
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// Most 'real' file systems do not set inode_operations::getattr, and
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// therefore vfs_getattr_nosec calls generic_fillattr which simply grabs the
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// information from the inode struct. This makes sense because these file
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// systems cannot spontaneously change; all modifications go through the
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// kernel which can update the inode struct as appropriate.
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//
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// In contrast, a FUSE file system may have spontaneous changes, so it calls
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// out to user space to fetch attributes. However this is expensive, so the
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// FUSE layer in the kernel caches the attributes if requested.
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//
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// This field controls when the attributes returned in this response and
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// stashed in the struct inode should be re-queried. Leave at the zero value
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// to disable caching.
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//
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// More reading:
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// http://stackoverflow.com/q/21540315/1505451
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AttributesExpiration time.Time
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// The time until which the kernel may maintain an entry for this name to
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// inode mapping in its dentry cache. After this time, it will revalidate the
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// dentry.
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//
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// As in the discussion of attribute caching above, unlike real file systems,
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// FUSE file systems may spontaneously change their name -> inode mapping.
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// Therefore the FUSE VFS layer uses dentry_operations::d_revalidate
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// (http://goo.gl/dVea0h) to intercept lookups and revalidate by calling the
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// user-space LookUpInode method. However the latter may be slow, so it
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// caches the entries until the time defined by this field.
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//
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// Example code walk:
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//
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// * (http://goo.gl/M2G3tO) lookup_dcache calls d_revalidate if enabled.
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// * (http://goo.gl/ef0Elu) fuse_dentry_revalidate just uses the dentry's
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// inode if fuse_dentry_time(entry) hasn't passed. Otherwise it sends a
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// lookup request.
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//
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// Leave at the zero value to disable caching.
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EntryExpiration time.Time
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}
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type ForgetInodeRequest struct {
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// The inode to be forgotten. The kernel guarantees that the node ID will not
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// be used in further calls to the file system (unless it is reissued by the
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// file system).
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ID InodeID
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}
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type ForgetInodeResponse struct {
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}
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type OpenDirRequest struct {
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// The ID of the inode to be opened.
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Inode InodeID
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// Mode and options flags.
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Flags bazilfuse.OpenFlags
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}
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type OpenDirResponse struct {
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// An opaque ID that will be echoed in follow-up calls for this directory
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// using the same struct file in the kernel. In practice this usually means
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// follow-up calls using the file descriptor returned by open(2).
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//
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// The handle may be supplied to the following methods:
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//
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// * ReadDir
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// * ReleaseHandle
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//
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// The file system must ensure this ID remains valid until a later call to
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// ReleaseHandle.
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Handle HandleID
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}
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type ReadDirRequest struct {
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// The directory inode that we are reading, and the handle previously
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// returned by OpenDir when opening that inode.
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Inode InodeID
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Handle HandleID
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// The offset within the directory at which to read.
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//
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// Warning: this field is not necessarily a count of bytes. Its legal values
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// are defined by the results returned in ReadDirResponse. See the notes
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// below and the notes on that struct.
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//
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// In the Linux kernel this ultimately comes from file::f_pos, which starts
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// at zero and is set by llseek and by the final consumed result returned by
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// each call to ReadDir:
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//
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// * (http://goo.gl/2nWJPL) iterate_dir, which is called by getdents(2) and
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// readdir(2), sets dir_context::pos to file::f_pos before calling
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// f_op->iterate, and then does the opposite assignment afterward.
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//
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// * (http://goo.gl/rTQVSL) fuse_readdir, which implements iterate for fuse
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// directories, passes dir_context::pos as the offset to fuse_read_fill,
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// which passes it on to user-space. fuse_readdir later calls
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// parse_dirfile with the same context.
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//
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// * (http://goo.gl/vU5ukv) For each returned result (except perhaps the
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// last, which may be truncated by the page boundary), parse_dirfile
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// updates dir_context::pos with fuse_dirent::off.
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//
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// It is affected by the Posix directory stream interfaces in the following
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// manner:
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//
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// * (http://goo.gl/fQhbyn, http://goo.gl/ns1kDF) opendir initially causes
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// filepos to be set to zero.
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//
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// * (http://goo.gl/ezNKyR, http://goo.gl/xOmDv0) readdir allows the user
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// to iterate through the directory one entry at a time. As each entry is
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// consumed, its d_off field is stored in __dirstream::filepos.
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//
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// * (http://goo.gl/WEOXG8, http://goo.gl/rjSXl3) telldir allows the user
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// to obtain the d_off field from the most recently returned entry.
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//
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// * (http://goo.gl/WG3nDZ, http://goo.gl/Lp0U6W) seekdir allows the user
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// to seek backward to an offset previously returned by telldir. It
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// stores the new offset in filepos, and calls llseek to update the
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// kernel's struct file.
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//
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// * (http://goo.gl/gONQhz, http://goo.gl/VlrQkc) rewinddir allows the user
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// to go back to the beginning of the directory, obtaining a fresh view.
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// It updates filepos and calls llseek to update the kernel's struct
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// file.
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//
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// Unfortunately, FUSE offers no way to intercept seeks
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// (http://goo.gl/H6gEXa), so there is no way to cause seekdir or rewinddir
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// to fail. Additionally, there is no way to distinguish an explicit
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// rewinddir followed by readdir from the initial readdir, or a rewinddir
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// from a seekdir to the value returned by telldir just after opendir.
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//
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// Luckily, Posix is vague about what the user will see if they seek
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// backwards, and requires the user not to seek to an old offset after a
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// rewind. The only requirement on freshness is that rewinddir results in
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// something that looks like a newly-opened directory. So FUSE file systems
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// may e.g. cache an entire fresh listing for each ReadDir with a zero
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// offset, and return array offsets into that cached listing.
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Offset DirOffset
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// The maximum number of bytes to return in ReadDirResponse.Data.
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Size uint64
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}
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type ReadDirResponse struct {
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// A buffer consisting of a sequence of FUSE directory entries in the format
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// generated by fuse_add_direntry (http://goo.gl/qCcHCV), which is consumed
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// by parse_dirfile (http://goo.gl/2WUmD2). Use fuseutil.AppendDirent to
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// generate this data.
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//
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// The buffer must not exceed the length specified in ReadDirRequest.Size. It
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// is okay for the final entry to be truncated; parse_dirfile copes with this
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// by ignoring the partial record.
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//
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// Each entry returned exposes a directory offset to the user that may later
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// show up in ReadDirRequest.Offset. See notes on that field for more
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// information.
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Data []byte
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}
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type ReleaseHandleRequest struct {
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// The handle ID to be released. The kernel guarantees that this ID will not
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// be used in further calls to the file system (unless it is reissued by the
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// file system).
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Handle HandleID
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}
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type ReleaseHandleResponse struct {
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}
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