Edit report at http://bugs.php.net/bug.php?id=44164&edit=1
ID: 44164
Updated by: cataphr...@php.net
Reported by: mplomer at gmx dot de
Summary: Handle "Content-Length" HTTP header when
zlib.output_compression active
Status: Assigned
Type: Bug
Package: *General Issues
Operating System: *
PHP Version: 5.2.5
Assigned To: cataphract
Block user comment: N
Private report: N
New Comment:
> Thatâs an error. Both scripts set the correct CL (that they know
very well),
> just the way the specification says they SHOULD. I donât agree that
it would
> be the responsibility of the script to counteract the setting (zlib
output
> compression in this case) of the executing framework (PHP in this
case). If
> the scripts should take care for every such situation then using the
header()
> would be completely illegal, because a future output handler might
interact
> with the output in such a way that invalidates the headers set. This
isnât a
> portable phylosophy since it implicitly requires the script being
aware of
> every aspects of plugins and settings in PHP.
> In fact it is the zlib output handler that was setting the wrong CL
header (by
> not removing the deprecated one). As I see, the handler is
constructing a new
> response entity instead the one it receives from the script; the
consistency of
> this response is entirely the responsibility of the handler. As I
understand
> this has now been patched so that the handler always removes the CL
header, and
> by that it assures correctness. Note: hereâs no refutation of the
correctness
> of the patched handler.
The problem is the zlib.output_compression is not presented as an output
handler that rewrites the response and creates a new entity. It is
presented as an inoffensive performance option that compresses the
output for better performance. And it does so, generally, without the
express assent of the programmer. The programmer can always use
ob_gzhandler to force compression.
Your thesis is that the output handler should not be deactivated;
instead it ought to remove the old header and write a new one, whenever
possible. This looks good. But consider this script:
if (empty($_SERVER["HTTP_RANGE"])) {
$offset = 0;
}
else { //violates rfc2616, which demands ignoring the header if invalid
preg_match("/^bytes=(\d+)-/i",$_SERVER["HTTP_RANGE"], $matches);
if (empty($matches[1]))
$offset = 0;
if (is_num_int($matches[1]) && $matches[1] < $filesize &&
$matches[1]>=0) {
$offset = $matches[1];
if (@fseek($fp,$offset,SEEK_SET) != 0)
InternalError();
header("HTTP/1.1 206 Partial Content");
header("Content-Range: bytes $offset-".($filesize -
1)."/$filesize");
}
elseif ($matches[1] > $filesize) {
header("HTTP/1.1 416 Requested Range Not Satisfiable");
die();
}
else $offset = 0;
}
$conlen = $filesize - $offset;
header("Content-Length: $conlen");
This is no way this script can work correctly under the zlib handler.
206 responses must have a content-length and the offsets are calculated
through the uncompressed size, while under zlib that should be
calculated under the compressed size, which is obviously impossible to
know without first compressing the file.
So actually the only option is to disable the zlib output handler.
Previous Comments:
------------------------------------------------------------------------
[2010-12-15 01:25:19] panczel dot levente at groware dot hu
Sorry for not being clear enough, let me explain! To put things simple
Iâll use two examples: [A] the one above with the 8K âAâ
characters and the following [B]:
<?php header(âContent-Length: 0â); ?>
> The problem is not the existence of a Content-length header
I never wrote that its existence would be a problem. On the contrary: I
think its correct presence is desirable wherever possible (most possible
requests and most possible layers of the runtime environment).
> it's the fact that you're setting a content-length header indicating a
size you cannot possibly know
Thatâs an error. Both scripts set the correct CL (that they know very
well), just the way the specification says they SHOULD. I donât agree
that it would be the responsibility of the script to counteract the
setting (zlib output compression in this case) of the executing
framework (PHP in this case). If the scripts should take care for every
such situation then using the header() would be completely illegal,
because a future output handler might interact with the output in such a
way that invalidates the headers set. This isnât a portable phylosophy
since it implicitly requires the script being aware of every aspects of
plugins and settings in PHP.
In fact it is the zlib output handler that was setting the wrong CL
header (by not removing the deprecated one). As I see, the handler is
constructing a new response entity instead the one it receives from the
script; the consistency of this response is entirely the responsibility
of the handler. As I understand this has now been patched so that the
handler always removes the CL header, and by that it assures
correctness. Note: hereâs no refutation of the correctness of the
patched handler.
> Apache already adds a Content-length header when it can (i.e. for
small responses), it's not necessary PHP does this
Didnât mean to suggest it would be necessary. It just yields better
performance (if the cost of generating the CL is not high).
> sending it on every compressed response is unpractical because it
would require buffering the entire response
Not for every compressed response; that would be impossible e.g. for
live streams. But on the other hand ALWAYS discarding CL is the worst
one among the correct solutions. Consider example [B]: I imagine that
the script has already finished once the handler receives control, thus
it is able to see that its input (from the script) is already closed. In
this case it does not have to use buffers or make heavy computations: by
skipping compression entirely everything is set, and a correct CL is
transmitted. Iâll get back to this.
> I suppose you can always
Yes, one can always make patches to avoid specific errors that a buggy
RTE produces. I just hope thereâs no software engineer who sees this
as a reason against fixing a bug.
Now back to example [B]. I see itâs not a common use case, but I think
it sheds light on other problems too. Letâs distinguish
administrators, who control webserver (or other environment) and PHP
settings but must not edit application code, and software designers who
have to create a versatile PHP application that can be run on any
platform efficiently without having influence on the specific settings
of the platform. So developer doesnât say âplease turn compression
offâ and admin doesnât add some new lines of code to the script.
Letâs assume the zlib handler has a small buffer (probably the one it
already has and is configurable with the value of
zlib.output_compression). The handler initially fills compressed output
into this buffer. If it has to flush the buffer before input EOF then it
clears CL flushes and replaces itself with the compressor-component in
the stream-chain (or does any other thing it does now to compress the
response body). Otherwise it computes and sets the correct CL and sends
the compressed body of the response.
In this manner correct applications can be written that have the benefit
of using CL without having to care for whether zlib is enabled; software
designers can rest assured that their code is good and runs efficient.
Admin can switch zlib on/off as he sees fit: he will neither break the
served apps, nor cripple their performance. And even better: when admin
sees that 1% of the responses is <4K (the default zlib buffer size) 98%
is between 4K and 20K and only 1% is >20K, he can just go âWhy
wouldnât I sacrifice that 16K/request RAM to have Content-Length
almost always sent to the client in contract to the current habit of
almost never sending?!â ⦠and how right he would be. As you can see
this solution is not only bright for the 0-long [B], not only to the
8K-long [A] but possibly for any environment, since sticking with the
defaults gives a good tradeoff while maintenance personnel has the
opportunity to fine-tune this behavior without adding modifications or
posing constraint to the code.
The answer showed that my previous post wasnât verbose enough to
express my opinion: striving towards such quality solutions as sketched
in the last part _might_ be a better option than choosing the simplest
solution (as the current one is). And Iâm pretty sure that the ones
who wrote the zlib handler can think of solutions that are both more
elegant and more efficient and provide the Web with as many correct CL
headers as possible.
------------------------------------------------------------------------
[2010-12-14 04:59:20] cataphr...@php.net
Sorry for the mess; I was betrayed by the browser's autocomplete.
------------------------------------------------------------------------
[2010-12-14 04:58:30] cataphr...@php.net
> Our projects make heavy use of Content-Length. Disabling it
unnecessarily is
> costly on networks with large RTT.
The problem is not the existence of a Content-length header, it's the
fact that you're setting a content-length header indicating a size you
cannot possibly know. A wrong Content-length header is worse than none.
Apache already adds a Content-length header when it can (i.e. for small
responses), it's not necessary PHP does this; sending it on every
compressed response is unpractical because it would require buffering
the entire response. If you need this, I suppose you can always
explicitly start the zlib output handler and call ob_get_length.
------------------------------------------------------------------------
[2010-12-14 04:57:52] cataphr...@php.net
> Our projects make heavy use of Content-Length. Disabling it
unnecessarily is
> costly on networks with large RTT.
The problem is not the existence of a Content-length header, it's the
fact that you're setting a content-length header indicating a size you
cannot possibly know. A wrong Content-length header is worse than none.
Apache already adds a Content-length header when it can (i.e. for small
responses), it's not necessary PHP does this; sending it on every
compressed response is unpractical because it would require buffering
the entire response. If you need this, I suppose you can always
explicitly start the zlib output handler and call ob_get_length.
------------------------------------------------------------------------
[2010-12-13 17:45:21] panczel dot levente at groware dot hu
Our projects make heavy use of Content-Length. Disabling it
unnecessarily is costly on networks with large RTT.
I also do not agree that manipulating Content-Length is a bad thing to
do for output handlers. To give a correct Content-Length (whenever
possible) is the task of the handler just as setting the
"Content-Encoding: gzip" is.
I'd vote for a solution where zlib output generates a correct
Content-Length whenever it has the opportunity (regarding the current
settings). The most straightforward solution I can imagine is that the
output compression module waits until the first buffer flush and then
right before writing to its output it checks whether the input has
finished [i.e. the whole page is buffered] and that the
compressed+encoded length is known; then it sets the correct
Content-Length. If, but _only_ if any of the above requirements are not
met (input still pending, compressed size is unknown for compression
cannot complete without flushing first) then clear Content-Length and
flush (so it cannot be set anymore).
I think this would maintain correctness, does not need additional
resources (like extra buffering), but keeps the benefits of sending
Content-Length whenever possible. (This last one I find to be a huge
benefit with pages that include many generated CSS-parts or for pages
that dynamically load many files, like dojo.)
------------------------------------------------------------------------
The remainder of the comments for this report are too long. To view
the rest of the comments, please view the bug report online at
http://bugs.php.net/bug.php?id=44164
--
Edit this bug report at http://bugs.php.net/bug.php?id=44164&edit=1
