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-rw-r--r--internal/openfiletable/open_file_table.go111
1 files changed, 111 insertions, 0 deletions
diff --git a/internal/openfiletable/open_file_table.go b/internal/openfiletable/open_file_table.go
new file mode 100644
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--- /dev/null
+++ b/internal/openfiletable/open_file_table.go
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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)
+}