[jira] [Commented] (IO-279) Tailer erroneously considers file as new

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[jira] [Commented] (IO-279) Tailer erroneously considers file as new

AD_LB (Jira)

    [ https://issues.apache.org/jira/browse/IO-279?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13633820#comment-13633820 ]

Herman Meerlo commented on IO-279:
----------------------------------

Well, this is the world turned upside down. I can only reopen the issue if I have a patch for it. That doesn't make sense. The problem still exists and therefore the ticket should be reopened. I have patched it locally for myself but I doubt that my patch is ok for everyone because I completely removed the last else if statement. For me it makes no sense to check if the file is newer. The only use case would be that the file had been overwritten with exactly the same amount of data. Truncation is not an issue because that would mean that the length and position must have been 0 anyway. For me it is way more likely that the file's modified time has been updated than that the content has been overwritten with the same amount of bytes.
               

> Tailer erroneously considers file as new
> ----------------------------------------
>
>                 Key: IO-279
>                 URL: https://issues.apache.org/jira/browse/IO-279
>             Project: Commons IO
>          Issue Type: Bug
>    Affects Versions: 2.0.1
>            Reporter: Sergio Bossa
>             Fix For: 2.4
>
>         Attachments: IO-279.patch
>
>
> Tailer sometimes erroneously considers the tailed file as new, forcing a repositioning at the start of the file: I'm still unable to reproduce this in a test case, because it only happens to me with huge log files during Apache Tomcat startup.
> This is the piece of code causing the problem:
> {code}
> // See if the file needs to be read again
> if (length > position) {
>     // The file has more content than it did last time
>     last = System.currentTimeMillis();
>     position = readLines(reader);
> } else if (FileUtils.isFileNewer(file, last)) {
>     /* This can happen if the file is truncated or overwritten
>         * with the exact same length of information. In cases like
>         * this, the file position needs to be reset
>         */
>     position = 0;
>     reader.seek(position); // cannot be null here
>     // Now we can read new lines
>     last = System.currentTimeMillis();
>     position = readLines(reader);
> }
> {code}
> What probably happens is that the new file content is about to be written on disk, the date is already updated but content is still not flushed, so actual length is untouched and there you go.
> In other words, I think there should be some better method to verify the condition above, rather than relying only on dates: keeping and comparing the hash code of the latest line may be a solution, but may hurt performances ... other ideas?

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