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.

include/internal/context.hpp

@@ -27,6 +27,7 @@
 #include <map>
 #include <memory>
 #include <string>
+#include <vector>
 
 #include "internal/metadata.hpp"
 #include "internal/modules.hpp"
@@ -73,6 +74,9 @@ struct dpm_ctx {
 
     /** Reason for the most recent failure. */
     std::string last_error;
+
+    /** Envelopes of the dispatches in progress, innermost last. */
+    std::vector<dpm_result*> result_frames;
 };
 
 /**