200 lines
7.2 KiB
Go
200 lines
7.2 KiB
Go
// Copyright 2015 Google Inc. All Rights Reserved.
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// Author: jacobsa@google.com (Aaron Jacobs)
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package fuseutil
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import (
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"time"
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"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 NotImplementedFileSystem to inherit defaults that return ENOSYS to the
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// 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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// Open a file or directory identified by an inode ID. The kernel calls this
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// method when setting up a struct file for a particular inode, usually in
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// response to an open(2) call from a user-space process. This may have side
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// effects, depending on the flags passed.
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Open(
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ctx context.Context,
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req *OpenRequest) (*OpenResponse, error)
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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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Lookup(
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ctx context.Context,
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req *LookupRequest) (*LookupResponse, error)
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// Forget an inode ID previously issued (e.g. by Lookup). The kernel calls
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// this when removing an inode from its internal caches.
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//
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// The kernel guarantees that the node 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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Forget(
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ctx context.Context,
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req *ForgetRequest) (*ForgetResponse, 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 Open or Lookup. Unlike all other inode IDs, which are minted
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// by the file system, the FUSE VFS layer may send a request for this ID
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// without the file system ever having referenced it in a previous response.
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const RootInodeID InodeID = InodeID(fuse.RootID)
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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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// 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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////////////////////////////////////////////////////////////////////////
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// Requests and responses
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////////////////////////////////////////////////////////////////////////
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type OpenRequest 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 fuse.OpenFlags
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}
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// Currently nothing interesting here. The file system should perform any
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// checking and side effects necessary as part of FileSystem.Open, and return
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// an error if appropriate.
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type OpenResponse struct {
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}
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type LookupRequest 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 LookupResponse 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 Forget.
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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 Lookup method. However the latter may be slow, so it caches the
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// 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 ForgetRequest 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 ForgetResponse struct {
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}
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