README.wget404 -- adding a return code of 4 for 404 errors to wget

[Rather than bloat the udeb with comments, the wget404 wrapper is documented here.]

This was originally inspired by a patch submitted by Alex Owen in bug #422088

As mentioned by Joey, the fragile bit of a script like this is due to
it's dependence on wget's output not changing.  That being the case,
I've made the search string quite long so that it's very unlikely to
match something that's not a 404 error, and also ensured that if the
output does change, the sed will fail safe by returning 1 (i.e. general
error) if no specific error is found.

Alex Owen alerts us (#491098) to the fact that:
From etch to lenny busybox wget error output has changed.
For lenny busybox wget 404 output is for example:
"server returned error: HTTP/1.1 404 Not Found"
This comprises the static string "server returned error: " 
followed by the server response which should follow rfc2616 section 6.1.
Thus the output may say HTTP/1.0 instead of HTTP/1.1 and the string "Not Found"
may also change. Thus the regular expression:
  /server returned error: HTTP\/[0-9.]\+ 404 /
should catch all possible output for lenny.

For the ftp method the error string is different. The following regexp should work:
 /bad response to RETR: 550 /

Here is a copy of the function being documented (since it's bound to
get out of sync with the one in the fetch-url-methods/http file, so you
might as well see the one that's being documented as well ;-)

        wget404() {
        # see README.wget404 in the debian-installer-utils udeb source for more info about this
                if [ "ftp" = "$proto" ] ; then
                        local file_not_found_pattern='bad response to RETR: 550 '
                else
                        local file_not_found_pattern='server returned error: HTTP\/[0-9.]\+ 404 '
                fi

                local RETVAL=$( {
                        echo 1
                        wget "$@" 2>&1 >&3 && echo %OK%
                        echo %EOF%
                        } | ( sed -ne '1{h;d};/'"$file_not_found_pattern"'/{p;s/.*/4/;h;d};/^%OK%$/{s/.*/0/;h;d};$!p;$x;$w /proc/self/fd/4' >&2 ) 4>&1
                ) 3>&1
                return $RETVAL
        }

The heart of this is the sed, which looks for the $file_not_found_pattern,
while also outputting the original STDOUT/ERR from wget untouched so that
it's available for the caller.

It manages this by doing the following:

  1{h;d}  --  take the first line (provided by the echo 1) and put it in sed's hold space
              this will provide a default return value of 1 unless something else happens

  $file_not_found_pattern{p;s/.*/4/;h;d}
          as mentioned above, the wget output is protocol dependent,
          so we've previously set $file_not_found_pattern depending
          on the protocol in use, then if we see the matching string,
          we print it, then turn it into a "4" and stuff it in the sed
          hold space, and finally, delete the "4" This is where our
          return value of 4 comes from.

  /^%OK%$/{s/.*/0/;h;d}
          If we see a "%OK%" line (provided by the echo %OK% if wget returns 0)
          then we replace it with "0", stuff that in the hold space, and discard it

  $!p    --  for all but the last line, print them if not yet matched

  $x     -- when we get to the last line (provided by the echo %EOF%) swap the pattern
            with the hold space -- so now we have our return code in the pattern space

  $w /proc/self/fd/4 -- write the result to file handle 4

The rest of this is just making sure that the standard error, and standard
out pops out of the function as if it were just wget, while allowing us
to do things with the results in between.

So, we call wget and put it's STDERR on STDOUT, and it's STDOUT on file
handle 3

   2>&1 >&3

that allows us to put the STDERR into sed to look for the 404 error

then, we make sure that the sed outputs everything that it was given,
and redirect that back onto STDERR:

   >&2

additionally, we're running the sed in a subshell, and the return value
is going to pop out on file handle 4, so we need to make that STDOUT so
that the $(...) can pick up on it and shove it into RETVAL, hence the:

  4>&1

finally, we've still got the wget's original STDOUT floating around on
file handle 3, so to get it back we do:

  3>&1

I did think that one might be able to do this without the variable
assignment, but that seems not to work for some reason.

Of course, having managed to preserve the STDOUT & STDERR, I note that
it's always called with a -q (quiet) and therefore shouldn't be producing
STDOUT anyway -- Doh!

Phil Hands -- 2008-07-18
