Add process cleanup functionality

Introduced a `close` method to ensure proper cleanup of child processes and shared memory resources, preventing zombie processes. Added a `clean` method to iterate through unfinished promises and apply the close operation, enhancing resource management.

src/PsyncLib/Psync.php

@@ -101,6 +101,8 @@
         {
             // Wait for the process to finish
             pcntl_waitpid($p->getPid(), $status);
+            pcntl_signal(SIGTERM, SIG_DFL);
+            posix_kill($p->getPid(), SIGTERM);
 
             // Read the serialized data from shared memory
             $shm = $p->getShm();
@@ -126,6 +128,24 @@
             return $result;
         }
 
+        /**
+         * Closes and cleans up resources associated with the given process.
+         *
+         * @param P $p The process instance to be closed.
+         * @return void
+         */
+        private static function close(P $p): void
+        {
+            // Clean up the child process to prevent zombie processes
+            pcntl_waitpid($p->getPid(), $status);
+            pcntl_signal(SIGTERM, SIG_DFL);
+            posix_kill($p->getPid(), SIGTERM);
+
+            $shm = $p->getShm();
+            shmop_delete($shm);
+            unset(self::$promises[$p->getUuid()]);
+        }
+
         /**
          * Waits for the completion of all promises and returns their results.
          *
@@ -178,6 +198,29 @@
             return $count;
         }
 
+        /**
+         * Cleans up resources associated with promises that are not completed.
+         *
+         * Iterates through a collection of promises, and for each promise that is not
+         * yet marked as done, calls a method to close and clean up the resource.
+         *
+         * @return int The number of promises that were closed and cleaned up.
+         */
+        public static function clean(): int
+        {
+            $count = 0;
+            foreach(self::$promises as $uuid => $p)
+            {
+                if(!self::isDone($p))
+                {
+                    self::close($p);
+                    $count++;
+                }
+            }
+
+            return $count;
+        }
+
         /**
          * Returns the size of the shared memory.
          *