From e62a18304e4bae13f2ffb77f94d9665a06c19d39 Mon Sep 17 00:00:00 2001
From: Calle Wilund <calle@scylladb.com>
Date: Wed, 10 Sep 2025 13:43:02 +0000
Subject: [PATCH] utils::seekable_source: Add a seekable IO source type

Extension of data_source, with the ability to
a.) Seek in any direction, i.e. move backwards. Thus not pure stream.
b.) Read a limited number of bytes.

The very transparent reason for the interface is to have a base
abstraction for providing a read-only file layer for networked
resources.
---
 utils/seekable_source.hh | 59 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 59 insertions(+)
 create mode 100644 utils/seekable_source.hh

diff --git a/utils/seekable_source.hh b/utils/seekable_source.hh
new file mode 100644
index 0000000000..acddcb2b52
--- /dev/null
+++ b/utils/seekable_source.hh
@@ -0,0 +1,59 @@
+/*
+ * Copyright (C) 2025-present ScyllaDB
+ */
+
+/*
+ * SPDX-License-Identifier: LicenseRef-ScyllaDB-Source-Available-1.0
+ */
+
+#pragma once
+
+#include <chrono>
+#include <seastar/core/iostream.hh>
+
+class seekable_data_source_impl : public seastar::data_source_impl {
+public:
+    virtual seastar::future<seastar::temporary_buffer<char>> get(size_t limit) = 0;
+    virtual seastar::future<> seek(uint64_t pos) = 0;
+    virtual seastar::future<uint64_t> size() {
+        return seastar::make_ready_future<uint64_t>(0);
+    }
+    virtual seastar::future<std::chrono::system_clock::time_point> timestamp() {
+        return seastar::make_ready_future<std::chrono::system_clock::time_point>();
+    }
+};
+
+class seekable_data_source : public seastar::data_source {
+public:
+    seekable_data_source(std::unique_ptr<seekable_data_source_impl> src)
+        : seastar::data_source(std::move(src))
+    {}
+    seastar::future<seastar::temporary_buffer<char>> get(size_t limit) noexcept {
+        try {
+            return static_cast<seekable_data_source_impl*>(impl())->get(limit);
+        } catch (...) {
+            return seastar::current_exception_as_future<seastar::temporary_buffer<char>>();
+        }
+    }
+    seastar::future<> seek(uint64_t pos) noexcept {
+        try {
+            return static_cast<seekable_data_source_impl*>(impl())->seek(pos);
+        } catch (...) {
+            return seastar::current_exception_as_future<>();
+        }
+    }
+    seastar::future<uint64_t> size() noexcept {
+        try {
+            return static_cast<seekable_data_source_impl*>(impl())->size();
+        } catch (...) {
+            return seastar::current_exception_as_future<uint64_t>();
+        }
+    }
+    seastar::future<std::chrono::system_clock::time_point> timestamp() noexcept {
+        try {
+            return static_cast<seekable_data_source_impl*>(impl())->timestamp();
+        } catch (...) {
+            return seastar::current_exception_as_future<std::chrono::system_clock::time_point>();
+        }
+    }
+};