Check for MAXLOGNAME and UT_NAMESIZE in configure and use them (in that order)

if available, falling back to a hard-coded maximum login name length of 255 if
neither is available.

Author: duaneg

Modules/posixmodule.c

@@ -9539,15 +9539,21 @@ 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];
+# if defined (HAVE_MAXLOGNAME)
+    char name[MAXLOGNAME + 1];
+# elif defined (HAVE_UT_NAMESIZE)
+    char name[UT_NAMESIZE + 1];
+# else
+    char name[256];
+# endif
     int err = getlogin_r(name, sizeof(name));
     if (err) {
         int old_errno = errno;
         errno = -err;
         posix_error();
         errno = old_errno;
-    } else {
+    }
+    else {
         result = PyUnicode_DecodeFSDefault(name);
     }
 #else

configure

@@ -5427,6 +5427,18 @@ PY_CHECK_FUNC([setgroups], [
 #endif
 ])
 
+AC_CHECK_DECL([MAXLOGNAME],
+              [AC_DEFINE([HAVE_MAXLOGNAME], [1],
+                         [Define if you have the 'MAXLOGNAME' constant.])],
+              [],
+              [@%:@include <sys/params.h>])
+
+AC_CHECK_DECLS([UT_NAMESIZE],
+              [AC_DEFINE([HAVE_UT_NAMESIZE], [1],
+                         [Define if you have the 'HAVE_UT_NAMESIZE' constant.])],
+              [],
+              [@%:@include <utmp.h>])
+
 # check for openpty, login_tty, and forkpty
 
 AC_CHECK_FUNCS([openpty], [],

configure.ac

@@ -1633,6 +1633,12 @@
 /* Define to 1 if the system has the type '__uint128_t'. */
 #undef HAVE___UINT128_T
 
+/* Define to 1 if you have the MAXLOGNAME constant. */
+#undef HAVE_MAXLOGNAME
+
+/* Define to 1 if you have the UT_NAMESIZE constant. */
+#undef HAVE_UT_NAMESIZE
+
 /* Define to 1 if 'major', 'minor', and 'makedev' are declared in <mkdev.h>.
    */
 #undef MAJOR_IN_MKDEV