The existing implementation of PyOS_mystricmp increments both pointers as long as the first string hasn't reached the end yet. If the second string ends first, then we increment past the null byte. If there is a difference in the middle of the two strings, then the result actually compares the following pair of letters.

e.g.
PyOS_mystricmp("a", "\0") => 0 (should be positive)
PyOS_mystricmp("foo", "fro") => 0 (should be negative)

Similarly, PyOS_mystrnicmp increments the pointers in a condition before breaking out of the loop. It's possible to increment the first pointer without incrementing the second, and the result is the character past the null byte.

e.g.
PyOS_mystrnicmp("\0a", "\0b", 2) => 97 (should be negative)

Good catch! Do you mind to provide a PR for this William?

New changeset 97eaf2b5e5c826b9abe59896a363853bef55c5d9 by wmeehan in branch 'master':
bpo-41524: fix pointer bug in PyOS_mystr{n}icmp (GH-21845)
https://github.com/python/cpython/commit/97eaf2b5e5c826b9abe59896a363853bef55c5d9

New changeset 901c2eae6e27ee7793e5a3c638664e01a3bf8de8 by Miss Islington (bot) in branch '3.9':
bpo-41524: fix pointer bug in PyOS_mystr{n}icmp (GH-21845) (GH-21978)
https://github.com/python/cpython/commit/901c2eae6e27ee7793e5a3c638664e01a3bf8de8

New changeset 85ca9c049c5a490d143d28933bbb02ab80394ed8 by Miss Islington (bot) in branch '3.8':
bpo-41524: fix pointer bug in PyOS_mystr{n}icmp (GH-21845) (GH-22016)
https://github.com/python/cpython/commit/85ca9c049c5a490d143d28933bbb02ab80394ed8

Thanks William