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