#15159: Segfault after deepcopy of MixedIntegerLinearProgram
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Reporter: jvkersch | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-6.4
Component: linear programming | Resolution:
Keywords: | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: #20414 | Stopgaps:
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Changes (by mkoeppe):
* dependencies: => #20414
Comment:
These segfaults happen because `deepcopy` does not copy the backend at
all:
{{{
sage: p = MixedIntegerLinearProgram()
sage: w = p.new_variable()
sage: q = deepcopy(p)
sage: p.get_backend()
<sage.numerical.backends.coin_backend.CoinBackend object at 0x19e7be670>
sage: q.get_backend()
}}}
#20414 supplies the necessary `__deepcopy__` methods.
However, the same example with `copy` instead of `deepcopy` also does not
work, but for a different reason.
{{{
sage: p = MixedIntegerLinearProgram(solver='glpk')
sage: w = p.new_variable()
sage: q = copy(p)
sage: q.add_constraint(w[0] >= 0)
...
GLPKError: glp_set_mat_row: i = 1; len = 1; invalid row length
Error detected in file glpapi01.c at line 760
}}}
What's happening in this example is that the variable w[0] is created in
the wrong MIP -- in `p`; because `w` belongs to `p` and knows nothing
about `q`. Then using that variable in `q` leads to an out-of-bounds error
(with GLPK) or crash (with COIN) as reported in #19523, #19525, #20360.
If one creates the variable `w[0]` before copying (simply by "mentioning"
it), then this code goes through:
{{{
sage: p = MixedIntegerLinearProgram()
sage: w = p.new_variable()
sage: w[0]
x_0
sage: sage: q = deepcopy(p)
sage: sage: q.add_constraint(w[0] >= 0)
}}}
We should conclude that `copy` (or `deepcopy`) of a
`MixedIntegerLinearProgram` is not very useful because we can no longer
(safely) access its variables. See #19523.
--
Ticket URL: <http://trac.sagemath.org/ticket/15159#comment:6>
Sage <http://www.sagemath.org>
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