[v26.1.x] cloud_topics: skip reset_size_checksum_metadata() in batch building path

src/v/cloud_topics/level_zero/stm/placeholder.cc

@@ -48,7 +48,8 @@ model::record_batch encode_placeholder_batch(
         builder.add_raw_kv(std::nullopt, std::nullopt);
     }
 
-    auto ph = std::move(builder).build();
+    auto ph = std::move(builder).build(
+      storage::record_batch_builder::reset_size_checksum::no);
     // In order for timequeries to work correctly, we need to ensure we never
     // look inside the batch to answer the query. If the time is append time we
     // don't need to unpack the batch, but instead all records have the

src/v/storage/record_batch_builder.cc

@@ -10,6 +10,7 @@
 #include "storage/record_batch_builder.h"
 
 #include "model/batch_compression.h"
+#include "model/compression.h"
 #include "model/record.h"
 #include "model/record_utils.h"
 #include "model/timeout_clock.h"
@@ -56,15 +57,21 @@ record_batch_builder& record_batch_builder::add_raw_kw(
     return *this;
 }
 
-model::record_batch record_batch_builder::build() && {
+model::record_batch record_batch_builder::build(reset_size_checksum reset) && {
+    vassert(
+      reset == reset_size_checksum::yes
+        || _compression == model::compression::none,
+      "reset_size::checksum::no is only valid for compression::none");
     if (!_timestamp) {
         _timestamp = model::timestamp::now();
     }
     auto header = build_header();
     auto batch = model::record_batch(
       header, std::move(_records), model::record_batch::tag_ctor_ng{});
     if (_compression == model::compression::none) {
-        batch.header().reset_size_checksum_metadata(batch.data());
+        if (reset == reset_size_checksum::yes) {
+            batch.header().reset_size_checksum_metadata(batch.data());
+        }
         return batch;
     }
     return model::compress_batch_sync(_compression, std::move(batch));

src/v/storage/record_batch_builder.h

@@ -15,6 +15,8 @@
 #include "model/record.h"
 #include "utils/vint.h"
 
+#include <seastar/util/bool_class.hh>
+
 namespace storage {
 class record_batch_builder {
 public:
@@ -30,7 +32,14 @@ class record_batch_builder {
       std::optional<iobuf>&& key,
       std::optional<iobuf>&& value,
       chunked_vector<model::record_header> headers);
-    model::record_batch build() &&;
+    // Whether `build()` should finalize the batch by computing its size and
+    // checksums. Only valid if compression mode is compression::none. If set to
+    // `no`, it is the callers responsibility to invoke
+    // `batch.reset_checksum_metadata()`.
+    using reset_size_checksum = ss::bool_class<struct reset_size_checksum_tag>;
+
+    model::record_batch
+    build(reset_size_checksum reset = reset_size_checksum::yes) &&;
     ss::future<model::record_batch> build_async() &&;
     virtual ~record_batch_builder();