feat: add merge append

While adding merge append tests, it was discovered that FastAppend
did not populate manifest count summary fields, thus add
SnapshotUpdate::BuildManifestCountSummary and use it from
FastAppend and MergingSnapshotUpdate, which is consistent with
java implementation.

Author: zhjwpku

src/iceberg/CMakeLists.txt

@@ -101,6 +101,7 @@ set(ICEBERG_SOURCES
     type.cc
     update/expire_snapshots.cc
     update/fast_append.cc
+    update/merge_append.cc
     update/merging_snapshot_update.cc
     update/pending_update.cc
     update/set_snapshot.cc

src/iceberg/meson.build

@@ -126,6 +126,7 @@ iceberg_sources = files(
     'type.cc',
     'update/expire_snapshots.cc',
     'update/fast_append.cc',
+    'update/merge_append.cc',
     'update/merging_snapshot_update.cc',
     'update/pending_update.cc',
     'update/set_snapshot.cc',

src/iceberg/table.cc

@@ -33,6 +33,7 @@
 #include "iceberg/transaction.h"
 #include "iceberg/update/expire_snapshots.h"
 #include "iceberg/update/fast_append.h"
+#include "iceberg/update/merge_append.h"
 #include "iceberg/update/set_snapshot.h"
 #include "iceberg/update/snapshot_manager.h"
 #include "iceberg/update/update_location.h"
@@ -217,6 +218,12 @@ Result<std::shared_ptr<FastAppend>> Table::NewFastAppend() {
   return FastAppend::Make(name().name, std::move(ctx));
 }
 
+Result<std::shared_ptr<MergeAppend>> Table::NewMergeAppend() {
+  ICEBERG_ASSIGN_OR_RAISE(
+      auto ctx, TransactionContext::Make(shared_from_this(), TransactionKind::kUpdate));
+  return MergeAppend::Make(name().name, std::move(ctx));
+}
+
 Result<std::shared_ptr<UpdateStatistics>> Table::NewUpdateStatistics() {
   ICEBERG_ASSIGN_OR_RAISE(
       auto ctx, TransactionContext::Make(shared_from_this(), TransactionKind::kUpdate));

src/iceberg/table.h

@@ -176,6 +176,9 @@ class ICEBERG_EXPORT Table : public std::enable_shared_from_this<Table> {
   /// \brief Create a new FastAppend to append data files and commit the changes.
   virtual Result<std::shared_ptr<FastAppend>> NewFastAppend();
 
+  /// \brief Create a new MergeAppend to append data files and merge manifests.
+  virtual Result<std::shared_ptr<MergeAppend>> NewMergeAppend();
+
   /// \brief Create a new SnapshotManager to manage snapshots and snapshot references.
   virtual Result<std::shared_ptr<SnapshotManager>> NewSnapshotManager();
 

src/iceberg/test/CMakeLists.txt

@@ -225,6 +225,7 @@ if(ICEBERG_BUILD_BUNDLE)
                    expire_snapshots_test.cc
                    fast_append_test.cc
                    manifest_filter_manager_test.cc
+                   merge_append_test.cc
                    merging_snapshot_update_test.cc
                    name_mapping_update_test.cc
                    snapshot_manager_test.cc