http clients repeatedly read the tags file

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Martin Pool wrote:
> Public bug reported:
>
> In looking at the Launchpad codehost logs, it seems that bzr http dumb
> clients reading the tags file unreasonably often. (Sorry, I'm not sure
> yet what operation is doing this, but spm can provide anonymized logs.)
>
> ** Affects: bzr
> Importance: Low
> Status: Confirmed
>
>
> ** Tags: http performance
>

Bzr doesn't cache the tags field. I worked around this in loggerhead in
a recent patch. On trunk:
  413 John Arbash Meinel 2010-04-27 [merge]
      Cache the Branch reverse-tag dict, since Branch does not.

We don't cache the result of "Branch.tags.get_tag_dict()" nor
"Branch.tags.get_reverse_tag_dict()". So we have to read and deserialize
the tags file each time.

In the case of loggerhead, this was being done for each revision that
was being displayed (so that it could add the tags info if present).

John
=:->

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