Fix #75696: posix_getgrnam fails to print details of group

According to the POSIX specification of `getgrnam_r()` the result of
`sysconf(_SC_GETGR_R_SIZE_MAX)` is an initial value suggested for the
size of the buffer, and `ERANGE` signals that insufficient storage was
supplied.  So if we get `ERANGE`, we try again with a buffer twice as
big, and so on, instead of failing.

NEWS

@@ -2,6 +2,8 @@ PHP                                                                        NEWS
 |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 ?? ??? 2018, PHP 7.1.23
 
+- POSIX:
+  Fixed bug #75696 (posix_getgrnam fails to print details of group). (cmb)
 
 13 Sep 2018, PHP 7.1.22
 

ext/posix/posix.c

@@ -1074,9 +1074,15 @@ PHP_FUNCTION(posix_getgrnam)
 		RETURN_FALSE;
 	}
 	buf = emalloc(buflen);
+try_again:
 	g = &gbuf;
 
 	if (getgrnam_r(name, g, buf, buflen, &g) || g == NULL) {
+		if (errno == ERANGE) {
+			buflen *= 2;
+			buf = erealloc(buf, buflen);
+			goto try_again;
+		}
 		POSIX_G(last_error) = errno;
 		efree(buf);
 		RETURN_FALSE;

ext/posix/tests/bug75696.phpt

@@ -0,0 +1,17 @@
+--TEST--
+Bug #75696 (posix_getgrnam fails to print details of group)
+--SKIPIF--
+<?php
+if (!extension_loaded('posix')) die('skip posix extension not available');
+?>
+--FILE--
+<?php
+$gid = posix_getgid();
+$name = posix_getgrgid($gid)['name'];
+$info = posix_getgrnam($name);
+var_dump(is_array($info));
+?>
+===DONE===
+--EXPECT--
+bool(true)
+===DONE===