feat: static table metadata access support

Author: shuxu.li

src/iceberg/table_impl.cc

@@ -19,6 +19,7 @@
 
 #include "iceberg/table_impl.h"
 
+#include "iceberg/exception.h"
 #include "iceberg/partition_spec.h"
 #include "iceberg/schema.h"
 #include "iceberg/snapshot.h"
@@ -30,23 +31,16 @@ namespace iceberg {
 BaseTable::BaseTable(std::string name, std::shared_ptr<TableMetadata> metadata)
     : name_(std::move(name)), metadata_(std::move(metadata)) {
   if (!metadata_) {
-    throw std::invalid_argument("Table metadata cannot be null");
+    throw IcebergError("Table metadata cannot be null");
   }
 }
 
 void BaseTable::InitSchema() const {
   for (const auto& schema : metadata_->schemas) {
     if (schema->schema_id()) {
       schemas_map_.emplace(schema->schema_id().value(), schema);
-      if (schema->schema_id().value() == metadata_->current_schema_id) {
-        schema_ = schema;
-      }
     }
   }
-  // compatible with V1 table schema
-  if (!schema_ && metadata_->schemas.size() == 1UL) {
-    schema_ = metadata_->schemas.front();
-  }
 }
 
 void BaseTable::InitPartitionSpec() const {
@@ -79,13 +73,7 @@ void BaseTable::InitSnapshot() const {
 
 const std::string& BaseTable::uuid() const { return metadata_->table_uuid; }
 
-Result<std::shared_ptr<Schema>> BaseTable::schema() const {
-  std::call_once(init_schema_once_, [this]() { InitSchema(); });
-  if (!schema_) {
-    return NotFound("Current schema is not defined for this table");
-  }
-  return schema_;
-}
+Result<std::shared_ptr<Schema>> BaseTable::schema() const { return metadata_->Schema(); }
 
 const std::unordered_map<int32_t, std::shared_ptr<Schema>>& BaseTable::schemas() const {
   std::call_once(init_schema_once_, [this]() { InitSchema(); });

src/iceberg/table_impl.h

@@ -84,7 +84,6 @@ class ICEBERG_EXPORT BaseTable : public Table {
 
   const std::string name_;
 
-  mutable std::shared_ptr<Schema> schema_;
   mutable std::unordered_map<int32_t, std::shared_ptr<Schema>> schemas_map_;
 
   mutable std::shared_ptr<PartitionSpec> partition_spec_;

src/iceberg/table_metadata.cc

@@ -47,13 +47,24 @@ std::string ToString(const MetadataLogEntry& entry) {
 }
 
 Result<std::shared_ptr<Schema>> TableMetadata::Schema() const {
-  auto iter = std::ranges::find_if(schemas, [this](const auto& schema) {
-    return schema->schema_id() == current_schema_id;
+  std::call_once(init_schema_once, [this]() {
+    auto iter = std::ranges::find_if(schemas, [this](const auto& schema) {
+      return schema->schema_id() == current_schema_id;
+    });
+    if (iter != schemas.end()) {
+      schema = *iter;
+    }
+
+    // compatible with V1 table schema
+    if (!schema && schemas.size() == 1UL) {
+      schema = schemas.front();
+    }
   });
-  if (iter == schemas.end()) {
-    return NotFound("Current schema is not found");
+
+  if (!schema) {
+    return NotFound("Current schema is not defined for this table");
   }
-  return *iter;
+  return schema;
 }
 
 Result<std::shared_ptr<PartitionSpec>> TableMetadata::PartitionSpec() const {

src/iceberg/table_metadata.h

@@ -92,6 +92,8 @@ struct ICEBERG_EXPORT TableMetadata {
   TimePointMs last_updated_ms;
   /// The highest assigned column ID for the table
   int32_t last_column_id;
+  /// The current schema for the table, or null if not set
+  mutable std::shared_ptr<Schema> schema;
   /// A list of schemas
   std::vector<std::shared_ptr<Schema>> schemas;
   /// ID of the table's current schema
@@ -127,6 +129,8 @@ struct ICEBERG_EXPORT TableMetadata {
   /// A `long` higher than all assigned row IDs
   int64_t next_row_id;
 
+  mutable std::once_flag init_schema_once;
+
   /// \brief Get the current schema, return NotFoundError if not found
   Result<std::shared_ptr<Schema>> Schema() const;
   /// \brief Get the current partition spec, return NotFoundError if not found