libflux/request: Rearch flux_request_decode

Rearchitect flux_request_decode.  User can now pass in a NULL
or non-NULL json_str pointer.  The function will succeed regardless
if a payload is available or not.  The function will no longer return
EPROTO under a payload & pointer mismatch.  User is responsible to
check for NULL or non-NULL per their expectations.

Fixes flux-framework#495

doc/man3/flux_request_decode.adoc

@@ -32,10 +32,9 @@ DESCRIPTION
 _topic_, if non-NULL, will be set the message's topic string. The storage
 for this string belongs to _msg_ and should not be freed.
 
-_json_str_, if non-NULL, will be set to the message's JSON payload. The
-storage for this string belongs to _msg_ and should not be freed.
-If non-NULL, decoding fails if the message doesn't have a JSON payload.
-If NULL, decoding fails if the message does have a JSON payload.
+_json_str_, if non-NULL, will be set to the message's JSON payload.
+If no payload exists, _json_str_ is set to NULL.  The storage for this
+string belongs to _msg_ and should not be freed.
 
 `flux_request_decodef()` decodes a request message with a JSON payload as
 above, parsing the payload using variable arguments with a format string

src/common/libflux/request.c

@@ -65,10 +65,6 @@ int flux_request_decode (const flux_msg_t *msg, const char **topic,
         goto done;
     if (flux_msg_get_json (msg, &js) < 0)
         goto done;
-    if ((json_str && !js) || (!json_str && js)) {
-        errno = EPROTO;
-        goto done;
-    }
     if (topic)
         *topic = ts;
     if (json_str)

src/common/libflux/request.h

@@ -8,9 +8,8 @@
 
 /* Decode a request message with optional json payload.
  * If topic is non-NULL, assign the request topic string.
- * If json_str is non-NULL, assign the payload.  This argument indicates whether
- * payload is expected and it is an EPROTO error if expectations are not met.
- * Returns 0 on success, or -1 on failure with errno set.
+ * If json_str is non-NULL, assign the payload or set to NULL if none
+ * exists.  Returns 0 on success, or -1 on failure with errno set.
  */
 int flux_request_decode (const flux_msg_t *msg, const char **topic,
                          const char **json_str);

src/common/libflux/test/request.c

@@ -35,8 +35,8 @@ int main (int argc, char *argv[])
     ok (flux_request_decode (msg, NULL, NULL) == 0,
         "flux_request_decode topic is optional");
     errno = 0;
-    ok (flux_request_decode (msg, NULL, &s) < 0 && errno == EPROTO,
-        "flux_request_decode returns EPROTO when expected payload is missing");
+    ok (flux_request_decode (msg, NULL, &s) == 0 && s == NULL,
+        "flux_request_decode returns s = NULL when expected payload is missing");
     flux_msg_destroy(msg);
 
     /* with JSON payload */
@@ -54,8 +54,8 @@ int main (int argc, char *argv[])
         "flux_request_decodef returns encoded payload");
 
     errno = 0;
-    ok (flux_request_decode (msg, NULL, NULL) < 0 && errno == EPROTO,
-        "flux_request_decode returns EPROTO when payload is unexpected");
+    ok (flux_request_decode (msg, NULL, NULL) == 0,
+        "flux_request_decode works with payload but don't want the payload");
     flux_msg_destroy(msg);
 
     /* without payload (raw) */