There's a very small race window in GetImageServlet, if the following interleaving occurs:
- The Storage object returns some local file in the storage directory (eg an edits file or image file)
- Race: some other process removes the file
- GetImageServlet calls file.length() which returns 0, since it doesn't exist. It thus faithfully sets the Content-Length header to 0
- getFileClient() throws FileNotFoundException when trying to open the file. But, since we call response.getOutputStream() before this, the headers have already been sent, so we fail to send the "404" or "500" response that we should.
Thus, the client sees a 0-length Content-Length followed by 0 lengths of content, and thinks it successfully has downloaded the target file, where in fact it downloads an empty one.
I saw this in practice during the "edits synchronization" phase of recovery while working on
HDFS-3077, though it could apply on existing code paths, as well, I believe.