Dispatch returns a result envelope

dpm_execute fills a dpm_result on every call: status and error_code
from the module's return value, and a payload the module hands back
through dpm_set_result. The payload's layout belongs to the module and
is documented per command; its release function is carried in the
envelope and invoked by dpm_result_release. The context keeps a stack
of the envelopes in progress, so a module calling a peer receives the
peer's payload in its own envelope and its caller sees only what the
module sets itself.

The good fixture returns a payload and counts its releases, and the
modules test covers the envelope fields, the payload round trip, single
release, discard on a NULL envelope, and a payload set outside any
dispatch. The prose documents describe the envelope and the payload
header a module ships.

tests/fixtures/src/good.cpp

@@ -24,6 +24,23 @@
  */
 #include <cstring>
 
+/* The one library call a module makes to return data, declared here
+   the way the rest of the contract is. */
+extern "C" void dpm_set_result(void* ctx, void* data, void (*release)(void*));
+
+namespace {
+    /* Counts how many times the payload below has been released, so a
+       test can observe the release path. */
+    int releases = 0;
+
+    void release_payload(void* data) {
+        (void)data;
+        releases++;
+    }
+
+    const char* payload = "pong";
+} // namespace
+
 extern "C" const char* dpm_module_version(void) {
     return "1.2.3";
 }
@@ -38,16 +55,23 @@ extern "C" const char* dpm_module_aliases(void) {
     return "goodie, gd";
 }
 
-/* Answers "ping" with 42 so a dispatch round trip is observable, and
-   0 for anything else. */
+/* Answers "ping" with 42 so a dispatch round trip is observable,
+   "payload" with the static payload above, "releases" with the number
+   of times that payload has been released, and 0 for anything else. */
 extern "C" int dpm_module_execute(void* ctx, const char* command,
                                   int argc, char** argv) {
-    (void)ctx;
     (void)argc;
     (void)argv;
 
     if (command && std::strcmp(command, "ping") == 0) {
         return 42;
     }
+    if (command && std::strcmp(command, "payload") == 0) {
+        dpm_set_result(ctx, const_cast<char*>(payload), release_payload);
+        return 0;
+    }
+    if (command && std::strcmp(command, "releases") == 0) {
+        return releases;
+    }
     return 0;
 }