Re: [RFC PATCHv2 04/10] gitweb: Use Cache::Cache compatibile (get, set) output caching

On Tue, 9 Feb 2010 at 11:30 +0100, Jakub Narebski wrote:

The cache_fetch subroutine captures output (from STDOUT only, as
STDERR is usually logged) using either ->push_layer()/->pop_layer()
from PerlIO::Util submodule (if it is available), or by setting and
restoring *STDOUT.  Note that only the former could be tested reliably
to be reliable in t9503 test!

Scratch that, I have just checked that (at least for Apache + mod_cgi,
but I don't think that it matters) the latter solution, with setting
and restoring *STDOUT doesn't work: I would get data in cache (so it
can be restored later), but instead of output I would get Internal Server
Error ("The server encountered an internal error or misconfiguration and
was unable to complete your request.") without even a hint what the
problem was.  Sprinkling "die ...: $!" didn't help to catch this error:
I suspect that the problem is with capturing.

So we either would have to live with non-core PerlIO::Util or (pure Perl)
Capture::Tiny, or do the 'print -> print $out' patch...

[....]

Capturing gitweb output
=======================
When output (response) caching is enabled, the caching mechanism has to
capture gitweb output (perhaps while printing it to standard output) to
save it to cache, unless the data is available in cache and not expired.

Because die_error uses 'exit', and because it uses git_header_html and
other printing subroutines (which output has to be captured in other
situations), it needs to disable caching, unless we are "tee"-ing.
Otherwise we would get no output from die_error (because it is captured),
but also we would not get data to be saved in cache and printed, because
'exit' in die_error would exit capture, too.  This restricts methods and
modules that can be used to capture output.

Below there are presented various considered and used ways of capturing the
output, or "tee"-ing it (capturing while printing), with their advantages
and disadvantages.


Capturing output (capture)
~~~~~~~~~~~~~~~~~~~~~~~~~~

[...]

5. Without 'print <sth>' to 'print $out <sth>' change to gitweb, one can try
   manipulating *STDOUT directly, first saving *STDOUT or \*STDOUT to some
   variable, then setting *STDOUT = $data_fh, where $data_fh is filehandle
   opened to in-memory file.

   This seems to work, does not require large patch to gitweb, and does not
   require extra (non-core) Perl module.  Nevertheless it seems fragile with
   respect to restoring STDOUT, and even though basic tests (not included)
   of this mechanism producted expected result, I wasn't able to write
   working tests when using this method.

   We could probably examine how Capture::Tiny does it, and reuse (copy)
   relevant parts of code, perhaps duplicating STDOUT, closing it and then
   reopening as in-memory filehandle.

   YMMV (Your Mileage May Vary).

-- 
Jakub Narebski
Poland