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
index 0000000..7cd401e
--- /dev/null
+++ b/internal/openfiletable/open_file_table.go
@@ -0,0 +1,111 @@
+// 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)
+}