Change MolienSeries to not hang on certain invalid inputs
#5919
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fingolfin
added
the
release notes: not needed
When a virtual character that is not an actual character was fed to MolienSeries, that could lead to an infinite loop. The reason in that case was that a quotient of two polynomials was computed that did not produce a polynomial, but rather a rational function. Calling `GcdRepresentation(F, f)` on that as a first step then tried to compute a ring containing both arguments, which ultimately caused the infinite loop. We fix this by passing a parent ring to `GcdRepresentation` which then raises an error in the above situation. The error message of course is not perfect, but note that other invalid inputs already can lead to different errors in other places in the code. Essentially the user is at fault here for passing in garbage. But we make this patch anyway because getting any error is certainly more helpful than an infinite loop.
Co-authored-by: Thomas Breuer <[email protected]>
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@ThomasBreuer please approve if you agree for this to be merged :-) |
ThomasBreuer
approved these changes
Jan 28, 2025
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When a virtual character that is not an actual character was fed to MolienSeries, that could lead to an infinite loop. The reason in that case was that a quotient of two polynomials was computed that did not produce a polynomial, but rather a rational function. Calling
GcdRepresentation(F, f)on that as a first step then tried to compute a ring containing both arguments, which ultimately caused the infinite loop.We fix this by passing a parent ring to
GcdRepresentationwhich then raises an error in the above situation. The error message of course is not perfect, but note that other invalid inputs already can lead to different errors in other places in the code. Essentially the user is at fault here for passing in garbage. But we make this patch anyway because getting any error is certainly more helpful than an infinite loop.I am not making any claims that this catches all potential hangs, but it certainly resolves #5284 for me.