refactor(lua-bridge) rewrite for full yielding support

Major refactor of the Lua bridge to support multiple concurrent yielding
Lua threads. The old implementation would break down when scheduling
more than one yielding Lua thread at a time.

The new implementation "tricks" OpenResty by scheduling uthreads via C
and passing these threads to the OpenResty runloop as if they were
created from Lua (via `ngx.thread`). Because all uthreads must resume
their "parent thread" when finished (as per OpenResty's implementation),
we schedule a stub "entry thread" whenever we are trying to use the Lua
bridge. This entry thread itself does nothing and is collected at
request pool cleanup.

List of significant changes for this refactor:

- **Breaking:** the `proxy_wasm.start()` FFI function is **removed**.
  Only `proxy_wasm.attach()` is now necessary, and the filter chain is
  only resumed once the ngx_http_wasm_module `rewrite` or `access`
  phases are entered. Prior, `proxy_wasm.start()` would resume the
  filter chain during the ngx_http_lua_module phase handlers, which was
  incompatible with Lua threads yielding.
- In ngx_wasm_socket_tcp, the `sock->env` member is now a pointer to the
  request's `env` instead of a copy so as to manipulate the `env->state`
  control variable.
- The `wasm_call` directive can now yield, which allows for sanity
  testing of the Lua bridge yielding functionality.
- A new `rctx->resume_handler` pointer holds the resume entry point back
  from yielding facilities into `ngx_http_core_run_phases`. For now,
  only the Lua bridge uses it, but other yielding facilities should be
  refactored to use it so as to factorize our resuming code.

Fix #524

.github/actions/setup-httpbin-server/action.yml

@@ -76,7 +76,8 @@ runs:
                 --no-resolv \
                 --port=${{ inputs.dns_port }} \
                 --server=${{ inputs.upstream_dns_server }} \
-                --address=/httpbin.org/127.0.0.1
+                --address=/httpbin.org/127.0.0.1 \
+                --address=/example.com/127.0.0.1
     - name: Start httpbin proxy + server
       shell: bash
       run: |

.github/workflows/job-unit-tests.yml

@@ -124,7 +124,7 @@ jobs:
           sudo bash -c 'echo "${{ github.workspace }}/coredumps/%e.%p.%t" > /proc/sys/kernel/core_pattern'
       - run: make setup
       - run: make
-      - name: Run make test
+      - name: Run tests
         run: |
           ulimit -c unlimited
           make test

lib/resty/wasmx/proxy_wasm.lua

@@ -53,7 +53,6 @@ if subsystem == "http" then
         int ngx_http_wasm_ffi_plan_attach(ngx_http_request_t *r,
                                           ngx_wasm_plan_t *plan,
                                           unsigned int isolation);
-        int ngx_http_wasm_ffi_start(ngx_http_request_t *r);
         int ngx_http_wasm_ffi_set_property(ngx_http_request_t *r,
                                            ngx_str_t *key,
                                            ngx_str_t *value,
@@ -160,6 +159,16 @@ function _M.load(c_plan)
         return nil, "failed loading plan"
     end
 
+    if get_request() then
+        -- ffi_gc: hold a reference tied to the request lifecycle so users
+        -- don't have to (like our test suite).
+        if not ngx.ctx[_M] then
+            ngx.ctx[_M] = {}
+        end
+
+        ngx.ctx[_M][c_plan] = true
+    end
+
     return true
 end
 
@@ -215,30 +224,6 @@ function _M.attach(c_plan, opts)
 end
 
 
-function _M.start()
-    local phase = ngx.get_phase()
-    if phase ~= "rewrite" and phase ~= "access" then
-        error("start must be called from 'rewrite' or 'access' phase", 2)
-    end
-
-    local r = get_request()
-    if not r then
-        error("no request found", 2)
-    end
-
-    local rc = C.ngx_http_wasm_ffi_start(r)
-    if rc == FFI_ERROR then
-        return nil, "unknown error"
-    end
-
-    if rc == FFI_DECLINED then
-        return nil, "plan not loaded and attached"
-    end
-
-    return true
-end
-
-
 function _M.set_property(key, value)
     if type(key) ~= "string" then
         error("key must be a string", 2)