Fixes #21 - adding bad example in G-8410

Author: PhilippSalvisberg

docs/4-language-usage/8-patterns/4-ensure-single-execution-at-a-time-of-a-program-unit/g-8410.md

@@ -12,6 +12,52 @@ The alternative using a table where a “Lock-Row” is stored has the disadvant
 ## Example (bad)
 
 ```
+-- create locking table
+CREATE TABLE application_locks (
+   lock_name VARCHAR2(128 CHAR) NOT NULL PRIMARY KEY
+  ,lock_time TIMESTAMP          NOT NULL
+) ORGANIZATION INDEX;
+-- definition
+CREATE OR REPLACE PACKAGE BODY lock_up IS
+   PROCEDURE request_lock (in_lock_name IN VARCHAR2) IS
+      PRAGMA AUTONOMOUS_TRANSACTION;
+   BEGIN
+      INSERT INTO application_locks (lock_name, lock_time) VALUES (in_lock_name, systimestamp);
+      COMMIT;
+   EXCEPTION
+      WHEN DUP_VAL_ON_INDEX THEN
+         RAISE err.e_lock_request_failed;
+   END request_lock;
+   
+   PROCEDURE release_lock (in_lock_name IN VARCHAR2) IS
+      PRAGMA AUTONOMOUS_TRANSACTION;
+   BEGIN
+      DELETE FROM application_locks
+       WHERE lock_name = in_lock_name;
+      COMMIT;
+   END release_lock;
+END lock_up;
+/
+-- call
+DECLARE
+   co_lock_name CONSTANT VARCHAR2(30 CHAR) := 'APPLICATION_LOCK';
+BEGIN
+   lock_up.request_lock(in_lock_name => co_lock_name);
+   -- processing
+   lock_up.release_lock(in_lock_name => co_lock_name);
+EXCEPTION
+   WHEN OTHERS THEN
+      -- log error
+      lock_up.release_lock(in_lock_name => co_lock_name);
+      RAISE;
+END;
+/
+```
+
+## Example (good)
+
+```
+-- definition
 CREATE OR REPLACE PACKAGE BODY lock_up IS
    FUNCTION request_lock (in_lock_name         IN VARCHAR2
                          ,in_release_on_commit IN BOOLEAN := FALSE) RETURN VARCHAR2 IS
@@ -40,11 +86,7 @@ CREATE OR REPLACE PACKAGE BODY lock_up IS
    END release_lock;
 END lock_up;
 /
-```
-
-## Example (good)
-
-```
+-- call
 DECLARE
    l_handle VARCHAR2(128 CHAR);
    co_lock_name CONSTANT VARCHAR2(30 CHAR) := 'APPLICATION_LOCK';
@@ -59,4 +101,4 @@ EXCEPTION
       RAISE;
 END;
 /
-```
+```