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From Stefan Fritsch>
Subject ap_expr problems
Date Sat, 23 Oct 2010 11:36:33 GMT

the idea to have a generic expression parser and evaluator in httpd 
that can be used by various modules is certainly execellent. However, 
after spending some time on the current ap_expr parser, I am convinced 
that mod_include's old parser is not the right choice for this:

It is optimized for using a parsed expression only once: If using a 
parsed expression more than once, it copies the whole parse tree which 
makes it use a lot more memory when evaluating than e.g. the ssl_expr 
parser. Also, using recursion during expression evaluation would 
result in much more readable code than what ap_expr currently does.

It does things during evaluation which should really be done during 
parsing: split strings into tokens for variable substitution, compile 
regexps, etc.

The parsing of strings during evaluation creates a load of problems:
- The syntax is very strange. E.g. one needs two backslashes to escape 
a $, the expression '$var'text is actually the same as '$var text' 
(i.e. a space is inserted), ...
- If the result of an expression evaluation depends on the value of a 
request header, the HTTP response needs a corresponding Vary: header. 
This seems to be rather difficult to implement with the current 

Since we won't be able to change the ap_expr syntax in any significant 
way after 2.4 has been released, we should try to do it right in 2.4. 
Therefore I suggest to replace it with something that is closer to 
ssl_expr. Initially, I thought it would be nice to have a SSI 
compatibility mode, but now I think it would be cleaner to just stick 
the SSI expression parser back into mod_include and not deal with it 
further. On the other hand, maybe it is possible to make ap_expr 
syntax compatible with ssl_expr and replace it. If not, at least the 
evaluation logic should be shared between ap_expr and ssl_expr.

Thoughts, comments?


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