16.4.1. GET Requests

But there is a certain situation that needs a workaround to achieve better cacheability. We need to deal with the "?" in the relative path part of the requested URI. Section 13.9 specifies that:

... caches MUST NOT treat responses to such URIs as fresh unless the server provides 
an explicit expiration time.  This specifically means that responses from HTTP/1.0 
servers for such URIs SHOULD NOT be taken from a cache.

Although it is tempting to imagine that if we are using HTTP/1.1 and send an explicit expiration time we are safe, the reality is unfortunately somewhat different. It has been common for quite a long time to misconfigure cache servers so that they treat all GET requests containing a question mark as uncacheable. People even used to mark anything that contained the string "cgi-bin" as uncacheable.

To work around this bug in HEAD requests, we have stopped calling CGI directories cgi-bin and we have written the following handler, which lets us work with CGI-like query strings without rewriting the software (e.g., Apache::Request and CGI.pm) that deals with them:

sub handler {
    my $r = shift;
    my $uri = $r->uri;
    if ( my($u1,$u2) = $uri =~ / ^ ([^?]+?) ; ([^?]*) $ /x ) {
        $r->uri($u1);
        $r->args($u2);
    }
    elsif ( my ($u1,$u2) = $uri =~ m/^(.*?)%3[Bb](.*)$/ ) {
        # protect against old proxies that escape volens nolens
        # (see HTTP standard section 5.1.2)
        $r->uri($u1);
        $u2 =~ s/%3[Bb]/;/g;
        $u2 =~ s/%26/;/g; # &
        $u2 =~ s/%3[Dd]/=/g;
        $r->args($u2);
    }
    DECLINED;
}

This handler must be installed as a PerlPostReadRequestHandler.

The handler takes any request that contains one or more semicolons but no question mark and changes it so that the first semicolon is interpreted as a question mark and everything after that as the query string. So now we can replace the request:

http://example.com/query?BGCOLOR=blue;FGCOLOR=red

with:

http://example.com/query;BGCOLOR=blue;FGCOLOR=red

This allows the coexistence of queries from ordinary forms that are being processed by a browser alongside predefined requests for the same resource. It has one minor bug: Apache doesn't allow percent-escaped slashes in such a query string. So instead of:

http://example.com/query;BGCOLOR=blue;FGCOLOR=red;FONT=%2Ffont%2Fpath

we must use:

http://example.com/query;BGCOLOR=blue;FGCOLOR=red;FONT=/font/path

To unescape the escaped characters, use the following code:

s/%([0-9A-Fa-f]{2})/chr hex $1/ge;