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From "Grzegorz Molenda (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (IO-279) Tailer erroneously considers file as new
Date Tue, 27 Nov 2012 14:31:59 GMT

    [ https://issues.apache.org/jira/browse/IO-279?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13504637#comment-13504637
] 

Grzegorz Molenda commented on IO-279:
-------------------------------------

I am tailing with the fixed Tailer (commons-io 2.4.0) a log4j log file and I still see the
issue. Despite the fact that the log file was neither rotated nor new data was added, the
position is being reset to 0, causing the Tailer re-reading the monitored file from the begining.


Since log4j's asynchronous logger is used to log into the monitored file, it might happen,
that the modifiedDate is set before the content is actually flushed to the file. 

I assume reseting position was added to cover the case, when the monitored file is overriden.
I think it is imposiilble for the Tailer to determine this. The current implementation covers
only the case, when the file length is equal to the last read position. If the file legth
after being overriden is higher than the last read position, then the Tailer will assume data
was normally appended and process the file from the last read position. 

Assuming the data is only appended to the file, I'd just get rid of the reseting position
feature from Tailer to resolve the issue finally.
                
> 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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