On Sat, Aug 31, 2019 at 04:55:26PM -0500, J. Lewis Muir wrote:
> On 08/31, Francis Daly wrote:
> > On Sat, Aug 31, 2019 at 09:10:09AM -0500, J. Lewis Muir wrote:
Hi there,
> > Using "realpath" should not affect nginx at all. nginx invites the
> > fastcgi server to use pathname2 instead of pathname1; so the fastcgi
> > server is the only thing that should care.
>
> Hmm, I might not be understanding this. The rationale of using
> $realpath_root instead of $document_root was to make it so that a
> new version of the web app could be deployed atomically at any time
> by changing the "current" symlink,
....
> fastcgi_param DOCUMENT_ROOT $realpath_root;
> fastcgi_param SCRIPT_FILENAME $realpath_root$fastcgi_script_name;
>
> So, does that make sense, or am I still not understanding this? I don't
> know what you mean by "nginx invites the fastcgi server to use pathname2
> instead of pathname1." What are pathname1 and pathname2?
nginx does not "do" php. nginx does not care what your fastcgi server
will do with the key/value pairs that it sends. nginx cares that the
fastcgi server gives a valid response to the request that nginx makes.
Typically, your fastcgi server will use the value associated with
SCRIPT_FILENAME as "the name of the file to execute". If your fastcgi
server fails to find / read / execute that file, it will return its own
error indication.
(So your "if", or the more common "try_files", is just an early-out,
to sometimes avoid involving the fastcgi server. It may happen that the
file is present when nginx looks for it, but is absent when the fastcgi
server looks for it -- so that case does have to be handled anyway.)
In this case, if $document_root is /srv/www/my-app/current/ and
$realpath_root is /srv/www/my-app/releases/1.0.2/, and the script
name is test.php, then with one config, nginx would send the string
"/srv/www/my-app/current/test.php", and with the other config nginx
would send the string "/srv/www/my-app/releases/1.0.2/test.php".
(That is "pathname1" vs "pathname2".)
So if "one request" involves the fastcgi server reading
"/srv/www/my-app/current/test.php", and then reading a bunch of other
files in the same directory -- then I guess that unfortunate timing
could lead to it reading some files from releases/1.0.1 and some from
releases/1.0.2. (Assuming that it opens the directory afresh each time --
which can't be ruled out.)
But if "the app" involves a http request to part1.php and then a http
request to part2.php (or: a second http request to part1.php), I don't
think that the symlink+realpath thing will prevent those two requests
going to different release versions.
All the best,
f
--
Francis Daly francis@daoine.org
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