nsq_to_file: refactor confusing Write interface

Part of this change is to only increment `f.filesize` with the actual
number of bytes written, rather than supplied.

apps/nsq_to_file/file_logger.go

@@ -110,11 +110,11 @@ func (f *FileLogger) router() {
 				f.updateFile()
 				sync = true
 			}
-			_, err := f.writer.Write(m.Body)
+			_, err := f.Write(m.Body)
 			if err != nil {
 				log.Fatalf("ERROR: writing message to disk - %s", err)
 			}
-			_, err = f.writer.Write([]byte("\n"))
+			_, err = f.Write([]byte("\n"))
 			if err != nil {
 				log.Fatalf("ERROR: writing newline to disk - %s", err)
 			}
@@ -205,9 +205,10 @@ func (f *FileLogger) Close() {
 	f.out = nil
 }
 
-func (f *FileLogger) Write(p []byte) (n int, err error) {
-	f.filesize += int64(len(p))
-	return f.out.Write(p)
+func (f *FileLogger) Write(p []byte) (int, error) {
+	n, err := f.writer.Write(p)
+	f.filesize += int64(n)
+	return err
 }
 
 func (f *FileLogger) Sync() error {
@@ -323,10 +324,10 @@ func (f *FileLogger) updateFile() {
 	}
 
 	if f.opts.GZIP {
-		f.gzipWriter, _ = gzip.NewWriterLevel(f, f.opts.GZIPLevel)
+		f.gzipWriter, _ = gzip.NewWriterLevel(f.out, f.opts.GZIPLevel)
 		f.writer = f.gzipWriter
 	} else {
-		f.writer = f
+		f.writer = f.out
 	}
 }