fix(protocol): release Data charge to its owning session

Data charged against a ProtocolSession kept only the charge amount, so
data_destroy released it from whatever session was thread-locally bound
at destroy time. Destroying a received Data on another thread, after the
session was unbound, or while a different session was bound leaked the
originating session's budget and underflowed the other's.

Add Data.owner, set it whenever protocol_receive_data_limited charges a
session, and have data_destroy release against that owner directly via
the newly-exported protocol_release_memory_for_session. Uncharged Data
(owner NULL) keeps the previous bound-session fallback.

Add a unit test proving a Data acquired on session A is released to A
even when unrelated session B is bound at destroy time.

src/shared/data.c

@@ -23,6 +23,7 @@ Data* data_create_reserve(size_t size) {
   d->data = NULL;
   d->size = size;
   d->protocol_charge = 0;
+  d->owner = NULL;
   return d;
 }
 
@@ -36,14 +37,19 @@ Data* data_create(void* data, size_t data_size) {
   new_data->data = data;
   new_data->size = data_size;
   new_data->protocol_charge = 0;
+  new_data->owner = NULL;
   return new_data;
 }
 
 void data_destroy(Data* data) {
   if (data == NULL)
     return;
-  if (data->protocol_charge != 0)
-    protocol_release_memory(data->protocol_charge);
+  if (data->protocol_charge != 0) {
+    if (data->owner != NULL)
+      protocol_release_memory_for_session(data->owner, data->protocol_charge);
+    else
+      protocol_release_memory(data->protocol_charge);
+  }
   free(data->data);
   free(data);
 }

src/shared/data.h

@@ -3,11 +3,19 @@
 
 #include <stdlib.h>
 
+/* Forward declaration for the connection budget a received Data is charged
+ * against; defined in protocol.h (which includes this header). */
+typedef struct ProtocolSession ProtocolSession;
+
 typedef struct {
   void* data;
   size_t size;
   /* Non-zero only for a buffer charged to the protocol connection budget. */
   size_t protocol_charge;
+  /* Session whose budget `protocol_charge` was reserved from.  The charge must
+   * always be returned to this session, regardless of which session (if any) is
+   * bound to the destroying thread.  NULL for uncharged Data. */
+  ProtocolSession* owner;
 } Data;
 
 Data* data_create_empty(size_t data_size);
@@ -15,5 +23,8 @@ Data* data_create_reserve(size_t size);
 Data* data_create(void* data, size_t data_size);
 void data_destroy(Data* data);
 void protocol_release_memory(size_t charge);
+/* Release `charge` against `session` directly instead of the thread-local bound
+ * session.  Used by data_destroy to honor Data.owner. */
+void protocol_release_memory_for_session(ProtocolSession* session, size_t charge);
 
 #endif

src/shared/protocol.c

@@ -40,7 +40,7 @@ static bool protocol_reserve_memory(ProtocolSession* session, size_t charge) {
   }
 }
 
-static void protocol_release_memory_for_session(ProtocolSession* session, size_t charge) {
+void protocol_release_memory_for_session(ProtocolSession* session, size_t charge) {
   unsigned long long allocated = atomic_load(&session->total_allocated_bytes);
   while (true) {
     unsigned long long remaining = (unsigned long long)charge >= allocated ? 0 : allocated - charge;
@@ -573,6 +573,7 @@ Data* protocol_receive_data_limited(ProtocolSession* session, unsigned long long
     return NULL;
   }
   result->protocol_charge = allocation_size;
+  result->owner = session;
   return result;
 }