fix(runloop): upstream ssl failure when plugins use response handler

[catbro666]

Summary

If a plugin has response() handler, in Kong.response it will emits a subrequest by calling ngx.location.capture("/kong_buffered_http", options). ngx.location.capture will create a new nginx request, so the overwritten ssl info (client key & cert etc.) get lost in the new nginx request.

To fix this, those ssl info need to be re-set in the new request context. We choose to do this in the early rewrite phase of the new request before Kong.balancer() getting executed.

The normal flow is as below:

Kong.access.before            Kong.access.after             Kong.balancer(first try)    Kong.balancer(retry)
       v                             v                                     |                 v
balancer_prepare              balancer.execute(try_count=0)                |          balancer.execute(try_count~=0)
       v                             v                                     |                 |
set service ssl config        set upstream ssl config                      v                 v
                 \                /                                  init upstream ssl connection
                  \              /                                              |
                   v            v                                               v
                   ---------------                   set onto              ------------
                   | downstream r|        --------------------------->     |upstream c|
                   ---------------                                         ------------

For the buffered_proxying case, ngx.location.capture will create a new request, so the flow diagram become:

Kong.access.before            Kong.access.after             Kong.balancer(first try)    Kong.balancer(retry)
       v                             v                                     |                 v
balancer_prepare              balancer.execute(try_count=0)                |          balancer.execute(try_count~=0)
       v                             v                                     |                 |
set service ssl config        set upstream ssl config                      v                 v
                 \                /                                  init upstream ssl connection
                  \              /                                              |
                   v            v                                               v
                   ---------------                   set onto              ------------
                   | downstream r|        --------------------------->     |upstream c|
                   ---------------                                         ------------
ngx.location.capture     |
.........................+.............................................................................................
                         | ctx
                         | vars
                         v
                   ---------------                   set onto              ------------
                   |    sub r    |        --------------------------->     |upstream c|
                   ---------------                                         ------------
                         ^                                                      ^
                         |                                                      |
            -----------------------------                               ------------------
            |       Kong.rewrite        |                               |  Kong.balancer |
            |  set service ssl config   |                               |  same as above |
            |  set upstream ssl config  |                               ------------------
            -----------------------------

FTI-5347

[oowl]

LGTM