Fix get_matched_publication_data on builtin topic

This crashed because the built-in topics are published by a "local
orphan writer" that is not contained in a participant, causing
dds_get_matched_publication_data to crash on dereferencing a null
pointer.

Signed-off-by: Erik Boasson <eb@ilities.com>

src/core/ddsc/src/dds_matched.c

@@ -76,22 +76,28 @@ static dds_builtintopic_endpoint_t *make_builtintopic_endpoint (const ddsi_guid_
   return ep;
 }
 
+static const struct ddsi_entity_common null_entity_common;
+
 dds_builtintopic_endpoint_t *dds_get_matched_subscription_data (dds_entity_t writer, dds_instance_handle_t ih)
 {
   dds_writer *wr;
   if (dds_writer_lock (writer, &wr))
     return NULL;
 
   dds_builtintopic_endpoint_t *ret = NULL;
-  struct ddsi_entity_common *rdc;
-  struct dds_qos *rdqos;
-  struct ddsi_entity_common *ppc;
+  const struct ddsi_entity_common *rdc;
+  const struct dds_qos *rdqos;
+  const struct ddsi_entity_common *ppc;
 
   // thread must be "awake" while pointers to DDSI entities are being used
   struct ddsi_domaingv * const gv = &wr->m_entity.m_domain->gv;
   ddsi_thread_state_awake (ddsi_lookup_thread_state (), gv);
   if (ddsi_writer_find_matched_reader (wr->m_wr, ih, &rdc, &rdqos, &ppc))
+  {
+    if (ppc == NULL)
+      ppc = &null_entity_common;
     ret = make_builtintopic_endpoint (&rdc->guid, &ppc->guid, ppc->iid, rdqos);
+  }
   ddsi_thread_state_asleep (ddsi_lookup_thread_state ());
 
   dds_writer_unlock (wr);
@@ -105,15 +111,19 @@ dds_builtintopic_endpoint_t *dds_get_matched_publication_data (dds_entity_t read
     return NULL;
 
   dds_builtintopic_endpoint_t *ret = NULL;
-  struct ddsi_entity_common *wrc;
-  struct dds_qos *wrqos;
-  struct ddsi_entity_common *ppc;
+  const struct ddsi_entity_common *wrc;
+  const struct dds_qos *wrqos;
+  const struct ddsi_entity_common *ppc;
 
   // thread must be "awake" while pointers to DDSI entities are being used
   struct ddsi_domaingv * const gv = &rd->m_entity.m_domain->gv;
   ddsi_thread_state_awake (ddsi_lookup_thread_state (), gv);
   if (ddsi_reader_find_matched_writer (rd->m_rd, ih, &wrc, &wrqos, &ppc))
+  {
+    if (ppc == NULL)
+      ppc = &null_entity_common;
     ret = make_builtintopic_endpoint (&wrc->guid, &ppc->guid, ppc->iid, wrqos);
+  }
   ddsi_thread_state_asleep (ddsi_lookup_thread_state ());
 
   dds_reader_unlock (rd);

src/core/ddsi/include/dds/ddsi/ddsi_endpoint_match.h

@@ -32,11 +32,33 @@ dds_return_t ddsi_writer_get_matched_subscriptions (struct ddsi_writer *wr, uint
 /** @component endpoint_matching */
 dds_return_t ddsi_reader_get_matched_publications (struct ddsi_reader *rd, uint64_t *wrs, size_t nwrs);
 
-/** @component endpoint_matching */
-bool ddsi_writer_find_matched_reader (struct ddsi_writer *wr, uint64_t ih, struct ddsi_entity_common **rdc, struct dds_qos **rdqos, struct ddsi_entity_common **ppc);
+/** @brief Lookup the instance handle of a matching reader and return it, it's qos and participant
+ * @component endpoint_matching
+ *
+ * @note Thread state must be "awake", returned pointers are aliases; lifetime ends on leaving the "awake" state; contents may not be modified.
+ *
+ * @param[in] wr writer for which to lookup ih in the setting of matching readers
+ * @param[in] ih instance handle of a reader
+ * @param[out] rdc pointer to the common entity data of the reader
+ * @param[out] rdqos pointer to the QoS of the reader
+ * @param[out] ppc participant, may be null for an orphan reader
+ * @return true iff ih corresponds to a matched reader, output parameters are undefined if not
+ */
+bool ddsi_writer_find_matched_reader (struct ddsi_writer *wr, uint64_t ih, const struct ddsi_entity_common **rdc, const struct dds_qos **rdqos, const struct ddsi_entity_common **ppc);
 
-/** @component endpoint_matching */
-bool ddsi_reader_find_matched_writer (struct ddsi_reader *rd, uint64_t ih, struct ddsi_entity_common **wrc, struct dds_qos **wrqos, struct ddsi_entity_common **ppc);
+/** @brief Lookup the instance handle of a matching writer and return it, it's qos and participant
+ * @component endpoint_matching
+ *
+ * @note Thread state must be "awake", returned pointers are aliases; lifetime ends on leaving the "awake" state; contents may not be modified.
+ *
+ * @param[in] rd reader for which to lookup ih in the setting of matching writers
+ * @param[in] ih instance handle of a writer
+ * @param[out] wrc pointer to the common entity data of the writer
+ * @param[out] wrqos pointer to the QoS of the writer
+ * @param[out] ppc participant, may be null for an orphan writer
+ * @return true iff ih corresponds to a matched writer, output parameters are undefined if not
+ */
+bool ddsi_reader_find_matched_writer (struct ddsi_reader *rd, uint64_t ih, const struct ddsi_entity_common **wrc, const struct dds_qos **wrqos, const struct ddsi_entity_common **ppc);
 
 #if defined (__cplusplus)
 }

src/core/ddsi/src/ddsi_endpoint_match.c

@@ -1864,7 +1864,7 @@ dds_return_t ddsi_reader_get_matched_publications (struct ddsi_reader *rd, uint6
   return (dds_return_t) nwrs_act;
 }
 
-bool ddsi_writer_find_matched_reader (struct ddsi_writer *wr, uint64_t ih, struct ddsi_entity_common **rdc, struct dds_qos **rdqos, struct ddsi_entity_common **ppc)
+bool ddsi_writer_find_matched_reader (struct ddsi_writer *wr, uint64_t ih, const struct ddsi_entity_common **rdc, const struct dds_qos **rdqos, const struct ddsi_entity_common **ppc)
 {
   /* FIXME: this ought not be so inefficient */
   struct ddsi_domaingv *gv = wr->e.gv;
@@ -1895,14 +1895,15 @@ bool ddsi_writer_find_matched_reader (struct ddsi_writer *wr, uint64_t ih, struc
       found = true;
       *rdc = &rd->e;
       *rdqos = rd->xqos;
-      *ppc = &rd->c.pp->e;
+      // The day orphan readers are introduced in addition to orphan writers, rd->c.pp may be a null pointer
+      *ppc = rd->c.pp ? &rd->c.pp->e : NULL;
     }
   }
   ddsrt_mutex_unlock (&wr->e.lock);
   return found;
 }
 
-bool ddsi_reader_find_matched_writer (struct ddsi_reader *rd, uint64_t ih, struct ddsi_entity_common **wrc, struct dds_qos **wrqos, struct ddsi_entity_common **ppc)
+bool ddsi_reader_find_matched_writer (struct ddsi_reader *rd, uint64_t ih, const struct ddsi_entity_common **wrc, const struct dds_qos **wrqos, const struct ddsi_entity_common **ppc)
 {
   /* FIXME: this ought not be so inefficient */
   struct ddsi_domaingv *gv = rd->e.gv;
@@ -1933,7 +1934,8 @@ bool ddsi_reader_find_matched_writer (struct ddsi_reader *rd, uint64_t ih, struc
       found = true;
       *wrc = &wr->e;
       *wrqos = wr->xqos;
-      *ppc = &wr->c.pp->e;
+      // Orphan writers have no participant
+      *ppc = wr->c.pp ? &wr->c.pp->e : NULL;
     }
   }
   ddsrt_mutex_unlock (&rd->e.lock);