117 lines
3.2 KiB
Go
117 lines
3.2 KiB
Go
// Package openfiletable maintains a table of currently opened files, identified
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// by the device number + inode number pair. This table is used by fusefrontend
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// to centrally store the current file ID and to lock files against concurrent
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// writes.
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package openfiletable
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import (
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"sync"
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"sync/atomic"
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"syscall"
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)
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// QIno = Qualified Inode number.
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// Uniquely identifies a backing file through the device number,
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// inode number pair.
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type QIno struct {
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// Stat_t.{Dev,Ino} is uint64 on 32- and 64-bit Linux
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Dev uint64
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Ino uint64
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}
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// QInoFromStat fills a new QIno struct with the passed Stat_t info.
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func QInoFromStat(st *syscall.Stat_t) QIno {
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return QIno{
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// There are some architectures that use 32-bit values here
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// (darwin, freebsd-32, maybe others). Add and explicit cast to make
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// this function work everywhere.
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Dev: uint64(st.Dev),
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Ino: uint64(st.Ino),
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}
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}
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// wlock - serializes write accesses to each file (identified by inode number)
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// Writing partial blocks means we have to do read-modify-write cycles. We
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// really don't want concurrent writes there.
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// Concurrent full-block writes could actually be allowed, but are not to
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// keep the locking simple.
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var t table
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func init() {
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t.entries = make(map[QIno]*Entry)
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}
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type table struct {
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// writeOpCount counts entry.ContentLock.Lock() calls. As every operation that
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// modifies a file should
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// call it, this effectively serves as a write-operation counter.
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// The variable is accessed without holding any locks so atomic operations
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// must be used. It must be the first element of the struct to guarantee
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// 64-bit alignment.
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writeOpCount uint64
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// Protects map access
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sync.Mutex
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// Table entries
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entries map[QIno]*Entry
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}
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// Entry is an entry in the open file table
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type Entry struct {
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// Reference count
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refCount int
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// ContentLock guards the file content from concurrent writes. Every writer
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// must take this lock before modifying the file content.
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ContentLock countingMutex
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// HeaderLock guards the file ID (in this struct) and the file header (on
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// disk). Take HeaderLock.RLock() to make sure the file ID does not change
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// behind your back. If you modify the file ID, you must take
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// HeaderLock.Lock().
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HeaderLock sync.RWMutex
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// ID is the file ID in the file header.
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ID []byte
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}
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// Register creates an open file table entry for "qi" (or incrementes the
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// reference count if the entry already exists) and returns the entry.
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func Register(qi QIno) *Entry {
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t.Lock()
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defer t.Unlock()
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e := t.entries[qi]
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if e == nil {
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e = &Entry{}
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t.entries[qi] = e
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}
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e.refCount++
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return e
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}
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// Unregister decrements the reference count for "qi" and deletes the entry from
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// the open file table if the reference count reaches 0.
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func Unregister(qi QIno) {
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t.Lock()
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defer t.Unlock()
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e := t.entries[qi]
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e.refCount--
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if e.refCount == 0 {
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delete(t.entries, qi)
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}
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}
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// countingMutex incrementes t.writeLockCount on each Lock() call.
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type countingMutex struct {
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sync.Mutex
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}
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func (c *countingMutex) Lock() {
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c.Mutex.Lock()
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atomic.AddUint64(&t.writeOpCount, 1)
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}
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// WriteOpCount returns the write lock counter value. This value is encremented
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// each time writeLock.Lock() on a file table entry is called.
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func WriteOpCount() uint64 {
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return atomic.LoadUint64(&t.writeOpCount)
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}
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