Trac #32095: DiFUB algorithm fails on some random graph

{{{
sage -t --long --random-seed=4 src/sage/graphs/digraph.py
**********************************************************************
File "src/sage/graphs/digraph.py", line 2475, in
sage.graphs.digraph.DiGraph.?
Failed example:
    d1 == d2
Expected:
    True
Got:
    False
**********************************************************************
1 item had failures:
   1 of 166 in sage.graphs.digraph.DiGraph.?
    [530 tests, 1 failure, 1.70 s]
}}}

{{{
sage: set_random_seed(4)
sage: G = graphs.RandomGNP(40, 0.4).to_directed()
sage: G.diameter()
3
sage: d1 = G.diameter(algorithm='DiFUB', by_weight=True)
sage: d1
2.0
}}}

Yet, `DiFUB` claims to be exact.

URL: https://trac.sagemath.org/32095
Reported by: gh-kliem
Ticket author(s): David Coudert
Reviewer(s): Jonathan Kliem

Author: Release Manager

src/sage/graphs/base/boost_graph.pyx

@@ -2381,8 +2381,8 @@ cdef double diameter_DiFUB(BoostVecWeightedDiGraphU g_boost,
     # distances respectively.
     # Now order_1 and order_2 will contain order of vertices in which
     # further distance computations will be done.
-    sorted(order_1, reverse=True)
-    sorted(order_2, reverse=True)
+    order_1 = sorted(order_1, reverse=True)
+    order_2 = sorted(order_2, reverse=True)
 
     LB = max(LB, LB_1, LB_2)
     if LB == sys.float_info.max:

src/sage/graphs/digraph.py

@@ -2490,6 +2490,17 @@ def diameter(self, by_weight=False, algorithm=None, weight_function=None,
             Traceback (most recent call last):
             ...
             ValueError: diameter is not defined for the empty DiGraph
+
+        :trac:`32095` is fixed::
+
+            sage: g6 = 'guQOUOQCW[IaDBCVP_IE\\RfxV@WMSaeHgheEIA@tfOJkB~@EpGLCrs'
+            sage: g6 += 'aPIpwgQI_`Abs_x?VWxNJAo@w\\hffCDAW]bYGMIZGC_PYOrIw[Gp['
+            sage: g6 += '@FTgc_O}E?fXAnGCB{gSaUcD'
+            sage: G = Graph(g6).to_directed()
+            sage: G.diameter(algorithm='DiFUB', by_weight=False)
+            3
+            sage: G.diameter(algorithm='DiFUB', by_weight=True)
+            3.0
         """
         if not self.order():
             raise ValueError("diameter is not defined for the empty DiGraph")