Add sig_on/sig_off to COINBackend

[mkoeppe]

The COIN backend needs sig_on, sig_off throughout so that bounds errors do not crash Sage.

sage: p = get_solver(solver = "Coin")
sage: p.is_variable_continuous(1234567)
True
sage: p.add_linear_constraint([(123, 456.7)], None, 0)
... 
Unhandled SIGSEGV: A segmentation fault occurred.

Another one, from #20326:

sage: coeffs = ([0, vector([1, 2])], [1, vector([2, 3])])
sage: upper = vector([5, 5])
sage: lower = vector([0, 0])
sage: from sage.numerical.backends.generic_backend import get_solver
sage: p = get_solver(solver = "Coin")       # optional - cbc
sage: p.add_linear_constraint_vector(2, coeffs, lower, upper)

CC: @dimpase @videlec @jdemeyer

Component: numerical

Issue created by migration from https://trac.sagemath.org/ticket/20360

[mkoeppe]

Description changed:

--- 
+++ 
@@ -9,3 +9,14 @@
 Unhandled SIGSEGV: A segmentation fault occurred.
 ```
 
+Another one, from #20326:
+
+```
+sage: coeffs = ([0, vector([1, 2])], [1, vector([2, 3])])
+sage: upper = vector([5, 5])
+sage: lower = vector([0, 0])
+sage: from sage.numerical.backends.generic_backend import get_solver
+sage: p = get_solver(solver = "Coin")       # optional - cbc
+sage: p.add_linear_constraint_vector(2, coeffs, lower, upper)
+```
+

[jdemeyer]

comment:2

I don't think this is a matter of adding sig_on/sig_off. Letting a program segfault and then catching the segfault is not a proper way of handling errors.

You should just check invalid input before calling COIN.

That being said, it might still be a good idea to add the sig_on/sig_off but not as a substitute for input checking.

[dimpase]

comment:3

Replying to @jdemeyer:

I don't think this is a matter of adding sig_on/sig_off. Letting a program segfault and then catching the segfault is not a proper way of handling errors.

You should just check invalid input before calling COIN.

IMHO it's rather the backend's task (and that's more pythonic, if you ask me). They should check for such things, and perhaps
they will fix it at some point.

That being said, it might still be a good idea to add the sig_on/sig_off but not as a substitute for input checking.