ext/gettext/gettext.c: fudge bindtextdomain() return value on musl

According to POSIX, bindtextdomain() returns "the implementation-
defined default directory pathname used by the gettext family of
functions" when its second parameter is NULL (i.e. when you are
querying the directory corresponding to some text domain and that
directory has not yet been set). But musl does not do this: when no
directory has been set, it consults an environment variable (that
itself may be unset), and there is no default:

  if (!libc.secure) path = getenv("MUSL_LOCPATH");
  /* FIXME: add a default path? */

Worse, the musl bindtextdomain(..., NULL) query always returns NULL,
even when MUSL_LOCPATH is set. This is segfaulting bindtextdomain() in
PHP because it tries to RETURN_STRING on the NULL pointer that the C
function returns.

To work around this, we check for a NULL return from the C function,
and then try to return the value of MUSL_LOCPATH. (This still isn't
perfect, because MUSL_LOCPATH can contain multiple paths, but at least
it's a string.) If MUSL_LOCPATH is unset, we RETURN_FALSE to indicate
failure (there's really no default) rather than segfaulting.

This partially addresses GH #13696

Author: orlitzky

ext/gettext/gettext.c

@@ -23,6 +23,7 @@
 #ifdef HAVE_LIBINTL
 
 #include <stdio.h>
+#include <stdlib.h> /* secure_getenv() */
 #include <locale.h>
 #include "ext/standard/info.h"
 #include "php_gettext.h"
@@ -180,7 +181,7 @@ PHP_FUNCTION(dcgettext)
 PHP_FUNCTION(bindtextdomain)
 {
 	zend_string *domain, *dir = NULL;
-	char *retval, dir_name[MAXPATHLEN];
+	char *retval, dir_name[MAXPATHLEN], *btd_result, *musl_locpath;
 
 	ZEND_PARSE_PARAMETERS_START(1, 2)
 		Z_PARAM_STR(domain)
@@ -191,7 +192,28 @@ PHP_FUNCTION(bindtextdomain)
 	PHP_GETTEXT_DOMAIN_LENGTH_CHECK(1, ZSTR_LEN(domain))
 
 	if (dir == NULL) {
-		RETURN_STRING(bindtextdomain(ZSTR_VAL(domain), NULL));
+		btd_result = bindtextdomain(ZSTR_VAL(domain), NULL);
+		if (btd_result == NULL) {
+			/* It's not POSIX compliant, but musl returns
+			 * NULL rather than an implementation-defined
+			 * default directory when no directory was
+			 * previously bound. Technically there is no
+			 * default directory on musl, but the user can
+			 * provide one (or more) via the MUSL_LOCPATH
+			 * environment variable. For lack of a better
+			 * idea, we return MUSL_LOCPATH if it is set,
+			 * even though it may contain multiple paths.
+			 * We use secure_getenv() because the musl docs
+			 * say that MUSL_LOCPATH will be ignored in the
+			 * scenarios where secure_getenv() will return
+			 * NULL rather than the value of the variable. */
+			musl_locpath = secure_getenv("MUSL_LOCPATH");
+			if (musl_locpath == NULL) {
+				RETURN_FALSE;
+			}
+			RETURN_STRING(musl_locpath);
+		}
+		RETURN_STRING(btd_result);
 	}
 
 	if (ZSTR_LEN(dir) != 0 && !zend_string_equals_literal(dir, "0")) {