Bug30416389 REVOKE ROLE WHEN USER HAS NO HOSTNAME

During lookup of the user, the hostname is set to nullptr.
This patch replace the nullptr with an empty string.

RB: 23229

Author: Kristofer Älvring

mysql-test/r/roles2.result

@@ -508,3 +508,17 @@ DROP DATABASE dddddb1;
 DROP DATABASE secdb1;
 DROP DATABASE secdb2;
 SET GLOBAL partial_revokes = @orig_partial_revokes;
+#
+# Empty hostnames not handled well
+#
+CREATE USER 'u1'@'' IDENTIFIED BY '123';
+GRANT SELECT ON *.* TO 'u1'@'';
+CREATE USER 'r1'@'' IDENTIFIED BY '123';
+CREATE USER 'r2'@'' IDENTIFIED BY '123';
+GRANT ROLE_ADMIN ON *.* TO current_user();
+GRANT 'r1'@'' TO 'u1'@'';
+GRANT 'r2'@'' TO 'u1'@'';
+SET DEFAULT ROLE 'r1'@'', 'r2'@'' TO 'u1'@'';
+REVOKE 'r1'@'' FROM 'u1'@'';
+REVOKE 'r2'@'' FROM 'u1'@'';
+DROP USER 'u1'@'','r1'@'','r2'@'';

mysql-test/t/roles2.test

@@ -564,3 +564,19 @@ disconnect con1;
 --disable_warnings
 SET GLOBAL partial_revokes = @orig_partial_revokes;
 --enable_warnings
+
+--echo #
+--echo # Empty hostnames not handled well
+--echo #
+CREATE USER 'u1'@'' IDENTIFIED BY '123';
+GRANT SELECT ON *.* TO 'u1'@'';
+CREATE USER 'r1'@'' IDENTIFIED BY '123';
+CREATE USER 'r2'@'' IDENTIFIED BY '123';
+GRANT ROLE_ADMIN ON *.* TO current_user();
+GRANT 'r1'@'' TO 'u1'@'';
+GRANT 'r2'@'' TO 'u1'@'';
+SET DEFAULT ROLE 'r1'@'', 'r2'@'' TO 'u1'@'';
+REVOKE 'r1'@'' FROM 'u1'@'';
+REVOKE 'r2'@'' FROM 'u1'@'';
+DROP USER 'u1'@'','r1'@'','r2'@'';
+

sql/auth/sql_auth_cache.h

@@ -79,11 +79,11 @@ class ACL_HOST_AND_IP {
  public:
   ACL_HOST_AND_IP()
       : hostname(nullptr), hostname_length(0), ip(0), ip_mask(0) {}
-  const char *get_host() const { return hostname; }
+  const char *get_host() const { return hostname ? hostname : ""; }
   size_t get_host_len() const { return hostname_length; }
 
   bool has_wildcard() {
-    return (strchr(hostname, wild_many) || strchr(hostname, wild_one) ||
+    return (strchr(get_host(), wild_many) || strchr(get_host(), wild_one) ||
             ip_mask);
   }