[pending] Add concept of "pending operations"

iPXE is fundamentally asynchronous in operation: some operations
continue in the background even after the foreground has continued to
a new task.  For example, the closing FIN/ACK exchanges of a TCP
connection will take place in the background after an HTTP download
has completed.

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

src/include/ipxe/errfile.h

@@ -62,6 +62,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #define ERRFILE_parseopt	       ( ERRFILE_CORE | 0x00160000 )
 #define ERRFILE_test		       ( ERRFILE_CORE | 0x00170000 )
 #define ERRFILE_xferbuf		       ( ERRFILE_CORE | 0x00180000 )
+#define ERRFILE_pending		       ( ERRFILE_CORE | 0x00190000 )
 
 #define ERRFILE_eisa		     ( ERRFILE_DRIVER | 0x00000000 )
 #define ERRFILE_isa		     ( ERRFILE_DRIVER | 0x00010000 )

src/include/ipxe/pending.h

@@ -0,0 +1,34 @@
+#ifndef _IPXE_PENDING_H
+#define _IPXE_PENDING_H
+
+/** @file
+ *
+ * Pending operations
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+#include <ipxe/list.h>
+
+/** A pending operation */
+struct pending_operation {
+	/** Pending count */
+	unsigned int count;
+};
+
+/**
+ * Check if an operation is pending
+ *
+ * @v pending		Pending operation
+ * @v is_pending	Operation is pending
+ */
+static inline int is_pending ( struct pending_operation *pending ) {
+	return ( pending->count != 0 );
+}
+
+extern void pending_get ( struct pending_operation *pending );
+extern void pending_put ( struct pending_operation *pending );
+extern int pending_wait ( unsigned long timeout );
+
+#endif /* _IPXE_PENDING_H */