vfs: Fix missed concurrency control between some item operations and reset

Item reset is invoked by cache cleaner for synchronous recovery
from ENOSPC errors. The reset operation removes the cache file and
closes/reopens the downloaders.  Although most parts of reset and
other item operations are done with the item mutex held, the mutex
is released during fd.WriteAt and downloaders calls. We used preAccess
and postAccess calls to serialize Reset, ReadAt, and Open, but missed
some other item operations. The patch adds preAccess/postAccess
calls in Sync, Truncate, Close, WriteAt, and rename.

vfs/vfscache/item.go

@@ -43,6 +43,13 @@ import (
 // be taken before Item.mu. writeback may call into Item but Item may
 // **not** call writeback methods with Item.mu held
 
+// LL Item reset is invoked by cache cleaner for synchronous recovery
+// from ENOSPC errors. The reset operation removes the cache file and
+// closes/reopens the downloaders.  Although most parts of reset and
+// other item operations are done with the item mutex held, the mutex
+// is released during fd.WriteAt and downloaders calls. We use preAccess
+// and postAccess calls to serialize reset and other item operations.
+
 // Item is stored in the item map
 //
 // The Info field is written to the backing store to store status
@@ -310,6 +317,8 @@ func (item *Item) _truncateToCurrentSize() (err error) {
 // extended and the extended data will be filled with zeros. The
 // object will be marked as dirty in this case also.
 func (item *Item) Truncate(size int64) (err error) {
+	item.preAccess()
+	defer item.postAccess()
 	item.mu.Lock()
 	defer item.mu.Unlock()
 
@@ -429,6 +438,8 @@ func (item *Item) _dirty() {
 
 // Dirty marks the item as changed and needing writeback
 func (item *Item) Dirty() {
+	item.preAccess()
+	defer item.postAccess()
 	item.mu.Lock()
 	item._dirty()
 	item.mu.Unlock()
@@ -612,6 +623,8 @@ func (item *Item) store(ctx context.Context, storeFn StoreFn) (err error) {
 // Close the cache file
 func (item *Item) Close(storeFn StoreFn) (err error) {
 	// defer log.Trace(item.o, "Item.Close")("err=%v", &err)
+	item.preAccess()
+	defer item.postAccess()
 	var (
 		downloaders   *downloaders.Downloaders
 		syncWriteBack = item.c.opt.WriteBack <= 0
@@ -1213,6 +1226,8 @@ func (item *Item) readAt(b []byte, off int64) (n int, err error) {
 
 // WriteAt bytes to the file at off
 func (item *Item) WriteAt(b []byte, off int64) (n int, err error) {
+	item.preAccess()
+	defer item.postAccess()
 	item.mu.Lock()
 	if item.fd == nil {
 		item.mu.Unlock()
@@ -1303,6 +1318,8 @@ func (item *Item) WriteAtNoOverwrite(b []byte, off int64) (n int, skipped int, e
 // this means flushing the file system's in-memory copy of recently written
 // data to disk.
 func (item *Item) Sync() (err error) {
+	item.preAccess()
+	defer item.postAccess()
 	item.mu.Lock()
 	defer item.mu.Unlock()
 	if item.fd == nil {
@@ -1322,6 +1339,8 @@ func (item *Item) Sync() (err error) {
 
 // rename the item
 func (item *Item) rename(name string, newName string, newObj fs.Object) (err error) {
+	item.preAccess()
+	defer item.postAccess()
 	item.mu.Lock()
 
 	// stop downloader
@@ -1351,6 +1370,5 @@ func (item *Item) rename(name string, newName string, newObj fs.Object) (err err
 		_ = downloaders.Close(nil)
 	}
 	item.c.writeback.Rename(id, newName)
-
 	return err
 }