gh-127081: use getlogin_r if available

The getlogin function is not thread-safe: replace with getlogin_r where
available.

Note that this function is untested (unit test is skipped with a note it caused
CI failures as behaviour differs between NIX environments).

Author: duaneg

Misc/NEWS.d/next/Library/2025-04-21-01-03-15.gh-issue-127081.WXRliX.rst

@@ -0,0 +1,2 @@
+Fix libc thread safety issues with :mod:`os` by replacing ``getlogin`` with
+``getlogin_r`` re-entrant version.

Modules/posixmodule.c

@@ -9538,6 +9538,18 @@ os_getlogin_impl(PyObject *module)
     }
     else
         result = PyErr_SetFromWindowsErr(GetLastError());
+#elif defined (HAVE_GETLOGIN_R)
+    /* AFAIK the maximum length should be 32, but this is not checkable */
+    char name[64];
+    int err = getlogin_r(name, sizeof(name));
+    if (err) {
+        int old_errno = errno;
+        errno = -err;
+        posix_error();
+        errno = old_errno;
+    } else {
+        result = PyUnicode_DecodeFSDefault(name);
+    }
 #else
     char *name;
     int old_errno = errno;

configure

@@ -5137,7 +5137,7 @@ AC_CHECK_FUNCS([ \
   faccessat fchmod fchmodat fchown fchownat fdopendir fdwalk fexecve \
   fork fork1 fpathconf fstatat ftime ftruncate futimens futimes futimesat \
   gai_strerror getegid geteuid getgid getgrent getgrgid getgrgid_r \
-  getgrnam_r getgrouplist gethostname getitimer getloadavg getlogin \
+  getgrnam_r getgrouplist gethostname getitimer getloadavg getlogin getlogin_r \
   getpeername getpgid getpid getppid getpriority _getpty \
   getpwent getpwnam_r getpwuid getpwuid_r getresgid getresuid getrusage getsid getspent \
   getspnam getuid getwd grantpt if_nameindex initgroups kill killpg lchown linkat \

configure.ac

@@ -530,6 +530,9 @@
 /* Define to 1 if you have the 'getlogin' function. */
 #undef HAVE_GETLOGIN
 
+/* Define to 1 if you have the 'getlogin_r' function. */
+#undef HAVE_GETLOGIN_R
+
 /* Define to 1 if you have the 'getnameinfo' function. */
 #undef HAVE_GETNAMEINFO