sagemathgh-38851: src/sage/graphs/generic_graph.py: work around doctest hang
    
One doctest in this file is "hanging" on ARM64 and RISC-V as GLPK tries
courageously to solve a MIP. A tweak to the solver options allows this
problem to be solved on those two architectures without affecting any
others. This is unlikely to solve the general problem, but it may buy us
some time.
    
URL: sagemath#38851
Reported by: Michael Orlitzky
Reviewer(s): David Coudert

Author: Release Manager

src/sage/graphs/generic_graph.py

@@ -7543,6 +7543,22 @@ def edge_disjoint_spanning_trees(self, k, algorithm=None, root=None, solver=None
                 p.add_constraint(pos[root, c] + BFS[u] <= pos[u, c])
 
         # We now solve this program and extract the solution
+
+        from sage.numerical.backends.glpk_backend import GLPKBackend
+        if isinstance(p.get_backend(), GLPKBackend):
+            # The MIP approach with GLPK is prone to compiler and
+            # optimization-level weirdness on some hardware:
+            #
+            #   * https://github.com/sagemath/sage/issues/34575
+            #   * https://github.com/sagemath/sage/issues/38831
+            #
+            # Disabling the presolver manages to perturb reality just
+            # enough in the one scenario that we doctest explicitly to
+            # "fix" the problem. It's also limited enough in scope
+            # that it probably hasn't badly broken some other use
+            # case.
+            p.solver_parameter("presolve_intopt", False)
+
         try:
             p.solve(log=verbose)
         except MIPSolverException: