libmdsclient: add function to find unused message id

Signed-off-by: Mattias Andrée <maandree@operamail.com>

1 files changed, 46 insertions, 0 deletions

diff --git a/src/libmdsclient/proto-util.c b/src/libmdsclient/proto-util.c
index f104d05..85b755f 100644
--- a/src/libmdsclient/proto-util.c
+++ b/src/libmdsclient/proto-util.c
@@ -743,3 +743,49 @@ int libmds_compose_v(char** restrict buffer, size_t* restrict buffer_size, size_
   return 0;
 }
 
+
+/**
+ * Increase the message ID counter
+ * 
+ * @param   message_id  Pointer to the current message ID, will be update with
+ *                      the next free message ID. Must not be `NULL`.
+ * @param   test        Function that tests whether an message ID is free,
+ *                      it takes the message ID to test as its first argument,
+ *                      and `data` as its second argument, and returns zero if
+ *                      it is in used, a positive integer if it is free, and a
+ *                      negative integer if an error occurred. `NULL` if there
+ *                      should be no testing.
+ * @param   data        Argument to pass to `test` so that it can deal with
+ *                      threading or any other issues. Unused if `test` is `NULL`.
+ * @return              Zero on success, -1 on error, `errno` will have been set
+ *                      accordingly on error.
+ * 
+ * @throws  EAGAIN      If there are no free message ID to be used.
+ *                      It is advisable to make `test` throw this immediately if
+ *                      there are no free message ID:s.
+ * @throws              Any error that `test` may throw.
+ */
+int libmds_next_message_id(uint32_t* restrict message_id,
+			   int (*test)(uint32_t message_id, void* data), void* data)
+{
+  uint32_t id = *message_id;
+  int r;
+  
+  id = id == UINT32_MAX ? 0 : (id + 1);
+  if (test != NULL)
+    for (;;)
+      {
+	r = test(id, data);
+	if (r < 0)
+	  return -1;
+	if (r)
+	  break;
+	id = id == UINT32_MAX ? 0 : (id + 1);
+	if (id == *message_id)
+	  return errno = EAGAIN, -1;
+      }
+  
+  *message_id = id;
+  return 0;
+}
+