1 files changed, 101 insertions, 0 deletions
diff --git a/internal/fusefrontend/open_file_table.go b/internal/fusefrontend/open_file_table.go
new file mode 100644
index 0000000..053298d
--- /dev/null
+++ b/internal/fusefrontend/open_file_table.go
@@ -0,0 +1,101 @@
+package fusefrontend
+
+import (
+	"sync"
+	"sync/atomic"
+	"syscall"
+)
+
+// DevInoStruct uniquely identifies a backing file through device number and
+// inode number.
+type DevInoStruct struct {
+	dev uint64
+	ino uint64
+}
+
+// DevInoFromStat fills a new DevInoStruct with the passed Stat_t info
+func DevInoFromStat(st *syscall.Stat_t) DevInoStruct {
+	// Explicit cast to uint64 to prevent build problems on 32-bit platforms
+	return DevInoStruct{
+		dev: uint64(st.Dev),
+		ino: uint64(st.Ino),
+	}
+}
+
+func init() {
+	openFileMap.entries = make(map[DevInoStruct]*openFileEntryT)
+}
+
+// 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 openFileMap openFileMapT
+
+// wlockMap - usage:
+// 1) register
+// 2) lock ... unlock ...
+// 3) unregister
+type openFileMapT struct {
+	// opCount counts 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.
+	opCount uint64
+	// Protects map access
+	sync.Mutex
+	entries map[DevInoStruct]*openFileEntryT
+}
+
+type opCountMutex struct {
+	sync.Mutex
+	// Points to the opCount variable of the parent openFileMapT
+	opCount *uint64
+}
+
+func (o *opCountMutex) Lock() {
+	o.Mutex.Lock()
+	atomic.AddUint64(o.opCount, 1)
+}
+
+// refCntMutex - mutex with reference count
+type openFileEntryT struct {
+	// Reference count
+	refCnt int
+	// Write lock for this inode
+	writeLock *opCountMutex
+	// ID is the file ID in the file header.
+	ID     []byte
+	IDLock sync.RWMutex
+}
+
+// register creates an entry for "ino", or incrementes the reference count
+// if the entry already exists.
+func (w *openFileMapT) register(di DevInoStruct) *openFileEntryT {
+	w.Lock()
+	defer w.Unlock()
+
+	r := w.entries[di]
+	if r == nil {
+		o := opCountMutex{opCount: &w.opCount}
+		r = &openFileEntryT{writeLock: &o}
+		w.entries[di] = r
+	}
+	r.refCnt++
+	return r
+}
+
+// unregister decrements the reference count for "di" and deletes the entry if
+// the reference count has reached 0.
+func (w *openFileMapT) unregister(di DevInoStruct) {
+	w.Lock()
+	defer w.Unlock()
+
+	r := w.entries[di]
+	r.refCnt--
+	if r.refCnt == 0 {
+		delete(w.entries, di)
+	}
+}