Gate W3C trace support to request phases with ngx.var access

[mtrop-godaddy]

The http library reads ngx.var.http_traceparent in send_request to propagate W3C trace context. ngx.var is only readable in request-bearing phases, so this crashes code running in init/init_worker.

Gate the read on ngx.get_phase() being in a request-bearing phase via a whitelist, so request-less phases (init, init_worker, timer, etc.) are excluded by default. Behavior is unchanged for real requests.

[sriemer]

Looks very good. Thank you very much! 😃
Just the phases should be revisited. TIA

[sriemer]

Phases reviewed. Those look mostly good. Thanks.
But what about adding "precontent" as seen in t/089-phase.t#L227?
I'd like to drop balancer and preread, because balancer has no full API support and preread is unlikely to be triggered.

[sriemer]

Reviewed documentation and code:

• https://openresty-reference.readthedocs.io/en/latest/Lua_Nginx_API/#ngxvarvariable
• https://openresty-reference.readthedocs.io/en/latest/Lua_Nginx_API/#ngxget_phase
• grep -R "NGX_HTTP_[A-Z_]\+PHASE" on latest NGINX source
• find . \( -name "*.lua" -o -name "*.t" -o -name "*.c" -o -name "*.h" \) -exec grep --color -RHi "phase" {} \; on latest lua-nginx-module source
• checked https://github.com/openresty/lua-nginx-module#directives for "*by_lua*"

[sriemer]

Phases not listed as supported in https://openresty-reference.readthedocs.io/en/latest/Lua_Nginx_API/#ngxvarvariable:

• server_rewrite
• preread
• balancer
• precontent

Accessible via *by_lua*: server_rewrite, precontent, balancer

[sriemer]

server_rewrite and precontent may do full API calls in a sandbox. balancer has a limited subset. But preread is unlikely to be triggered.
So I would like to go for dropping balancer and preread.
I would like to add precontent as well.
@mtrop-godaddy Is this plan fine for you? TIA