fs/io/FileOutputStream: seek end-of-file in AppendFileOutputStream ctor

src/fs/io/FileOutputStream.cxx

@@ -226,6 +226,12 @@ AppendFileOutputStream::AppendFileOutputStream(Path _path, Error &error)
 	if (!IsDefined())
 		error.FormatLastError("Failed to append to %s",
 				      GetPath().ToUTF8().c_str());
+
+	if (!SeekEOF()) {
+		error.FormatLastError("Failed seek end-of-file of %s",
+				      GetPath().ToUTF8().c_str());
+		Close();
+	}
 #else
 	if (!SetFD().Open(GetPath().c_str(),
 			  O_WRONLY|O_APPEND))

src/fs/io/FileOutputStream.hxx

@@ -95,6 +95,12 @@ protected:
 #endif
 	}
 
+#ifdef WIN32
+	bool SeekEOF() {
+		return SetFilePointer(handle, 0, nullptr, FILE_END);
+	}
+#endif
+
 public:
 	bool IsDefined() const {
 #ifdef WIN32