Update position in dma_read(iovec) in create_file_for_seekable_source

Fixes: SCYLLADB-1523

The returned file object does not increment file pos as is. One line fix.
Added test to make sure this read path works as expected.

Closes scylladb/scylladb#29456

test/boost/gcp_object_storage_test.cc

@@ -89,16 +89,7 @@ static future<> create_object_of_size(storage::client& c
     co_await sink.close();
 }
 
-static future<> compare_object_data(const local_gcs_wrapper& env, std::string_view object_name, std::vector<temporary_buffer<char>>&& bufs) {
-    auto& c = env.client();
-    auto total = std::accumulate(bufs.begin(), bufs.end(), size_t{}, [](size_t s, auto& buf) {
-        return s + buf.size();
-    });
-
-    auto source = c.create_download_source(env.bucket, object_name);
-    auto is1 = seastar::input_stream<char>(std::move(source));
-    auto is2 = seastar::input_stream<char>(create_memory_source(std::move(bufs)));
-
+static future<> compare_stream_data(seastar::input_stream<char>& is1, seastar::input_stream<char>& is2, size_t total) {
     uint64_t read = 0;
     while (!is1.eof()) {
         auto buf = co_await is1.read();
@@ -113,6 +104,23 @@ static future<> compare_object_data(const local_gcs_wrapper& env, std::string_vi
     BOOST_REQUIRE_EQUAL(read, total);
 }
 
+static std::tuple<seastar::input_stream<char>, size_t> stream_from_buffers(std::vector<temporary_buffer<char>>&& bufs) {
+    auto total = std::accumulate(bufs.begin(), bufs.end(), size_t{}, [](size_t s, auto& buf) {
+        return s + buf.size();
+    });
+    auto is = seastar::input_stream<char>(create_memory_source(std::move(bufs)));
+    return std::make_tuple(std::move(is), total);
+}
+
+static future<> compare_object_data(const local_gcs_wrapper& env, std::string_view object_name, std::vector<temporary_buffer<char>>&& bufs) {
+    auto& c = env.client();
+    auto source = c.create_download_source(env.bucket, object_name);
+    auto is1 = seastar::input_stream<char>(std::move(source));
+    auto [is2, total] = stream_from_buffers(std::move(bufs));
+
+    co_await compare_stream_data(is1, is2, total);
+}
+
 using namespace std::string_literals;
 static constexpr auto prefix = "bork/ninja/"s;
 
@@ -277,4 +285,34 @@ SEASTAR_FIXTURE_TEST_CASE(test_merge_objects, local_gcs_wrapper, *check_gcp_stor
     co_await compare_object_data(env, name, std::move(bufs));
 }
 
+
+SEASTAR_FIXTURE_TEST_CASE(test_gcp_storage_read_large_object_iov, local_gcs_wrapper, *check_gcp_storage_test_enabled()) {
+    auto& c = client();
+    auto name = make_name();
+    std::vector<temporary_buffer<char>> written;
+
+    auto dest_size = 32*1024*1024 + 357 + 1022*67;
+
+    // ensure we remove the object
+    objects_to_delete.emplace_back(name);
+    co_await create_object_of_size(c, bucket, name, dest_size, &written);
+    auto source = c.create_download_source(bucket, name);
+    auto f = create_file_for_seekable_source(std::move(source));
+    auto [is2, total] = stream_from_buffers(std::move(std::move(written)));
+    std::vector<temporary_buffer<char>> bufs;
+    std::vector<iovec> vecs;
+    for (size_t i = 0; i < total; ) {
+        auto n = std::min(total - i, size_t(8192));
+        bufs.emplace_back(n);
+        vecs.emplace_back(iovec{ .iov_base = bufs.back().get_write(), .iov_len = n });
+        i += n;
+    }
+    auto read = co_await f.dma_read(0, std::move(vecs));
+    BOOST_REQUIRE_EQUAL(read, total);
+    auto [is1, total2] = stream_from_buffers(std::move(std::move(bufs)));
+    BOOST_REQUIRE_EQUAL(total, total2);
+    co_await compare_stream_data(is1, is2, total);
+
+}
+
 BOOST_AUTO_TEST_SUITE_END()

utils/io-wrappers.cc

@@ -270,6 +270,7 @@ seastar::file create_file_for_seekable_source(seekable_data_source src, seekable
                     break;
                 }
                 res += n;
+                pos += n;
             }
             co_return res;
         }