ngx.var can't set variables prefixed with http_

[adamonduty]

I ran into this issue today:

2014/12/05 21:49:52 [error] 27389#0: *1 lua entry thread aborted: runtime error: access_by_lua:5: variable "http_cache_purge" cannot be assigned a value
stack traceback:
coroutine 0:
        [C]: in function '__newindex'
        access_by_lua:5: in function <access_by_lua:1>, client: 127.0.0.1,

The problem appears to be that the variable was prefixed with http, as changing it to cache_purge fixes the issue. I note the documentation for ngx.var.VARIABLE mentions

Some special nginx variables like $args and $limit_rate can be assigned a value, some are not, like $arg_PARAMETER

but nothing about http_. Are there any other prefixes that will cause the same issue? I can throw together a pull request to fix the docs if needed.

[agentzh]

@adamonduty Not all the nginx built-in variables are changeable. Actually many of them like http_ are read-only and cannot be assigned to.

[adamonduty]

@agentzh Oh, are you saying http_cache_purge is a built-in variable? I was following this post, and it sets $http_cache_purge as a normal variable. I don't think it is built-in, since when I try ngx.var.http_blah = "test", I get the same error.

[agentzh]

@adamonduty Maybe what you need is just the ngx.req.set_header API function? See https://github.com/openresty/lua-nginx-module#ngxreqset_header

[agentzh]

@adamonduty All nginx variables with the http_ prefix are considered builtin. See http://nginx.org/en/docs/http/ngx_http_core_module.html#var_http_ Setting to such variables is always illegal.

[agentzh]

@adamonduty The post you referenced does not write to $http_cache_purge either. It just reads its value in the proxy_cache_bypass config directive, which is certainly valid.

[adamonduty]

@agentzh Ah yes, of course, I didn't know the entire http_ prefix was built-in. At any rate, I can take a stab at a pull request to the docs, or we can just close the issue. Thanks for the feedback!

[agentzh]

@adamonduty Okay, cool, thanks :)

[agentzh]

Consider it resolved.