Expose query timeout checking to modules

Summary:
- Add the `mgp_must_abort(const mgp_graph *graph)` C API.
- Add the `ProcCtx.must_abort()` Python API.

The usage is very simple -- the function returns a boolean indicating whether
the procedure should abort.

Reviewers: mferencevic, dsantl

Reviewed By: mferencevic

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D2742

include/mg_procedure.h

@@ -798,6 +798,25 @@ int mgp_proc_add_deprecated_result(struct mgp_proc *proc, const char *name,
                                    const struct mgp_type *type);
 ///@}
 
+/// @name Execution
+///
+/// The following functions are used to control the execution of the procedure.
+///
+/// @{
+
+/// Return non-zero if the currently executing procedure should abort as soon as
+/// possible.
+///
+/// Procedures which perform heavyweight processing run the risk of running too
+/// long and going over the query execution time limit. To prevent this, such
+/// procedures should periodically call this function at critical points in
+/// their code in order to determine whether they should abort or not. Note that
+/// this mechanism is purely cooperative and depends on the procedure doing the
+/// checking and aborting on its own.
+int mgp_must_abort(const struct mgp_graph *graph);
+
+/// @}
+
 #ifdef __cplusplus
 }  // extern "C"
 #endif

include/mgp.py

@@ -531,6 +531,11 @@ class Graph:
         return Vertices(self._graph)
 
 
+class AbortError(Exception):
+    '''Signals that the procedure was asked to abort its execution.'''
+    pass
+
+
 class ProcCtx:
     '''Context of a procedure being executed.
 
@@ -554,6 +559,15 @@ class ProcCtx:
             raise InvalidContextError()
         return self._graph
 
+    def must_abort(self) -> bool:
+        if not self.is_valid():
+            raise InvalidContextError()
+        return self._graph._graph.must_abort()
+
+    def check_must_abort(self):
+        if self.must_abort():
+            raise AbortError
+
 
 # Additional typing support