strerror_r-posix: Fix for Hurd.

* lib/strerror_r.c (strerror_r): Interpret return value of
__xpg_strerror_r correctly. Remove assumption about how strerror_r
behaves.

ChangeLog

@@ -1,3 +1,10 @@
+2022-09-02  Bruno Haible  <bruno@clisp.org>
+
+	strerror_r-posix: Fix for Hurd.
+	* lib/strerror_r.c (strerror_r): Interpret return value of
+	__xpg_strerror_r correctly. Remove assumption about how strerror_r
+	behaves.
+
 2022-09-02  Bruno Haible  <bruno@clisp.org>
 
 	ptsname tests, ptsname_r tests: Fix test failures on Hurd.

lib/strerror_r.c

@@ -166,16 +166,19 @@ strerror_r (int errnum, char *buf, size_t buflen)
 
 # if HAVE___XPG_STRERROR_R
       ret = __xpg_strerror_r (errnum, buf, buflen);
-      if (ret < 0)
-        ret = errno;
+      /* ret is 0 upon success, or EINVAL or ERANGE upon failure.  */
 # endif
 
       if (!*buf)
         {
-          /* glibc 2.13 would not touch buf on err, so we have to fall
-             back to GNU strerror_r which always returns a thread-safe
-             untruncated string to (partially) copy into our buf.  */
-          char *errstring = strerror_r (errnum, buf, buflen);
+          /* glibc 2.13 ... 2.34 (at least) don't touch buf upon failure.
+             Therefore we have to fall back to strerror_r which, for valid
+             errnum, returns a thread-safe untruncated string.  For invalid
+             errnum, though, it returns a truncated string, which does not
+             allow us to determine whether to return ERANGE or 0.  Thus we
+             need to pass a sufficiently large buffer.  */
+          char stackbuf[80];
+          char *errstring = strerror_r (errnum, stackbuf, sizeof stackbuf);
           ret = errstring ? safe_copy (buf, buflen, errstring) : errno;
         }
     }