feat(proxy-wasm) add get_property, supporting Nginx variables

Implements the `get_property` call for the proxy-wasm SDK.

The actual properties themselves are not listed by the proxy-wasm
specification, so they seem to be proxy-specific.

In this initial iteration, the only properties implemented are Nginx
variables, obtained using the Nginx API via `ngx_http_get_variable` and hence
exposed in the `ngx.http.*` property path.

The ABI for property path values (that is, how they should be parsed by a host
implementation) is not specified in the proxy-wasm spec docs either, so I have
followed an implementation compatible with the input produced by the
proxy-wasm-rust-sdk, which expects from the user a path as a vector of strings
(`vec!["ngx", "http", "pid"]`) and produces a byte string to the host by
joining them using `\0` bytes.

With regard to property support, we could mimic the support for many Envoy
attributes which are exposed via `get_property` by mapping them to the
equivalent Nginx data. The list is available here:
https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/advanced/attributes.html?highlight=attributes

However, the future of `get_property` and `set_property` is currently put into
question: they are not included in the vNEXT docs, at least as of their state
in 2020: see https://github.com/proxy-wasm/spec/pull/1/files#r472951567 —
"I've dropped `{get,set}_property` from this iteration, since the current
implementation is not really portable (it's an opaque pass-through to CEL),
and I wanted to get a consensus on this MVP first... but we should definitely
add something that can be standardized in its place as soon as this is
merged.". More info about the current lack of standardization of properties
here: proxy-wasm/proxy-wasm-cpp-host#90

The Envoy properties seem to be the "de facto" properties for proxy-wasm and
which properties are proxy-specific or proxy-independent are ill-defined
(they just map to the Envoy internals), but on our side, we start from a clean
slate by using the `ngx` namespace for entries that map 1-to-1 to Nginx values.

src/common/proxy_wasm/ngx_proxy_wasm_host.c

@@ -854,6 +854,92 @@ ngx_proxy_wasm_hfuncs_get_configuration(ngx_wavm_instance_t *instance,
 }
 
 
+static ngx_uint_t
+hash_str(u_char *src, size_t n)
+{
+    ngx_uint_t  key;
+
+    key = 0;
+
+    while (n--) {
+        key = ngx_hash(key, *src);
+        src++;
+    }
+
+    return key;
+}
+
+
+static ngx_int_t
+ngx_proxy_wasm_hfuncs_get_property(ngx_wavm_instance_t *instance,
+    wasm_val_t args[], wasm_val_t rets[])
+{
+    const char                   *path_data;
+    int32_t                       path_size;
+    ngx_wavm_ptr_t               *ret_data;
+    int32_t                      *ret_size;
+    u_char                       *prop_data = NULL;
+    int32_t                       prop_size = 0;
+    ngx_wavm_ptr_t                p = 0;
+    ngx_proxy_wasm_filter_ctx_t  *fctx;
+#ifdef NGX_WASM_HTTP
+    ngx_http_wasm_req_ctx_t      *rctx;
+    ngx_str_t                     name;
+    ngx_uint_t                    hash;
+    ngx_http_variable_value_t    *vv;
+    static const char            *http_prefix = "ngx\0http\0";
+    static const int              http_prefix_len = 9;
+#endif
+
+    fctx = ngx_proxy_wasm_instance2fctx(instance);
+
+    path_data = ngx_wavm_memory_lift(instance->memory, args[0].of.i32);
+    path_size = args[1].of.i32;
+    ret_data = ngx_wavm_memory_lift(instance->memory, args[2].of.i32);
+    ret_size = ngx_wavm_memory_lift(instance->memory, args[3].of.i32);
+
+#ifdef NGX_WASM_HTTP
+    if (path_size > http_prefix_len &&
+        memcmp(path_data, http_prefix, http_prefix_len) == 0)
+    {
+        name.data = (u_char *)(path_data + http_prefix_len);
+        name.len = path_size - http_prefix_len;
+
+        hash = hash_str(name.data, name.len);
+
+        rctx = ngx_http_proxy_wasm_get_rctx(instance);
+
+        vv = ngx_http_get_variable(rctx->r, &name, hash);
+
+        if (vv && !vv->not_found) {
+            prop_data = vv->data;
+            prop_size = vv->len;
+        }
+    }
+#endif
+
+    if (!prop_data) {
+        return ngx_proxy_wasm_result_notfound(rets);
+    }
+
+    p = ngx_proxy_wasm_alloc(fctx, prop_size);
+    if (p == 0) {
+        return ngx_proxy_wasm_result_err(rets);
+    }
+
+    if (!ngx_wavm_memory_memcpy(instance->memory, p,
+                                prop_data, prop_size))
+    {
+        return ngx_proxy_wasm_result_invalid_mem(rets);
+    }
+
+    *ret_data = p;
+    *ret_size = prop_size;
+
+    return ngx_proxy_wasm_result_ok(rets);
+}
+
+
 static ngx_int_t
 ngx_proxy_wasm_hfuncs_resume_http_request(ngx_wavm_instance_t *instance,
     wasm_val_t args[], wasm_val_t rets[])
@@ -1296,7 +1382,7 @@ static ngx_wavm_host_func_def_t  ngx_proxy_wasm_hfuncs[] = {
      ngx_wavm_arity_i32 },
 
    { ngx_string("proxy_get_property"),
-     &ngx_proxy_wasm_hfuncs_nop,
+     &ngx_proxy_wasm_hfuncs_get_property,
      ngx_wavm_arity_i32x4,
      ngx_wavm_arity_i32 },
 

t/03-proxy_wasm/132-proxy_get_property.t

@@ -0,0 +1,50 @@
+# vim:set ft= ts=4 sts=4 sw=4 et fdm=marker:
+
+use strict;
+use lib '.';
+use t::TestWasm;
+
+skip_valgrind();
+
+plan tests => repeat_each() * (blocks() * 4);
+
+run_tests();
+
+__DATA__
+
+=== TEST 1: proxy_wasm - get_property() gets Nginx $hostname variable
+--- wasm_modules: hostcalls
+--- load_nginx_modules: ngx_http_echo_module
+--- config
+    location /t {
+        proxy_wasm hostcalls 'test=/t/log/property name=ngx.http.hostname';
+        echo ok;
+    }
+--- response_body
+ok
+--- error_log eval
+[
+    qr/\[info\] .*? ngx.http.hostname: [a-z0-9]+/,
+]
+--- no_error_log
+[error]
+
+
+=== TEST 2: proxy_wasm - get_property() gets an Nginx $pid variable
+All get_property calls for Nginx variables return strings, so this
+should print the $pid as ASCII numbers.
+--- wasm_modules: hostcalls
+--- load_nginx_modules: ngx_http_echo_module
+--- config
+    location /t {
+        proxy_wasm hostcalls 'test=/t/log/property name=ngx.http.pid';
+        echo ok;
+    }
+--- response_body
+ok
+--- error_log eval
+[
+    qr/\[info\] .*? ngx.http.pid: [0-9]+/,
+]
+--- no_error_log
+[error]

t/lib/proxy-wasm-tests/hostcalls/src/filter.rs

@@ -124,6 +124,7 @@ impl TestHttpHostcalls {
             "/t/log/response_headers" => test_log_response_headers(self),
             "/t/log/response_body" => test_log_response_body(self),
             "/t/log/current_time" => test_log_current_time(self),
+            "/t/log/property" => test_log_property(self),
 
             /* send_local_response */
             "/t/send_local_response/status/204" => test_send_status(self, 204),

t/lib/proxy-wasm-tests/hostcalls/src/test_cases.rs

@@ -80,6 +80,21 @@ pub(crate) fn test_log_request_path(ctx: &mut TestHttpHostcalls) {
     info!("path: {}", path);
 }
 
+pub(crate) fn test_log_property(ctx: &mut TestHttpHostcalls) {
+    let name = ctx.config.get("name")
+        .expect("expected a name argument");
+
+    match ctx.get_property(name.split(".").collect()) {
+        Some(p) => {
+            match std::str::from_utf8(&p) {
+                Ok(value) => info!("{}: {}", name, value),
+                Err(_) => panic!("failed converting {} to UTF-8", name),
+            }
+        },
+        None => panic!("{} property not found", name),
+    }
+}
+
 pub(crate) fn test_send_status(ctx: &mut TestHttpHostcalls, status: u32) {
     ctx.send_http_response(status, vec![], None)
 }