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From "Nuutti Kotivuori (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (COUCHDB-887) _log handler has very odd semantics for bytes/offset, probably a bug
Date Mon, 31 Oct 2011 08:14:32 GMT

    [ https://issues.apache.org/jira/browse/COUCHDB-887?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13139993#comment-13139993
] 

Nuutti Kotivuori commented on COUCHDB-887:
------------------------------------------

Even your explanation is not really exact. What it does is (abstractly):
- start reading from the end
- seek `bytes` backwards from the end
- seek `offset` bytes forward into the file
- read until a maximum of `bytes` bytes is returned
- stop

The result is that:
- bytes=10000&offset=1000 -> returns last 9000 bytes of log file
- bytes=9000 -> returns last 9000 bytes of log file

Like I said in the bug report, the results are 100% equivalent of just doing 'bytes - offset'
on the clientside before making the request, except for the case of 'offset => bytes',
in which case EOF is returned because of the attempt to read past file end.

In general, I'm having trouble figuring out what the original motivation behind offset is...
I mean, *if* it would seek backwards, you could theoretically get the last 100kB of log in
1kB chunks by doing repeated queries with bytes=1024 and incrementing offset by 1024 each
time - but in practice the log file grows between each request, so there'll be lots of repeated
lines and such.
                
> _log handler has very odd semantics for bytes/offset, probably a bug
> --------------------------------------------------------------------
>
>                 Key: COUCHDB-887
>                 URL: https://issues.apache.org/jira/browse/COUCHDB-887
>             Project: CouchDB
>          Issue Type: Bug
>          Components: HTTP Interface
>    Affects Versions: 0.11, 0.11.1, 0.11.2, 1.0, 1.0.1
>            Reporter: Nuutti Kotivuori
>            Priority: Trivial
>         Attachments: couch_log.erl.diff
>
>
> The _log method for couchdb servers has query arguments "bytes" and "offset", but they
seem to behave really weirdly. If offset >= bytes, an eof error is returned - otherwise
offset is just substracted from bytes and the request behaves identically otherwise.
> A simple fix to the expected behaviour is included - offset counts backwards from bytes.

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