1. you first determine the size of the xattr list
2. then you alloc a buffer of that size
3. then you actually fetch the list into the buffer

if between 1 and 3 the xattr list changes in the filesystem, the call fails.

at other places in the code I've seen a while loop for such cases.

BTW, I was looking at your code because we just had such a race condition in our code, discovered in the wild, in practical use. (borgbackup has xattr code for linux, freebsd, osx).