sethostname tests: Avoid test failure on Cygwin

[Bruno Haible]

On the GitHub Cygwin machines (Cygwin 3.3.6 and 3.5.3), I see this test
failure:

FAIL: test-sethostname2
=======================

setting a too long hostname succeeded.
FAIL test-sethostname2.exe (exit status: 1)

The cause: Cygwin apparently does not reject a too long hostname argument.
While the LSB specifies it should fail in this case, I don't think it's
worth a workaround in Gnulib, since this function is rarely used.

So, let me just skip this test and notify the Cygwin people.


2024-05-23  Bruno Haible  <br...@clisp.org>

        sethostname tests: Avoid test failure on Cygwin.
        * tests/test-sethostname2.c (main): On Cygwin, skip the "too long
        hostname" test.
        * doc/glibc-functions/sethostname.texi: Mention the Cygwin problem.

diff --git a/doc/glibc-functions/sethostname.texi 
b/doc/glibc-functions/sethostname.texi
index 1aff926cd4..cef0edf268 100644
--- a/doc/glibc-functions/sethostname.texi
+++ b/doc/glibc-functions/sethostname.texi
@@ -40,4 +40,8 @@
 The second parameter is @code{int} instead of @code{size_t}
 on some platforms:
 macOS 12.5, FreeBSD 14.0, MidnightBSD 3.0, Solaris 11 2010-11, Solaris 11 
OpenIndiana, Solaris 11 OmniOS.
+@item
+This function does not reject a too long host name on some platforms:
+@c https://cygwin.com/pipermail/cygwin/2024-May/255986.html
+Cygwin 3.5.3.
 @end itemize
diff --git a/tests/test-sethostname2.c b/tests/test-sethostname2.c
index 33d7da76de..25a3ae1857 100644
--- a/tests/test-sethostname2.c
+++ b/tests/test-sethostname2.c
@@ -45,7 +45,6 @@ main (int argc, _GL_UNUSED char *argv[])
 {
   char origname[HOST_NAME_MAX];
   char newname[HOST_NAME_MAX];
-  char longname[HOST_NAME_MAX + 2];
   int rcs, i;
 
   /* skip the tests if we don't have root privilege.  this does not
@@ -108,23 +107,31 @@ main (int argc, _GL_UNUSED char *argv[])
 #endif
     }
 
-  /* glibc does allow setting a zero length name, so the lower bound
-     needs no test. validate that we are constrained by
-     HOST_NAME_MAX */
-  for (i = 0; i < (HOST_NAME_MAX + 1); i++)
-    longname[i] = 'a';
-
-  longname[i] = '\0';
-
-  rcs = sethostname (longname, (HOST_NAME_MAX + 1));
-
-  if (rcs != -1)
-    {
-      /* attempt to restore the original name. */
-      ASSERT (sethostname (origname, strlen (origname)) == 0);
-      fprintf (stderr, "setting a too long hostname succeeded.\n");
-      return 1;
-    }
+  /* Known Cygwin bug:
+     <https://cygwin.com/pipermail/cygwin/2024-May/255986.html>  */
+#if !defined __CYGWIN__
+  {
+    char longname[HOST_NAME_MAX + 2];
+
+    /* glibc does allow setting a zero length name, so the lower bound
+       needs no test. validate that we are constrained by
+       HOST_NAME_MAX */
+    for (i = 0; i < (HOST_NAME_MAX + 1); i++)
+      longname[i] = 'a';
+
+    longname[i] = '\0';
+
+    rcs = sethostname (longname, (HOST_NAME_MAX + 1));
+
+    if (rcs != -1)
+      {
+        /* attempt to restore the original name. */
+        ASSERT (sethostname (origname, strlen (origname)) == 0);
+        fprintf (stderr, "setting a too long hostname succeeded.\n");
+        return 1;
+      }
+  }
+#endif
 
   /* restore the original name. */
   ASSERT (sethostname (origname, strlen (origname)) == 0);