Re: kernel.org mirroring (Re: [GIT PULL] MMC update)

Jeff Garzik wrote:

Jakub Narebski wrote:

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Jeff Garzik wrote:

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Jakub Narebski wrote:

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P.S. Any hints to how to do this with CGI Perl module?

It's impossible, Apache doesn't supply e-tag info to CGI programs.  (it 
does supply HTTP_CACHE_CONTROL though apparently)

By ETag info you mean access to HTTP headers sent by browser
If-Modified-Since:, If-Match:, If-None-Match: do you?

Adn in CGI standard there is a way to access additional HTTP headers
info from CGI script: the envirionmental variables are HTTP_HEADER,
for example if browser sent If-Modified-Since: header it's value
can be found in HTTP_IF_MODIFIED_SINCE environmental variable.

But of course gitweb should rather use mod_perl if possible, so
somewhere in gitweb there would be the following line:

  $in_date = $ENV{'MOD_PERL'} ?
    $r->header('If-Modified-Since') :
    $ENV{'HTTP_IF_MODIFIED_SINCE'};

or something like that...
 

You can use this attached shell script as a CGI script, to see precisely 
what information Apache gives you.  You can even experiment with passing 
back headers other than Content-type (such as E-tag), to see what sort 
of results are produced.  The script currently passes back both E-Tag 
and Last-Modified of a sample file; modify or delete those lines to suit 
your experiments.

It is ETag, not E-tag. Besides, I don't see what the attached script is
meant to do: it does not output the sample file anyway.

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It's a pity that CGI interface doesn't cover that...

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You could probably do it via mod_perl.

So the cache verification should be wrapped in if ($ENV{MOD_PERL}) ?

Sorry, I was /assuming/ mod_perl would make this available.  The HTTP 
header info is available to all Apache modules, but I confess I have no 
idea how mod_perl passes that info to scripts.

Also, an interesting thing while I was testing the attached shell 
script:  even though repeated hits to the script generate a proper 304 
response to the browse, the CGI script and its output run to completion. 
  So, it didn't save work on the CGI side; the savings was solely in not 
transmitting the document from server to client.  The server still went 
through the work of generating the document (by running the CGI), as one 
would expect.

The idea is of course to stop processing in CGI script / mod_perl script
as soon as possible if cache validates.

I don't know if Apache intercepts and remembers ETag and Last-Modified
headers, adds 304 Not Modified HTTP response on finding that cache validates
and cuts out CGI script output. I.e. if browser provided If-Modified-Since:,
script wrote Last-Modified: header, If-Modified-Since: is no earlier than
Last-Modified: (usually is equal in the case of cache validation), then
Apache provides 304 Not Modified response instead of CGI script output.

-- 
Jakub Narebski