question around ngx_http_get_flushed_variable()

[Chris Newton]

Thanks Maxim - that does help.

I do have a follow up question though, around when to mark variables as
no_cacheable.

Looking for instance at the request_method variable, it is set via the
ngx_http_variable_request_method()
function which on success sets v->no_cacheable to 0 - that makes sense to
me, since it looks at r->main->method_name and so I would expect it to be
fixed for the lifetime of a request

However, the declaration of the variable within ngx_http_core_variables has
the NGX_HTTP_VAR_NOCACHEABLE flag set.

I guess I have two questions here :

1. it seems like having this variable marked as non-cacheable is
unnecessary; in this case, there is little work to be done to set it, but
I'm trying to understand the criteria being used - I'd imagine that any
variable that is not expected to change over the lifetime of the request
should be cacheable - in which case variables like msec would be reasonably
marked non-cacheable, but I don't see why those like request_method would
be? It seems similar in scope to me as http_host which is *not* marked
non-cachable

2. is there a reason why the get handler should ever set the variable as
non-cacheable, or should it always set it that flag to 0 and instead rely
on the call to the ngx_http_add_variable() function to set it?  (coding
style?)

Thank you again for your help

Chris

On Tue, Apr 5, 2022 at 9:02 AM Maxim Dounin <mdounin at mdounin.ru> wrote:

> Hello!
>
> On Mon, Apr 04, 2022 at 12:44:38PM -0700, Chris Newton via nginx-devel
> wrote:
>
> > I'm a little unclear as to when ngx_http_get_flushed_variable() should be
> > used rather than ngx_http_get_indexed_variable(). The dev guide is a
> little
> > sparse, so I'm hoping someone could help.
>
> [...]
>
> > Are there any guidelines on when ngx_http_get_flushed_variable() should
> be
> > used instead of ngx_http_get_indexed_variable()?
>
> The ngx_http_get_indexed_variable() does not flush non-cacheable
> variables by itself, and should be used when flushes are not
> needed and/or handled separately.  For example, it is used by the
> script codes, and this makes it possible to evaluate multiple
> variables with a single cache flush, such as by
> ngx_http_script_flush_no_cacheable_variables().
>
> The ngx_http_get_flushed_variable() does a flush automatically
> when needed, that is, it does not use cache for non-cacheable
> variables.  This is to be used when flushes are not handled
> separately and might be important.
>
> Unless you have some explanation on why using
> ngx_http_get_indexed_variable() in the particular place is safe,
> you probably should use ngx_http_get_flushed_variable() instead,
> or you are risking to end up with incorrect behaviour if you'll
> try to lookup non-cacheable variables.
>
> Just in case, looking through all uses of the
> ngx_http_get_indexed_variable() function in nginx:
>
> - ngx_http_charset_filter_module.c: not sure it is safe, might be
>   a bug;
>
> - ngx_http_log_module.c: used with a separate flush by
>   ngx_http_script_flush_no_cacheable_variables();
>
> - ngx_http_memcached_module.c: the $memcached_key variable is
>   expected to be set by "set", so the code assumes no flushes are
>   needed;
>
> - ngx_http_userid_filter_module.c: the $uid_reset variables is
>   expected to be set by "set", so the code assumes no flushes are
>   needed.
>
> - ngx_http_variables.c: in the ngx_http_get_flushed_variable()
>   function, with automatic flush when needed;
>
> - ngx_http_script.c: used when a flush is handled separately.
>
> > It seems like it is 'practically never', but why for example does
> > ngx_http_get_variable() use
> >
> > ngx_http_get_flushed_variable()?
>
> The ngx_http_get_variable() is to be used for dynamic variables
> lookup, when you don't know the variable name in advance, notably
> in SSI and embedded Perl.  It is expected to be used without
> external flushes and so uses ngx_http_get_flushed_variable() to
> ensure automatic flushes when needed.
>
> Hope this helps.
>
> --
> Maxim Dounin
> http://mdounin.ru/
>
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