io/uring/Operation: add method ReplaceUring()

src/io/uring/CancellableOperation.hxx

@@ -67,6 +67,16 @@ public:
 		// TODO: io_uring_prep_cancel()
 	}
 
+	void Replace(Operation &old_operation,
+		     Operation &new_operation) noexcept {
+		assert(operation == &old_operation);
+		assert(old_operation.cancellable == this);
+
+		old_operation.cancellable = nullptr;
+		operation = &new_operation;
+		new_operation.cancellable = this;
+	}
+
 	void OnUringCompletion(int res) noexcept {
 		if (operation == nullptr)
 			return;

src/io/uring/Operation.cxx

@@ -33,6 +33,7 @@
 #include "Operation.hxx"
 #include "CancellableOperation.hxx"
 
+#include <cassert>
 #include <utility>
 
 namespace Uring {
@@ -46,4 +47,14 @@ Operation::CancelUring() noexcept
 	std::exchange(cancellable, nullptr)->Cancel(*this);
 }
 
+void
+Operation::ReplaceUring(Operation &new_operation) noexcept
+{
+	assert(IsUringPending());
+
+	cancellable->Replace(*this, new_operation);
+
+	assert(cancellable == nullptr);
+}
+
 } // namespace Uring

src/io/uring/Operation.hxx

@@ -62,6 +62,12 @@ public:
 	 */
 	void CancelUring() noexcept;
 
+	/**
+	 * Replace this pending operation with a new one.  This method
+	 * is only legal if IsUringPending().
+	 */
+	void ReplaceUring(Operation &new_operation) noexcept;
+
 	/**
 	 * This method is called when the operation completes.
 	 *