fs: Convert repeated code in rwpool.Open() into a single function. (#4864)

Refer https://github.com/minio/minio/issues/4658 for more information.

cmd/fs-v1-rwpool.go

@@ -31,24 +31,16 @@ type fsIOPool struct {
 	readersMap map[string]*lock.RLockedFile
 }
 
-// Open is a wrapper call to read locked file which
-// returns a ReadAtCloser.
+// lookupToRead - looks up an fd from readers map and
+// returns read locked fd for caller to read from, if
+// fd found increments the reference count. If the fd
+// is found to be closed then purges it from the
+// readersMap and returns nil instead.
 //
-// ReaderAt is provided so that the fd is non seekable, since
-// we are sharing fd's with concurrent threads, we don't want
-// all readers to change offsets on each other during such
-// concurrent operations. Using ReadAt allows us to read from
-// any offsets.
-//
-// Closer is implemented to track total readers and to close
-// only when there no more readers, the fd is purged if the lock
-// count has reached zero.
-func (fsi *fsIOPool) Open(path string) (*lock.RLockedFile, error) {
-	if err := checkPathLength(path); err != nil {
-		return nil, err
-	}
-
-	fsi.Lock()
+// NOTE: this function is not protected and it is callers
+// responsibility to lock this call to be thread safe. For
+// implementation ideas look at the usage inside Open() call.
+func (fsi *fsIOPool) lookupToRead(path string) (*lock.RLockedFile, bool) {
 	rlkFile, ok := fsi.readersMap[path]
 	// File reference exists on map, validate if its
 	// really closed and we are safe to purge it.
@@ -69,15 +61,33 @@ func (fsi *fsIOPool) Open(path string) (*lock.RLockedFile, error) {
 			rlkFile.IncLockRef()
 		}
 	}
-	fsi.Unlock()
+	return rlkFile, ok
+}
 
-	// Locked path reference doesn't exist, freshly open the file in
-	// read lock mode.
+// Open is a wrapper call to read locked file which
+// returns a ReadAtCloser.
+//
+// ReaderAt is provided so that the fd is non seekable, since
+// we are sharing fd's with concurrent threads, we don't want
+// all readers to change offsets on each other during such
+// concurrent operations. Using ReadAt allows us to read from
+// any offsets.
+//
+// Closer is implemented to track total readers and to close
+// only when there no more readers, the fd is purged if the lock
+// count has reached zero.
+func (fsi *fsIOPool) Open(path string) (*lock.RLockedFile, error) {
+	if err := checkPathLength(path); err != nil {
+		return nil, err
+	}
+
+	fsi.Lock()
+	rlkFile, ok := fsi.lookupToRead(path)
+	fsi.Unlock()
+	// Locked path reference doesn't exist, acquire a read lock again on the file.
 	if !ok {
-		var err error
-		var newRlkFile *lock.RLockedFile
-		// Open file for reading.
-		newRlkFile, err = lock.RLockedOpenFile(path)
+		// Open file for reading with read lock.
+		newRlkFile, err := lock.RLockedOpenFile(path)
 		if err != nil {
 			if os.IsNotExist(err) {
 				return nil, errFileNotFound
@@ -93,34 +103,29 @@ func (fsi *fsIOPool) Open(path string) (*lock.RLockedFile, error) {
 			return nil, err
 		}
 
-		// Save new reader on the map.
+		/// Save new reader on the map.
+
+		// It is possible by this time due to concurrent
+		// i/o we might have another lock present. Lookup
+		// again to check for such a possibility. If no such
+		// file exists save the newly opened fd, if not
+		// reuse the existing fd and close the newly opened
+		// file
 		fsi.Lock()
-		rlkFile, ok = fsi.readersMap[path]
-		if ok && rlkFile != nil {
-			// If the file is closed and not removed from map is a bug.
-			if rlkFile.IsClosed() {
-				// Log this as an error.
-				errorIf(errUnexpected, "Unexpected entry found on the map %s", path)
-
-				// Purge the cached lock path from map.
-				delete(fsi.readersMap, path)
-
-				// Save the newly acquired read locked file.
-				rlkFile = newRlkFile
-			} else {
-				// Increment the lock ref, since the file is not closed yet
-				// and caller requested to read the file again.
-				rlkFile.IncLockRef()
-				newRlkFile.Close()
-			}
+		rlkFile, ok = fsi.lookupToRead(path)
+		if ok {
+			// Close the new fd, since we already seem to have
+			// an active reference.
+			newRlkFile.Close()
 		} else {
-			// Save the newly acquired read locked file.
+			// Save the new rlk file.
 			rlkFile = newRlkFile
 		}
 
-		// Save the rlkFile back on the map.
+		// Save the new fd on the map.
 		fsi.readersMap[path] = rlkFile
 		fsi.Unlock()
+
 	}
 
 	// Success.