Fix to append rather than overwrite data in existing files (#184)

* Fix to append rather than overwrite data in existing files

* Update #availability flag for all supported platforms

Co-authored-by: Danny Sung <[email protected]>

Author: dannys42

Sources/Segment/Utilities/OutputFileStream.swift

@@ -53,6 +53,14 @@ internal class OutputFileStream {
         if fileHandle != nil { return }
         do {
             fileHandle = try FileHandle(forWritingTo: fileURL)
+            if #available(iOS 13.4, macOS 10.15.4, tvOS 13.4, *) {
+                _ = try? fileHandle?.seekToEnd()
+            } else if #available(tvOS 13.0, *) {
+                try? fileHandle?.seek(toOffset: .max)
+            } else {
+                // unsupported
+                throw OutputStreamError.unableToOpen(fileURL.path)
+            }
         } catch {
             throw OutputStreamError.unableToOpen(fileURL.path)
         }