From 18e79d730eb136ea625e21fe4e8f30b4b5a0422b Mon Sep 17 00:00:00 2001
From: Calle Wilund <calle@scylladb.com>
Date: Wed, 20 Sep 2023 13:44:16 +0000
Subject: [PATCH] commitlog: Add iterator adaptor for doing buffer splitting
 into sub-page ranges

With somewhat less overhead than creating 100+ temporary_buffer proxies
---
 db/commitlog/commitlog.cc       | 41 ++++++++++++++++++++++++
 db/commitlog/commitlog_entry.hh | 56 +++++++++++++++++++++++++++++++++
 2 files changed, 97 insertions(+)

diff --git a/db/commitlog/commitlog.cc b/db/commitlog/commitlog.cc
index f0c00d8ce7..84eb6ed0d2 100644
--- a/db/commitlog/commitlog.cc
+++ b/db/commitlog/commitlog.cc
@@ -624,6 +624,47 @@ std::enable_if_t<std::is_fundamental<T>::value, T> read(Input& in) {
     return net::ntoh(in.template read<T>());
 }
 
+detail::sector_split_iterator::sector_split_iterator(const sector_split_iterator&) noexcept = default;
+
+detail::sector_split_iterator::sector_split_iterator()
+    : _ptr(nullptr)
+    , _size(0)
+    , _sector_size(0)
+{}
+
+detail::sector_split_iterator::sector_split_iterator(base_iterator i, base_iterator e, size_t sector_size)
+    : _iter(i)
+    , _end(e)
+    , _ptr(i != e ? const_cast<char*>(i->get()) : nullptr)
+    , _size(i != e ? sector_size - sector_overhead_size : 0)
+    , _sector_size(sector_size)
+{}
+
+detail::sector_split_iterator& detail::sector_split_iterator::operator++() {
+    assert(_iter != _end);
+    _ptr += _sector_size;
+    // check if we have more pages in this temp-buffer (in out case they are always aligned + sized in page units)
+    auto rem = _iter->size() - std::distance(_iter->get(), const_cast<const char*>(_ptr));
+    if (rem == 0) {
+        if (++_iter == _end) {
+            _ptr = nullptr;
+            _size = 0; 
+            return *this;
+        }
+        rem = _iter->size();
+        assert(rem >= _sector_size);
+        // booh. ugly.
+        _ptr = const_cast<char*>(_iter->get());
+    }
+    return *this;
+}
+
+detail::sector_split_iterator detail::sector_split_iterator::operator++(int) {
+    auto res = *this;
+    ++(*this);
+    return res;
+}
+
 /*
  * A single commit log file on disk. Manages creation of the file and writing mutations to disk,
  * as well as tracking the last mutation position of any "dirty" CFs covered by the segment file. Segment
diff --git a/db/commitlog/commitlog_entry.hh b/db/commitlog/commitlog_entry.hh
index b3c994935b..3ee5716c80 100644
--- a/db/commitlog/commitlog_entry.hh
+++ b/db/commitlog/commitlog_entry.hh
@@ -13,6 +13,62 @@
 #include "commitlog_types.hh"
 #include "mutation/frozen_mutation.hh"
 #include "schema/schema_fwd.hh"
+#include "replay_position.hh"
+
+namespace detail {
+
+    using buffer_type = fragmented_temporary_buffer;
+    using base_iterator = typename std::vector<temporary_buffer<char>>::const_iterator;
+
+    static constexpr auto sector_overhead_size = sizeof(uint32_t) + sizeof(db::segment_id_type);
+
+    // iterator adaptor to enable splitting normal
+    // frag-buffer temporary buffer objects into 
+    // sub-disk-page sized chunks.
+    class sector_split_iterator {
+        base_iterator _iter, _end;
+        char* _ptr;
+        size_t _size;
+        size_t _sector_size;
+    public:
+        sector_split_iterator(const sector_split_iterator&) noexcept;
+        sector_split_iterator(base_iterator i, base_iterator e, size_t sector_size);
+        sector_split_iterator();
+
+        char* get_write() const {
+            return _ptr;
+        }
+        size_t size() const {
+            return _size;
+        }
+        char* begin() {
+            return _ptr;
+        }
+        char* end() {
+            return _ptr + _size;
+        }
+        const char* begin() const {
+            return _ptr;
+        }
+        const char* end() const {
+            return _ptr + _size;
+        }
+
+        bool operator==(const sector_split_iterator& rhs) const {
+            return _iter == rhs._iter && _ptr == rhs._ptr;
+        }
+
+        auto& operator*() const {
+            return *this;
+        }
+        auto* operator->() const {
+            return this;
+        }
+
+        sector_split_iterator& operator++();
+        sector_split_iterator operator++(int);
+    };
+}
 
 class commitlog_entry {
     std::optional<column_mapping> _mapping;