fix(proxy-wasm) only inject shim headers after on_response_headers

Due to some Nginx internals (some response headers are added after our
module is executed but before the response is sent) we inject "shim
headers" into the `get_http_response_headers()` API. Some heuristics are
used to determine whether or not `Transfer-Encoding`, `Connection`, or
other "last minute" headers will be injected or not.

The issue is that the heuristics used are only valid during response
production and after (e.g. `r->chunked` or
`r->headers_out.content_length_n` may only be relevant during response
production). If a user uses `get_http_response_headers()` in request
phases, the returned headers may include elements that will not actually
be included in the response (e.g. `Transfer-Encoding`).

We now enforce that shim response headers should only be injected during
and after `on_http_response_headers`.

src/common/proxy_wasm/ngx_proxy_wasm_maps.c

@@ -143,6 +143,8 @@ ngx_proxy_wasm_maps_get_all(ngx_wavm_instance_t *instance,
 #ifdef NGX_WASM_HTTP
     ngx_table_elt_t            *shim;
     ngx_array_t                *shims;
+    ngx_proxy_wasm_ctx_t       *pwctx;
+    ngx_proxy_wasm_exec_t      *pwexec;
 #endif
 
     list = ngx_proxy_wasm_maps_get_map(instance, map_type);
@@ -178,10 +180,22 @@ ngx_proxy_wasm_maps_get_all(ngx_wavm_instance_t *instance,
         }
 
 #ifdef NGX_WASM_HTTP
-        if (map_type == NGX_PROXY_WASM_MAP_HTTP_RESPONSE_HEADERS) {
-            /* inject shim response headers
-             * (produced by ngx_http_header_filter)
+        pwexec = ngx_proxy_wasm_instance2pwexec(instance);
+        pwctx = pwexec->parent;
+
+        if (map_type == NGX_PROXY_WASM_MAP_HTTP_RESPONSE_HEADERS
+            && (pwctx->step == NGX_PROXY_WASM_STEP_RESP_HEADERS
+                || pwctx->step == NGX_PROXY_WASM_STEP_RESP_BODY
+                || pwctx->step == NGX_PROXY_WASM_STEP_LOG))
+        {
+            /**
+             * Inject shim response headers during and after
+             * on_http_response_headers step as they are produced by
+             * ngx_http_header_filter.
              */
+            ngx_log_debug0(NGX_LOG_DEBUG_WASM, pwexec->log, 0,
+                           "injecting shim headers in response headers map");
+
             shims = ngx_http_wasm_get_shim_headers(
                         ngx_http_proxy_wasm_get_rctx(instance));
 

t/03-proxy_wasm/hfuncs/104-proxy_get_http_response_headers.t

@@ -46,8 +46,6 @@ response
 --- grep_error_log eval: qr/(testing in|resp) .*/
 --- grep_error_log_out eval
 qr/testing in "RequestHeaders".*
-resp Transfer-Encoding: chunked.*
-resp Connection: close.*
 testing in "ResponseHeaders".*
 resp :status: 200.*
 resp Content-Type: text\/plain.*