[process] Add support for one-shot processes

Some processes execute only once, and exist solely in order to defer
execution until after the relevant instantiator method has returned.
Such processes do not need to be automatically rescheduled when
executing.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/process.c

 		ref_get ( process->refcnt ); /* Inhibit destruction mid-step */
 		desc = process->desc;
 		object = process_object ( process );
-		list_del ( &process->list );
-		list_add_tail ( &process->list, &run_queue );
+		if ( desc->reschedule ) {
+			list_del ( &process->list );
+			list_add_tail ( &process->list, &run_queue );
+		} else {
+			process_del ( process );
+		}
 		DBGC2 ( PROC_COL ( process ), "PROCESS " PROC_FMT
 			" executing\n", PROC_DBG ( process ) );
 		desc->step ( object );

src/include/ipxe/process.h

 	 * CPU to another process.
 	 */
 	void ( * step ) ( void *object );
+	/** Automatically reschedule the process */
+	int reschedule;
 };
 
 /**
 #define PROC_DESC( object_type, process, _step ) {			      \
 		.offset = process_offset ( object_type, process ),	      \
 		.step = PROC_STEP ( object_type, _step ),		      \
+		.reschedule = 1,					      \
+	}
+
+/**
+ * Define a process descriptor for a process that runs only once
+ *
+ * @v object_type	Containing object data type
+ * @v process		Process name (i.e. field within object data type)
+ * @v step		Process' step() method
+ * @ret desc		Object interface descriptor
+ */
+#define PROC_DESC_ONCE( object_type, process, _step ) {			      \
+		.offset = process_offset ( object_type, process ),	      \
+		.step = PROC_STEP ( object_type, _step ),		      \
+		.reschedule = 0,					      \
 	}
 
 /**
 #define PROC_DESC_PURE( _step ) {					      \
 		.offset = 0,						      \
 		.step = PROC_STEP ( struct process, _step ),		      \
+		.reschedule = 1,					      \
 	}
 
 extern void * __attribute__ (( pure ))