Fix HPC-GAP crash when calling IntFFE repeatedly
#5500
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The kernel function `INT_FF` had a bug under HPC-GAP that could lead to
it allocating more and more memory (that it did not actually need),
eventually causing GAP to come to a halt or the system running out of
memory.
This kernel function is called by e.g. `IntFFE` and other user facing
code. As a result, this code did not terminate (with the fix it runs
in a few milliseconds):
for p in Primes do IntFFE(Z(p)); od;
For the experts: the code was directly calling `GrowPlist`, which
does not perform a check to see if the list is actually already big
enough. So each time `INT_FF` was called ended up increasing an
internal list by a certain small percentage. But doing this often
enough still leads to exponential growth of the size of that list...
fingolfin
added
kind: bug
ThomasBreuer
approved these changes
Sep 1, 2023
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The kernel function
INT_FFhad a bug under HPC-GAP that could lead to it allocating more and more memory (that it did not actually need), eventually causing GAP to come to a halt or the system running out of memory.This kernel function is called by e.g.
IntFFEand other user facing code. As a result, this code did not terminate (with the fix it runs in a few milliseconds):For the experts: the code was directly calling
GrowPlist, which does not perform a check to see if the list is actually already big enough. So each timeINT_FFwas called ended up increasing an internal list by a certain small percentage. But doing this often enough still leads to exponential growth of the size of that list...I did not include a test case (other than the one in the commit message) because the test really only can test whether GAP hangs and/or gets slow or not, which is not terribly useful by itself. Besides, PR #5495 adds a test which also triggers this problem.